<pre>Internet Engineering Task Force (IETF)                         J. Kelsey
Request for Comments: 5848                                          NIST
Category: Standards Track                                      J. Callas
ISSN: 2070-1721                                          PGP Corporation
                                                                A. Clemm
                                                           Cisco Systems
                                                                May 2010


                         <span class="h1">Signed Syslog Messages</span>

Abstract

   This document describes a mechanism to add origin authentication,
   message integrity, replay resistance, message sequencing, and
   detection of missing messages to the transmitted syslog messages.
   This specification is intended to be used in conjunction with the
   work defined in <a href="./rfc5424">RFC 5424</a>, "The Syslog Protocol".

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in <a href="./rfc5741#section-2">Section&nbsp;2 of RFC 5741</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="http://www.rfc-editor.org/info/rfc5848">http://www.rfc-editor.org/info/rfc5848</a>.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   <a href="#section-1">1</a>.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-3">3</a>
   <a href="#section-2">2</a>.  Conventions Used in This Document  . . . . . . . . . . . . . .  <a href="#page-5">5</a>
   <a href="#section-3">3</a>.  Syslog Message Format  . . . . . . . . . . . . . . . . . . . .  <a href="#page-5">5</a>
   <a href="#section-4">4</a>.  Signature Blocks . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-6">6</a>
     <a href="#section-4.1">4.1</a>.  Syslog Messages Containing a Signature Block . . . . . . .  <a href="#page-7">7</a>
     <a href="#section-4.2">4.2</a>.  Signature Block Format and Fields  . . . . . . . . . . . .  <a href="#page-7">7</a>
       <a href="#section-4.2.1">4.2.1</a>.  Version  . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-9">9</a>
       <a href="#section-4.2.2">4.2.2</a>.  Reboot Session ID  . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
       <a href="#section-4.2.3">4.2.3</a>.  Signature Group and Signature Priority . . . . . . . . <a href="#page-10">10</a>
       <a href="#section-4.2.4">4.2.4</a>.  Global Block Counter . . . . . . . . . . . . . . . . . <a href="#page-13">13</a>
       <a href="#section-4.2.5">4.2.5</a>.  First Message Number . . . . . . . . . . . . . . . . . <a href="#page-13">13</a>
       <a href="#section-4.2.6">4.2.6</a>.  Count  . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
       <a href="#section-4.2.7">4.2.7</a>.  Hash Block . . . . . . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
       <a href="#section-4.2.8">4.2.8</a>.  Signature  . . . . . . . . . . . . . . . . . . . . . . <a href="#page-14">14</a>
       <a href="#section-4.2.9">4.2.9</a>.  Example  . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-15">15</a>
   <a href="#section-5">5</a>.  Payload and Certificate Blocks . . . . . . . . . . . . . . . . <a href="#page-15">15</a>
     <a href="#section-5.1">5.1</a>.  Preliminaries: Key Management and Distribution Issues  . . <a href="#page-15">15</a>
     <a href="#section-5.2">5.2</a>.  Payload Block  . . . . . . . . . . . . . . . . . . . . . . <a href="#page-16">16</a>
       <a href="#section-5.2.1">5.2.1</a>.  Block Format and Fields  . . . . . . . . . . . . . . . <a href="#page-16">16</a>
       <a href="#section-5.2.2">5.2.2</a>.  Signer Authentication and Authorization  . . . . . . . <a href="#page-18">18</a>
     <a href="#section-5.3">5.3</a>.  Certificate Block  . . . . . . . . . . . . . . . . . . . . <a href="#page-19">19</a>
       <a href="#section-5.3.1">5.3.1</a>.  Syslog Messages Containing a Certificate Block . . . . <a href="#page-19">19</a>
       <a href="#section-5.3.2">5.3.2</a>.  Certificate Block Format and Fields  . . . . . . . . . <a href="#page-20">20</a>
   <a href="#section-6">6</a>.  Redundancy and Flexibility . . . . . . . . . . . . . . . . . . <a href="#page-24">24</a>
     <a href="#section-6.1">6.1</a>.  Configuration Parameters . . . . . . . . . . . . . . . . . <a href="#page-24">24</a>
       <a href="#section-6.1.1">6.1.1</a>.  Configuration Parameters for Certificate Blocks  . . . <a href="#page-24">24</a>
       <a href="#section-6.1.2">6.1.2</a>.  Configuration Parameters for Signature Blocks  . . . . <a href="#page-26">26</a>
     <a href="#section-6.2">6.2</a>.  Overlapping Signature Blocks . . . . . . . . . . . . . . . <a href="#page-27">27</a>
   <a href="#section-7">7</a>.  Efficient Verification of Logs . . . . . . . . . . . . . . . . <a href="#page-27">27</a>
     <a href="#section-7.1">7.1</a>.  Offline Review of Logs . . . . . . . . . . . . . . . . . . <a href="#page-28">28</a>
     <a href="#section-7.2">7.2</a>.  Online Review of Logs  . . . . . . . . . . . . . . . . . . <a href="#page-29">29</a>
   <a href="#section-8">8</a>.  Security Considerations  . . . . . . . . . . . . . . . . . . . <a href="#page-32">32</a>
     <a href="#section-8.1">8.1</a>.  Cryptographic Constraints  . . . . . . . . . . . . . . . . <a href="#page-32">32</a>
     <a href="#section-8.2">8.2</a>.  Packet Parameters  . . . . . . . . . . . . . . . . . . . . <a href="#page-33">33</a>



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     <a href="#section-8.3">8.3</a>.  Message Authenticity . . . . . . . . . . . . . . . . . . . <a href="#page-33">33</a>
     <a href="#section-8.4">8.4</a>.  Replaying  . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-33">33</a>
     <a href="#section-8.5">8.5</a>.  Reliable Delivery  . . . . . . . . . . . . . . . . . . . . <a href="#page-34">34</a>
     <a href="#section-8.6">8.6</a>.  Sequenced Delivery . . . . . . . . . . . . . . . . . . . . <a href="#page-34">34</a>
     <a href="#section-8.7">8.7</a>.  Message Integrity  . . . . . . . . . . . . . . . . . . . . <a href="#page-34">34</a>
     <a href="#section-8.8">8.8</a>.  Message Observation  . . . . . . . . . . . . . . . . . . . <a href="#page-34">34</a>
     <a href="#section-8.9">8.9</a>.  Man-in-the-Middle Attacks  . . . . . . . . . . . . . . . . <a href="#page-34">34</a>
     <a href="#section-8.10">8.10</a>. Denial of Service  . . . . . . . . . . . . . . . . . . . . <a href="#page-35">35</a>
     <a href="#section-8.11">8.11</a>. Covert Channels  . . . . . . . . . . . . . . . . . . . . . <a href="#page-35">35</a>
   <a href="#section-9">9</a>.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . <a href="#page-35">35</a>
     <a href="#section-9.1">9.1</a>.  Structured Data and Syslog Messages  . . . . . . . . . . . <a href="#page-35">35</a>
     <a href="#section-9.2">9.2</a>.  Version Field  . . . . . . . . . . . . . . . . . . . . . . <a href="#page-36">36</a>
     <a href="#section-9.3">9.3</a>.  SG Field . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-38">38</a>
     <a href="#section-9.4">9.4</a>.  Key Blob Type  . . . . . . . . . . . . . . . . . . . . . . <a href="#page-38">38</a>
   <a href="#section-10">10</a>. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-39">39</a>
   <a href="#section-11">11</a>. References . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-39">39</a>
     <a href="#section-11.1">11.1</a>. Normative References . . . . . . . . . . . . . . . . . . . <a href="#page-39">39</a>
     <a href="#section-11.2">11.2</a>. Informative References . . . . . . . . . . . . . . . . . . <a href="#page-40">40</a>

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   This document describes a mechanism, called syslog-sign in this
   document, that adds origin authentication, message integrity, replay
   resistance, message sequencing, and detection of missing messages to
   syslog.  Essentially, this is accomplished by sending a special
   syslog message.  The content of this syslog message is called a
   Signature Block.  Each Signature Block contains, in effect, a
   detached signature on some number of previously sent messages.  It is
   cryptographically signed and contains the hashes of previously sent
   syslog messages.  The originator of syslog-sign messages is simply
   referred to as a "signer".  The signer can be the same originator as
   the originator whose messages it signs, or it can be a separate
   originator.

   While most implementations of syslog involve only a single originator
   and a single collector of each message, provisions need to be made to
   cover situations in which messages are sent to multiple collectors.
   This concerns, in particular, situations in which different messages
   from the same originator are sent to different collectors, which
   means that some messages are sent to some collectors but not to
   others.  The required differentiation of messages is generally
   performed based on the Priority value of the individual messages.
   For example, messages from any Facility with a Severity value of 3,
   2, 1, or 0 may be sent to one collector while all messages of
   Facilities 4, 10, 13, and 14 may be sent to another collector.
   Appropriate syslog-sign messages must be kept with their proper
   syslog messages.  To address this, syslog-sign uses a Signature
   Group.  A Signature Group identifies a group of messages that are all



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   kept together for signing purposes by the signer.  A Signature Block
   always belongs to exactly one Signature Group and always signs
   messages belonging only to that Signature Group.

   Additionally, a signer sends Certificate Blocks to provide key
   management information between the signer and the collector.  A
   Certificate Block has a field to denote the type of key material
   which may be such things as a Public Key Infrastructure using X.509
   (PKIX) certificate, an OpenPGP (Pretty Good Privacy) certificate, or
   even an indication that a key had been pre-distributed.  In the cases
   of certificates being sent, the certificates may have to be split
   across multiple Certificate Blocks carried in separate messages.

   It is possible that the same host contains multiple signers that each
   use their own keys to sign syslog messages.  In this case, each
   signer sends its own Certificate Block and Signature Blocks.
   Furthermore, each signer defines its own Signature Groups.  Each
   signer on a given host needs to use a distinct combination of APP-
   NAME, and PROCID for its Signature Block and Certificate Block
   message.  (This implies that the combination of HOSTNAME, APP-NAME,
   and PROCID uniquely distinguishes originators of syslog-sign messages
   across hosts, provided that the signers use a unique HOSTNAME.)

   The collector may verify that the hash of each received message
   matches the signed hash contained in the corresponding Signature
   Block.  A collector may process these Signature Blocks as they
   arrive, building an authenticated log file.  Alternatively, it may
   store all the log messages in the order they were received.  This
   allows a network operator to authenticate the log file at the time
   the logs are reviewed.

   The process of signing works as long as the collector accepts the
   syslog messages, the Certificate Blocks and the Signature Blocks.
   Once that is done, the process is complete.  After that, anyone can
   go back, find the key material, and validate the received messages
   using the information in the Signature Blocks.  Finding the key
   material is very easily done with Key Blob Types C, P, and K (see
   <a href="#section-4.2">Section 4.2</a>) since the public key is in the Payload Block.  If Key
   Blob Types N or U are used, some poking around may be required to
   find the key material.  The only way to have a vendor-specific
   implementation is through N or U; however, also in that case, the key
   material will have to be available in some form which could be used
   by implementations of other vendors.

   Because the mechanism that is described in this specification uses
   the concept of STRUCTURED-DATA elements defined in [<a href="./rfc5424" title="&quot;The syslog Protocol&quot;">RFC5424</a>],
   compliant implementations of this specification MUST also implement
   [<a href="./rfc5424" title="&quot;The syslog Protocol&quot;">RFC5424</a>].  It is conceivable that the concepts underlying this



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   specification could also be used in conjunction with other message-
   delivery mechanisms.  Designers of other efforts to define event
   notification mechanisms are therefore encouraged to consider this
   specification in their designs.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Conventions Used in This Document</span>

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [<a href="./rfc2119" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">RFC2119</a>].

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Syslog Message Format</span>

   This specification is intended to be used in conjunction with the
   syslog protocol as defined in [<a href="./rfc5424" title="&quot;The syslog Protocol&quot;">RFC5424</a>].  The syslog protocol
   therefore MUST be supported by implementations of this specification.

   Because the originator generating the Signature Block message, also
   simply referred to as "signer", signs each message in its entirety,
   the messages MUST NOT be changed in transit.  By the same token, the
   syslog-sign messages MUST NOT be changed in transit.  One of the
   effects of such behavior, including message alteration by relays,
   would be to render any signing invalid and hence make the mechanism
   useless.  Likewise, any truncation of messages that occurs between
   sending and receiving renders the mechanism useless.  For this
   reason, syslog signer and collector implementations implementing this
   specification MUST support messages of up to and including 2048
   octets in length, in order to minimize the chance of truncation.
   While syslog signer and collector implementations MAY support
   messages with a length longer than 2048 octets, implementers need to
   be aware that any message truncations that occur render the mechanism
   useless.  In such cases, it is up to the operator to ensure that the
   syslog messages can be received properly and can be validated.

   [<a id="ref-RFC5426">RFC5426</a>] recommends using the Transport Layer Security (TLS)
   transport and deliberately constrains the use of UDP.  UDP is NOT
   RECOMMENDED for use with signed syslog because its recommended
   payload size of 480 octets is too restrictive for the purposes of
   syslog-sign.  A 480-octet Signature Block could sign only 9 normal
   messages, meaning that at a significant proportion of messages would
   be Signature Block messages.  The 480-octet limitation is primarily
   geared towards small embedded systems with significant resource
   constraints that, because of those constraints, would not implement
   syslog-sign in the first place.  In addition, the use of UDP is
   geared towards syslog messages that are primarily intended for
   troubleshooting, a very different purpose from the application
   targeted by syslog-sign.  Where syslog UDP transport is used, it is
   the responsibility of operators to ensure that network paths are



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   configured in a way that messages of sufficient length (up to and
   including 2048 octets) can be properly delivered.

   This specification uses the syslog message format described in
   [<a href="./rfc5424" title="&quot;The syslog Protocol&quot;">RFC5424</a>].  Along with other fields, that document describes the
   concept of Structured Data (SD).  Structured Data is defined in terms
   of SD ELEMENTS (SDEs).  An SDE consists of a name and a set of
   parameter name-value pairs.  The SDE name is referred to as SD-ID.
   The name-value pairs are referred to as SD-PARAM, or SD Parameters,
   with the name constituting the SD-PARAM-NAME, and the value
   constituting the SD-PARAM-VALUE.

   The syslog messages defined in this document carry the data that is
   associated with Signature Blocks and Certificate Blocks as Structured
   Data.  For this purpose, the special syslog messages defined in this
   document include definitions of SDEs to convey parameters that relate
   to the signing of syslog messages.  The MSG part of the syslog
   messages defined in this document SHOULD simply be empty -- the
   content of the messages is not intended for interpretation by humans
   but by applications that use those messages to build an authenticated
   log.

   Because the syslog messages defined in this document adhere to the
   format described in [<a href="./rfc5424" title="&quot;The syslog Protocol&quot;">RFC5424</a>], they identify the machine that
   originates the syslog message in the HOSTNAME field.  Therefore, the
   Signature Block and Certificate Block data do not need to include any
   additional parameter to identify the machine that originates the
   message.

   In addition, several signers MAY sign messages on a single host
   independently of each other, each using their own Signature Groups.
   In that case, each unique signer is distinguished by the combination
   of APP-NAME and PROCID.  (By the same token, the same message might
   be signed by multiple signers.)  Each unique signer MUST have a
   unique APP-NAME and PROCID on each host.  (This implies that the
   combination of HOSTNAME, APP-NAME and PROCID uniquely distinguishes
   the originator of syslog-sign messages, provided that the signers use
   a unique HOSTNAME.)  A Signature Block message MUST use the same
   combination of HOSTNAME, APP-NAME, and PROC-ID that was used to send
   the corresponding Certificate Block messages containing the Payload
   Block.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Signature Blocks</span>

   This section describes the format of the Signature Block and the
   fields used within the Signature Block, as well as the syslog
   messages used to carry the Signature Block.




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<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Syslog Messages Containing a Signature Block</span>

   There is a need to distinguish the Signature Block itself from the
   syslog message that is used to carry a Signature Block.  Signature
   Blocks MUST be encompassed within completely formed syslog messages.
   Syslog messages that contain a Signature Block are also referred to
   as Signature Block messages.

   A Signature Block message is identified by the presence of an SD
   ELEMENT with an SD-ID with the value "ssign".  In addition, a
   Signature Block message MUST contain valid APP-NAME, PROCID, and
   MSGID fields to be compliant with [<a href="./rfc5424" title="&quot;The syslog Protocol&quot;">RFC5424</a>].  This specification does
   not mandate particular values for these fields; however, for
   consistency, a signer MUST use the same values for APP-NAME, PROCID,
   and MSGID fields for every Signature Block message that is sent,
   whichever values are chosen.  It MUST also use the same value for its
   HOSTNAME field.  To allow for the possibility of multiple signers per
   host, the combination of APP-NAME and PROCID MUST be unique for each
   such signer on any given host.  If a signer daemon is restarted, it
   MAY use a new PROCID for what is otherwise the same signer but MUST
   continue to use the same APP-NAME.  If it uses a new PROCID, it MUST
   send a new Payload Block using Certificate Block messages that use
   the same new PROCID (and the same APP-NAME).  It is RECOMMENDED (but
   not required) to use 110 as value for the PRI field, corresponding to
   facility 13 (log audit) and severity 6 (informational).  The
   Signature Block is carried as Structured Data within the Signature
   Block message, per the definitions that follow in the next section.
   A Signature Block message MAY carry other Structured Data besides the
   Structured Data of the Signature Block itself.  The MSG part of a
   Signature Block message SHOULD be empty.

   The syslog messages defined as part of syslog-sign themselves
   (Signature Block messages and Certificate Block messages) MUST NOT be
   signed by a Signature Block.  Collectors that implement syslog-sign
   know to distinguish syslog messages that are associated with syslog-
   sign from those that are subjected to signing and process them
   differently.  The intent of syslog-sign is to sign a stream of syslog
   messages, not to alter it.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Signature Block Format and Fields</span>

   The content of a Signature Block message is the Signature Block
   itself.  The Signature Block MUST be encoded as an SD ELEMENT, as
   defined in [<a href="./rfc5424" title="&quot;The syslog Protocol&quot;">RFC5424</a>].

   The SD-ID MUST have the value of "ssign".





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   The SDE contains the fields of the Signature Block encoded as SD
   Parameters, as specified in the following.  The Signature Block is
   composed of the following fields.  The value of each field MUST be
   printable ASCII, and any binary values MUST be base64 encoded, as
   defined in [<a href="./rfc4648" title="&quot;The Base16, Base32, and Base64 Data Encodings&quot;">RFC4648</a>].

      Field                     SD-PARAM-NAME        Size in octets
      -----                     -------------        ---- -- ------

      Version                          VER                 4

      Reboot Session ID               RSID                1-10

      Signature Group                   SG                 1

      Signature Priority              SPRI                1-3

      Global Block Counter             GBC                1-10

      First Message Number             FMN                1-10

      Count                            CNT                1-2

      Hash Block                        HB      variable, size of hash
                                              times the number of hashes
                                               (base64 encoded binary)

      Signature                       SIGN             variable
                                               (base64 encoded binary)

   The fields MUST be provided in the order listed.  Each SD parameter
   MUST occur once and only once in the Signature Block.  New SD
   parameters MUST NOT be added unless a new Version of the protocol is
   defined.  (Implementations that wish to add proprietary extensions
   will need to define a separate SD ELEMENT.)  A Signature Block is
   accordingly encoded as follows, where xxx denotes a placeholder for
   the particular values:

   [ssign VER="xxx" RSID="xxx" SG="xxx" SPRI="xxx" GBC="xxx" FMN="xxx"
   CNT="xxx" HB="xxx" SIGN="xxx"]

   Values of the fields constitute SD parameter values and are hence
   enclosed in quotes, per [<a href="./rfc5424" title="&quot;The syslog Protocol&quot;">RFC5424</a>].  The fields are separated by
   single spaces and are described in the subsequent subsections.







<span class="grey">Kelsey, et al.               Standards Track                    [Page 8]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-9" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


<span class="h4"><a class="selflink" id="section-4.2.1" href="#section-4.2.1">4.2.1</a>.  Version</span>

   The Version field is an alphanumeric value that has a length of 4
   octets, which may include leading zeroes.  The first 2 octets and the
   last octet contain a decimal character in the range of "0" to "9",
   whereas the third octet contains an alphanumeric character in the
   range of "0" to "9", "a" to "z", or "A" to "Z".  The value in this
   field specifies the version of the syslog-sign protocol.  This is
   extensible to allow for different hash algorithms and signature
   schemes to be used in the future.  The value of this field is the
   grouping of the protocol version (2 octets), the hash algorithm (1
   octet), and the signature scheme (1 octet).

      Protocol Version - 2 octets, with "01" as the value for the
      protocol version that is described in this document.

      Hash Algorithm - 1 octet, where, in conjunction with Protocol
      Version 01, a value of "1" denotes SHA1 and a value of "2" denotes
      SHA256, as defined in [<a href="#ref-FIPS.180-2.2002" title="&quot;Secure Hash Standard&quot;">FIPS.180-2.2002</a>].  (This is the octet that
      can have a value of not just "0" to "9" but also "a" to "z" and
      "A" to "Z".)

      Signature Scheme - 1 octet, where, in conjunction with Protocol
      Version 01, a value of "1" denotes OpenPGP DSA, defined in
      [<a href="./rfc4880" title="&quot;OpenPGP Message Format&quot;">RFC4880</a>] and [<a href="#ref-FIPS.186-2.2000" title="&quot;Digital Signature Standard&quot;">FIPS.186-2.2000</a>].

   The version, hash algorithm, and signature scheme defined in this
   document would accordingly be represented as "0111" (if SHA1 is used
   as Hash Algorithm) and "0121" (if SHA256 is used as Hash Algorithm),
   respectively (without the quotation marks).

   The values of the Hash Algorithm and Signature Scheme are defined
   relative to the Protocol Version.  If the single-octet representation
   of the values for Hash Algorithm and Signature Scheme were to ever
   represent a limitation, this limitation could be overcome by defining
   a new Protocol Version with additional Hash Algorithms and/or
   Signature Schemes, and having implementations support both Protocol
   Versions concurrently.

   As long as the sender and receiver are both adhering to [<a href="./rfc5424" title="&quot;The syslog Protocol&quot;">RFC5424</a>],
   the prerequisites are in place so that signed messages can be
   received by the receiver and validated with a Signature Block.  To
   ensure immediate validation of received messages, all implementations
   MUST support SHA1, and SHA256 SHOULD be supported.







<span class="grey">Kelsey, et al.               Standards Track                    [Page 9]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-10" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


<span class="h4"><a class="selflink" id="section-4.2.2" href="#section-4.2.2">4.2.2</a>.  Reboot Session ID</span>

   The Reboot Session ID is a decimal value that has a length between 1
   and 10 octets.  The acceptable values for this are between 0 and
   9999999999.  Leading zeroes MUST be omitted.

   A Reboot Session ID is expected to strictly monotonically increase
   (i.e., to never repeat or decrease) whenever a signer reboots in
   order to allow collectors to distinguish messages and message
   signatures across reboots.  There are several ways in which this may
   be accomplished.  In one way, the Reboot Session ID may increase by
   1, starting with a value of 1.  Note that in this case, a signer is
   required to retain the previous Reboot Session ID across reboots.  In
   another way, a value of the Unix time (number of seconds since 1
   January 1970) may be used.  Implementers of this method need to
   beware of the possibility of multiple reboots occurring within a
   single second.  Implementers need to also beware of the year 2038
   problem, which will cause the 32-bit representation of Unix time to
   wrap in the year 2038.  In yet another way, implementations where the
   Simple Network Management Protocol (SNMP) engine and the signer
   always reboot at the same time might consider using the
   snmpEngineBoots value as a source for this counter as defined in
   [<a href="./rfc3414" title="&quot;User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3)&quot;">RFC3414</a>].

   In cases where a signer is not able to guarantee that the Reboot
   Session ID is always increased after a reboot, the Reboot Session ID
   MUST always be set to a value of 0.  If the value can no longer be
   increased (e.g., because it reaches 9999999999), it SHOULD be reset
   to a value of 1.  Implementations SHOULD ensure that such a reset
   does not go undetected, for example, by requesting operator
   acknowledgment when a reset is performed upon reboot.  (Operator
   acknowledgment may not be possible in all situations, e.g., in the
   case of embedded devices.)

   If a reboot of a signer takes place, Signature Block messages MAY use
   a new PROCID.  However, Signature Block messages of the same signer
   MUST continue to use the same HOSTNAME, APP-NAME, and MSGID.

<span class="h4"><a class="selflink" id="section-4.2.3" href="#section-4.2.3">4.2.3</a>.  Signature Group and Signature Priority</span>

   The SG parameter may take any value from 0-3 inclusive.  The SPRI
   parameter may take any value from 0-191 inclusive.  These fields
   taken together allow network administrators to associate groupings of
   syslog messages with appropriate Signature Blocks and Certificate
   Blocks.  Groupings of syslog messages that are signed together are
   also called Signature Groups.  A Signature Block contains only hashes
   of those syslog messages that are part of the same Signature Group.




<span class="grey">Kelsey, et al.               Standards Track                   [Page 10]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-11" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


   For example, in some cases, network administrators might have
   originators send syslog messages of Facilities 0 through 15 to one
   collector and those with Facilities 16 through 23 to another.  In
   such cases, associated Signature Blocks should likely be sent to the
   corresponding collectors as well, signing the syslog messages that
   are intended for each collector separately.  This way, each collector
   receives Signature Blocks for all syslog messages that it receives,
   and only for those.  The ability to associate different categories of
   syslog messages with different Signature Groups, signed in separate
   Signature Blocks, provides administrators with flexibility in this
   regard.

   Syslog-sign provides four options for handling Signature Groups,
   linking them with PRI values so they may be routed to the destination
   commensurate with the corresponding syslog messages.  In all cases,
   no more than 192 distinct Signature Groups (0-191) are permitted.

   The Signature Group to which a Signature Block pertains is indicated
   by the Signature Priority (SPRI) field.  The Signature Group (SG)
   field indicates how to interpret the Signature Priority field.  (Note
   that the SG field does not indicate the Signature Group itself, as
   its name might suggest.)  The SG field can have one of the following
   values:

   a.  "0" -- There is only one Signature Group.  In this case, the
       administrators want all Signature Blocks to be sent to a single
       destination; in all likelihood, all of the syslog messages will
       also be going to that same destination.  Signature Blocks contain
       signatures for all messages regardless of their PRI value.  This
       means that, in effect, the Signature Block's SPRI value can be
       ignored.  However, it is RECOMMENDED that a single SPRI value be
       used for all Signature Blocks.  Furthermore, it is RECOMMENDED to
       set that value to the same value as the PRI field of the
       Signature Block message.  This way, the PRI of the Signature
       Block message matches the SPRI of the Signature Block that it
       contains.

   b.  "1" -- Each PRI value is associated with its own Signature Group.
       Signature Blocks for a given Signature Group have SPRI = PRI for
       that Signature Group.  In other words, the SPRI of the Signature
       Block matches the PRI value of the syslog messages that are part
       of the Signature Group and hence signed by the Signature Block.
       An SG value of 1 can, for example, be used when the administrator
       of a signer does not know where any of the syslog messages will
       ultimately go but anticipates that messages with different PRI
       values will be collected and processed separately.  Having a
       Signature Group per PRI value provides administrators with a
       large degree of flexibility with regard to how to divide up the



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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


       processing of syslog messages and their signatures after they are
       received, at the same time allowing Signature Blocks to follow
       the corresponding syslog messages to their eventual destination.

   c.  "2" -- Each Signature Group contains a range of PRI values.
       Signature Groups are assigned sequentially.  A Signature Block
       for a given Signature Group has its own SPRI value denoting the
       highest PRI value of syslog messages in that Signature Group.
       The lowest PRI value of syslog messages in that Signature Group
       will be 1 larger than the SPRI value of the previous Signature
       Group or "0" in case there is no other Signature Group with a
       lower SPRI value.  The specific Signature Groups and ranges they
       are associated with are subject to configuration by a system
       administrator.

   d.  "3" -- Signature Groups are not assigned with any of the above
       relationships to PRI values of the syslog messages they sign.
       Instead, another scheme is used, which is outside the scope of
       this specification.  There has to be some predefined arrangement
       between the originator and the intended collectors as to which
       syslog messages are to be included in which Signature Group,
       requiring configuration by a system administrator.  This also
       provides administrators with the flexibility to group syslog
       messages into Signature Groups according to criteria that are not
       tied to the PRI value.  Note that this option is not intended for
       deployments that lack such an arrangement, as in those cases a
       collector could misinterpret the intended meaning of the
       Signature Group.  A collector that receives Signature Block
       messages of a Signature Group of whose scheme it is not aware
       SHOULD bring this fact to the attention of the system
       administrator.  The particular mechanism used for that is
       implementation-specific and outside the scope of this
       specification.

   One reasonable way to configure some installations is to have only
   one Signature Group, indicated with SG=0, and have the signer send a
   copy of each Signature Block to each collector.  In that case,
   collectors that are not configured to receive every syslog message
   will still receive signatures for every message, even ones they are
   not supposed to receive.  While the collector will not be able to
   detect gaps in the messages (because the presence of a signature of a
   message that is missing does not tell the collector whether or not
   the corresponding message would be of the collector's concern), it
   does allow all messages that do arrive at each collector to be put
   into the right order and to be verified.  It also allows each
   collector to detect duplicates.  Likewise, configuring only one





<span class="grey">Kelsey, et al.               Standards Track                   [Page 12]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-13" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


   Signature Group can be a reasonable way to configure installations
   that involve relay chains, where one or more interim relays may or
   may not relay all messages to the same destination.

<span class="h4"><a class="selflink" id="section-4.2.4" href="#section-4.2.4">4.2.4</a>.  Global Block Counter</span>

   The Global Block Counter is a decimal value representing the number
   of Signature Blocks sent by syslog-sign before the current one, in
   this reboot session.  This takes at least 1 octet and at most 10
   octets displayed as a decimal counter.  The acceptable values for
   this are between 0 and 9999999999, starting with 0.  Leading zeroes
   MUST be omitted.  If the value of the Global Block Counter has
   reached 9999999999 and the Reboot Session ID has a value other than 0
   (indicating the fact that persistence of the Reboot Session ID is
   supported), then the Reboot Session ID MUST be incremented by 1 and
   the Global Block Counter resumes at 0.  When the Reboot Session ID is
   0 (i.e., persistent Reboot Session IDs are not supported) and the
   Global Block Counter reaches its maximum value, then the Global Block
   Counter is reset to 0 and the Reboot Session ID MUST remain at 0.

   Note that the Global Block Counter crosses Signature Groups; it
   allows one to roughly synchronize when two messages were sent, even
   though they went to different collectors and are part of different
   Signature Groups.

   Because a reboot results in the start of a new reboot session, the
   signer MUST reset the Global Block Counter to 0 after a reboot
   occurs.  Applications need to take into account the possibility that
   a reboot occurred when authenticating a log, and situations in which
   reboots occur frequently may result in losing the ability to verify
   the proper sequence in which messages were sent, hence jeopardizing
   the integrity of the log.

<span class="h4"><a class="selflink" id="section-4.2.5" href="#section-4.2.5">4.2.5</a>.  First Message Number</span>

   This is a decimal value between 1 and 10 octets, with leading zeroes
   omitted.  It contains the unique message number within this Signature
   Group of the first message whose hash appears in this block.  The
   very first message of the reboot session is numbered "1".  This
   implies that when the Reboot Session ID increases, the message number
   is reset to 1.

   For example, if this Signature Group has processed 1000 messages so
   far and message number 1001 is the first message whose hash appears
   in this Signature Block, then this field contains 1001.  The message
   number is relative to the Signature Group to which it belongs; hence,
   a message number does not identify a message beyond its Signature
   Group.



<span class="grey">Kelsey, et al.               Standards Track                   [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


   Should the message number reach 9999999999 within the same reboot
   session and Signature Group, the message number subsequently restarts
   at 1.  In such an event, the Global Block Counter will be vastly
   different between two occurrences of the same message number.

<span class="h4"><a class="selflink" id="section-4.2.6" href="#section-4.2.6">4.2.6</a>.  Count</span>

   The count is a 1- or 2-octet field that indicates the number of
   message hashes to follow.  The valid values for this field are 1
   through 99.  The number of hashes included in the Signature Block
   MUST be chosen such that the length of the resulting syslog message
   does not exceed the maximum permissible syslog message length.

<span class="h4"><a class="selflink" id="section-4.2.7" href="#section-4.2.7">4.2.7</a>.  Hash Block</span>

   The hash block is a block of hashes, each separately encoded in
   base64.  Each hash in the hash block is the hash of the entire syslog
   message represented by the hash, independent of the underlying
   transport.  Hashes are ordered from left to right in the order of
   occurrence of the syslog messages that they represent.  The space
   character is used to separate the hashes.  Note, the hash block
   constitutes a single SD-PARAM; a Signature Block message MUST include
   all its hashes in a single hash block and MUST NOT spread its hashes
   across several hash blocks.

   The "entire syslog message" refers to what is described as the syslog
   message excluding transport parts that are described in [<a href="./rfc5425" title="&quot;TLS Transport Mapping for syslog&quot;">RFC5425</a>] and
   [<a href="./rfc5426" title="&quot;Transmission of syslog Messages over UDP&quot;">RFC5426</a>], and excluding other parts that may be defined in future
   transports.  The hash value will be the result of the hashing
   algorithm run across the syslog message, starting with the "&lt;" of the
   PRI portion of the header part of the message.  The hash algorithm
   used and indicated by the Version field determines the size of each
   hash, but the size MUST NOT be shorter than 160 bits without the use
   of padding.  It is base64 encoded as per [<a href="./rfc4648" title="&quot;The Base16, Base32, and Base64 Data Encodings&quot;">RFC4648</a>].

   The number of hashes in a hash block SHOULD be chosen such that the
   resulting Signature Block message does not exceed a length of 2048
   octets in order to avoid the possibility that truncation occurs.
   When more hashes need to be sent than fit inside a Signature Block
   message, it is advisable to start a new Signature Block.

<span class="h4"><a class="selflink" id="section-4.2.8" href="#section-4.2.8">4.2.8</a>.  Signature</span>

   This is a digital signature, encoded in base64 per [<a href="./rfc4648" title="&quot;The Base16, Base32, and Base64 Data Encodings&quot;">RFC4648</a>].  The
   signature is calculated over the completely formatted Signature Block
   message (starting from the first octet of PRI and continuing to the
   last octet of MSG, or STRUCTURED-DATA if MSG is not present), before
   the SIGN parameter (SD Parameter Name and the space before it



<span class="grey">Kelsey, et al.               Standards Track                   [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


   [" SIGN"], "=", and the corresponding value) is added.  (In other
   words, the digital signature is calculated over the whole message,
   with the "SIGN=value" portion removed.)  For the OpenPGP DSA
   signature scheme, the value of the signature field contains the DSA
   values r and s, encoded as two multiprecision integers (see
   [<a href="./rfc4880" title="&quot;OpenPGP Message Format&quot;">RFC4880</a>], Sections <a href="#section-5.2.2">5.2.2</a> and <a href="#section-3.2">3.2</a>), concatenated, and then encoded in
   base64 [<a href="./rfc4648" title="&quot;The Base16, Base32, and Base64 Data Encodings&quot;">RFC4648</a>].

<span class="h4"><a class="selflink" id="section-4.2.9" href="#section-4.2.9">4.2.9</a>.  Example</span>

   An example of a Signature Block message is depicted below, broken
   into lines to fit publication rules.  There is a space at the end of
   each line, with the exception of the last line, which ends with "]".

   &lt;110&gt;1 2009-05-03T14:00:39.529966+02:00 host.example.org syslogd
   2138 - [ssign VER="0111" RSID="1" SG="0" SPRI="0" GBC="2" FMN="1"
   CNT="7" HB="K6wzcombEvKJ+UTMcn9bPryAeaU= zrkDcIeaDluypaPCY8WWzwHpPok=
   zgrWOdpx16ADc7UmckyIFY53icE= XfopJ+S8/hODapiBBCgVQaLqBKg=
   J67gKMFl/OauTC20ibbydwIlJC8= M5GziVgB6KPY3ERU1HXdSi2vtdw=
   Wxd/lU7uG/ipEYT9xeqnsfohyH0="
   SIGN="AKBbX4J7QkrwuwdbV7Taujk2lvOf8gCgC62We1QYfnrNHz7FzAvdySuMyfM="]

   The message is of syslog-sign protocol version "01".  It uses SHA1 as
   hash algorithm and an OpenPGP DSA signature scheme.  Its reboot
   session ID is 1.  Its Signature Group is 0, which means that all
   syslog messages go to the same destination; its Signature Priority
   (which can effectively be ignored because all syslog messages will be
   signed regardless of their PRI value) is 0.  Its Global Block Counter
   is 2.  The first message number is 1; the message contains 7 message
   hashes.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Payload and Certificate Blocks</span>

   Certificate Blocks and Payload Blocks provide key management for
   syslog-sign.  Their purpose is to support key management that uses
   public key cryptosystems.

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Preliminaries: Key Management and Distribution Issues</span>

   A Payload Block contains public-key-certificate information that is
   to be conveyed to the collector.  A Payload Block is sent at the
   beginning of a new reboot session, carrying public key information in
   effect for the reboot session.  However, a Payload Block is not sent
   directly, but in (one or more) fragments.  Those fragments are termed
   Certificate Blocks.  Therefore, signers send at least one Certificate
   Block at the beginning of a new reboot session.





<span class="grey">Kelsey, et al.               Standards Track                   [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


   There are three key points to understand about Certificate Blocks:

   a.  They handle a variable-sized payload, fragmenting it if necessary
       and transmitting the fragments as legal syslog messages.  This
       payload is built (as described below) at the beginning of a
       reboot session and is transmitted in pieces with each Certificate
       Block carrying a piece.  There is exactly one Payload Block per
       reboot session.

   b.  The Certificate Blocks are digitally signed.  The signer does not
       sign the Payload Block, but the signatures on the Certificate
       Blocks ensure its authenticity.  Note that it may not even be
       possible to verify the signature on the Certificate Blocks
       without the information in the Payload Block; in this case, the
       Payload Block is reconstructed, the key is extracted, and then
       the Certificate Blocks are verified.  (This is necessary even
       when the Payload Block carries a certificate, because some other
       fields of the Payload Block are not otherwise verified.)  In
       practice, most installations keep the same public key over long
       periods of time, so that most of the time, it is easy to verify
       the signatures on the Certificate Blocks, and use the Payload
       Block to provide other useful per-session information.

   c.  The kind of Payload Block that is expected is determined by what
       kind of key material is on the collector that receives it.  The
       signer and collector (or offline log viewer) both have some key
       material (such as a root public key or pre-distributed public
       key) and an acceptable value for the Key Blob Type in the Payload
       Block, below.  The collector or offline log viewer MUST NOT
       accept a Payload Block of the wrong type.

<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>.  Payload Block</span>

   The Payload Block is built when a new reboot session is started.
   There is a one-to-one correspondence between reboot sessions and
   Payload Blocks.  A signer creates a new Payload Block after each
   reboot.  The Payload Block is used until the next reboot.

<span class="h4"><a class="selflink" id="section-5.2.1" href="#section-5.2.1">5.2.1</a>.  Block Format and Fields</span>

   A Payload Block MUST have the following fields:

   a.  Full local timestamp for the signer at the time the reboot
       session started.  This must be in the timestamp format specified
       in [<a href="./rfc5424" title="&quot;The syslog Protocol&quot;">RFC5424</a>] (essentially, timestamp format per [<a href="./rfc3339" title="&quot;Date and Time on the Internet: Timestamps&quot;">RFC3339</a>] with
       some further restrictions).





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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


   b.  Key Blob Type, a one-octet field containing one of five values:

       1.  'C' -- a PKIX certificate (per [<a href="./rfc5280" title="&quot;Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile&quot;">RFC5280</a>]).

       2.  'P' -- an OpenPGP KeyID and OpenPGP certificate (a
           Transferable Public Key as defined in [<a href="./rfc4880" title="&quot;OpenPGP Message Format&quot;">RFC4880</a>], <a href="#section-11.1">Section</a>
           <a href="#section-11.1">11.1</a>).  The first 8 octets of the key blob field contain the
           OpenPGP KeyID (identifying which key or subkey inside the
           OpenPGP certificate is used), followed by the OpenPGP
           certificate itself.

       3.  'K' -- the public key whose corresponding private key is
           being used to sign these messages.  For the OpenPGP DSA
           signature scheme, the key blob field contains the DSA prime
           p, DSA group order q, DSA group generator g, and DSA public-
           key value y, encoded as 4 multiprecision integers (see
           [<a href="./rfc4880" title="&quot;OpenPGP Message Format&quot;">RFC4880</a>], Sections <a href="#section-5.5.2">5.5.2</a> and <a href="#section-3.2">3.2</a>).

       4.  'N' -- no key information sent; key is pre-distributed.

       5.  'U' -- installation-specific key exchange information.

   c.  The key blob, if any, base64 encoded per [<a href="./rfc4648" title="&quot;The Base16, Base32, and Base64 Data Encodings&quot;">RFC4648</a>] and consisting
       of the raw key data.

   The fields are separated by single space characters.  Because a
   Payload Block is not carried in a syslog message directly, only the
   corresponding Certificate Blocks, it does not need to be encoded as
   an SD ELEMENT.  The Payload Block does not contain a field that
   identifies the reboot session; instead, the reboot session can be
   inferred from the Reboot Session ID parameter of the Certificate
   Blocks that are used to carry the Payload Block.

   To ensure that the sender and receiver have at least one common Key
   Blob Type, for immediate validation of received messages, all
   implementations MUST support Key Blob Type "C" (PKIX certificate).
   When a PKIX certificate is used ("C" Key Blob Type), it is the
   certificate specified in [<a href="./rfc5280" title="&quot;Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile&quot;">RFC5280</a>].  Per [<a href="./rfc5425" title="&quot;TLS Transport Mapping for syslog&quot;">RFC5425</a>], syslog messages
   may be transported over the TLS protocol, even where there is no PKI.
   If that transport is used, then the device will already have a PKIX
   certificate, and it MAY use the private key associated with that
   certificate to sign messages.  In the case where there is no PKI, the
   chain of trust of a PKIX certificate must still be established to
   meet conventional security requirements.  The methods for doing this
   are described in [<a href="./rfc5425" title="&quot;TLS Transport Mapping for syslog&quot;">RFC5425</a>].






<span class="grey">Kelsey, et al.               Standards Track                   [Page 17]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


<span class="h4"><a class="selflink" id="section-5.2.2" href="#section-5.2.2">5.2.2</a>.  Signer Authentication and Authorization</span>

   When the collector receives a Payload Block, it needs to determine
   whether the signatures are to be trusted.  The following methods are
   in scope of this specification:

   a.  X.509 certification path validation: The collector is configured
       with one or more trust anchors (typically root Certification
       Authority (CA) certificates), which allow it to verify a binding
       between the subject name and the public key.  Certification path
       validation is performed as specified in [<a href="./rfc5280" title="&quot;Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile&quot;">RFC5280</a>].

       If the HOSTNAME contains a Fully-Qualified Domain Name (FQDN) or
       an IP address, it is then compared against the certificate as
       described in <a href="./rfc5425#section-5.2">[RFC5425], Section&nbsp;5.2</a>.  Comparing other forms of
       HOSTNAMEs is beyond the scope of this specification.

       Collectors SHOULD support this method.  Note that due to message
       size restrictions, syslog-sign sends only the end-entity
       certificate in the Payload Block.  Depending on the PKI
       deployment, the collector may need to obtain intermediate
       certificates by other means (for example, from a directory).

   b.  X.509 end-entity certificate matching: The collector is
       configured with information necessary to identify the valid end-
       entity certificates of its valid peers, and for each peer, the
       HOSTNAME(s) it is authorized to use.

       To ensure interoperability, collectors MUST support fingerprints
       of X.509 certificates as described below.  Other methods MAY be
       supported.

       Collectors MUST support Key Blob Type 'C', and configuring the
       list of valid peers using certificate fingerprints.  The
       fingerprint is calculated and formatted as specified in
       <a href="./rfc5425#section-4.2.2">[RFC5425], Section&nbsp;4.2.2</a>.

       For each peer, the collector MUST support configuring a list of
       HOSTNAMEs that this peer is allowed to use either as FQDNs or IP
       addresses.  Other forms of HOSTNAMEs are beyond the scope of this
       specification.

       If the locally configured FQDN is an internationalized domain
       name, conforming implementations MUST convert it to the ASCII
       Compatible Encoding (ACE) format for performing comparisons as
       specified in <a href="./rfc5280#section-7">Section&nbsp;7 of [RFC5280]</a>.  An exact case-insensitive
       string match MUST be supported, but the implementation MAY also




<span class="grey">Kelsey, et al.               Standards Track                   [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


       support wildcards of any type ("*", regular expressions, etc.) in
       locally configured names.

       Signer implementations MUST provide a means to generate a key
       pair and self-signed certificate in the case that a key pair and
       certificate are not available through another mechanism, and MUST
       make the certificate fingerprint available through a management
       interface.

   c.  OpenPGP V4 fingerprints: Like X.509 fingerprints, except Key Blob
       Type 'P' is used, and the fingerprint is calculated as specified
       in <a href="./rfc4880#section-12.2">[RFC4880], Section&nbsp;12.2</a>.  When the fingerprint value is
       displayed or configured, each byte is represented in hexadecimal
       (using two uppercase ASCII characters), and space is added after
       every second byte.  For example: "0830 2A52 2CD1 D712 6E76 6EEC
       32A5 CAE1 03C8 4F6E".

       Signers and collectors MAY support this method.

   Other methods, such as "web of trust", are beyond the scope of this
   document.

<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>.  Certificate Block</span>

   This section describes the format of the Certificate Block and the
   fields used within the Certificate Block, as well as the syslog
   messages used to carry Certificate Blocks.

<span class="h4"><a class="selflink" id="section-5.3.1" href="#section-5.3.1">5.3.1</a>.  Syslog Messages Containing a Certificate Block</span>

   Certificate Blocks are used to get the Payload Block to the
   collector.  As with a Signature Block, each Certificate Block is
   carried in its own syslog message, called a Certificate Block
   message.  In case separate collectors are associated with different
   Signature Groups, Certificate Block messages need to be sent to each
   collector.

   Because certificates can legitimately be much longer than 2048
   octets, the Payload Block can be split up into several pieces, with
   each Certificate Block carrying a piece of the Payload Block.  Note
   that the signer MAY make the Certificate Blocks of any legal length
   (that is, any length that keeps the entire Certificate Block message
   within 2048 octets) that holds all the required fields.  Software
   that processes Certificate Blocks MUST deal correctly with blocks of
   any legal length.  The length of the fragment of the Payload Block
   that a Certificate Block carries MUST be at least one octet.  The
   length SHOULD be chosen such that the length of the Certificate Block
   message does not exceed 2048 octets.



<span class="grey">Kelsey, et al.               Standards Track                   [Page 19]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


   A Certificate Block message is identified by the presence of an SD
   ELEMENT with an SD-ID with the value "ssign-cert".  In addition, a
   Certificate Block message MUST contain valid APP-NAME, PROCID, and
   MSGID fields to be compliant with syslog protocol.  Syslog-sign does
   not mandate particular values for these fields; however, for
   consistency, a signer MUST use the same value for APP-NAME, PROCID,
   and MSGID fields for every Certificate Block message, whichever
   values are chosen.  It MUST also use the same value for its HOSTNAME
   field.  To allow for the possibility of multiple signers per host,
   the combination of APP-NAME and PROCID MUST be unique for each such
   originator.  If a signer daemon is restarted, it MAY use a new PROCID
   for what is otherwise the same signer.  The combination of APP-NAME
   and PROCID MUST be the same that is used for Signature Block messages
   of the same signer; however, a different MSGID MAY be used for
   Signature Block and Certificate Block messages.  It is RECOMMENDED to
   use 110 as the value for the PRI field, corresponding to facility 13
   (log audit) and severity 6 (informational).  The Certificate Block is
   carried as Structured Data within the Certificate Block message.  A
   Certificate Block message MAY carry other Structured Data besides the
   Structured Data of the Certificate Block itself.  The MSG part of a
   Certificate Block message SHOULD be empty.

<span class="h4"><a class="selflink" id="section-5.3.2" href="#section-5.3.2">5.3.2</a>.  Certificate Block Format and Fields</span>

   The contents of a Certificate Block message is the Certificate Block
   itself.  Like a Signature Block, the Certificate Block is encoded as
   an SD ELEMENT.  The SD-ID of the Certificate Block is "ssign-cert".
   The Certificate Block is composed of the following fields, each of
   which is encoded as an SD Parameter with parameter name as indicated.
   Each field must be printable ASCII, and any binary values are base64
   encoded per [<a href="./rfc4648" title="&quot;The Base16, Base32, and Base64 Data Encodings&quot;">RFC4648</a>].




















<span class="grey">Kelsey, et al.               Standards Track                   [Page 20]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-21" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


       Field                       SD-PARAM-NAME      Size in octets
       -----                       -------------      ---- -- ------

       Version                          VER                 4

       Reboot Session ID               RSID                1-10

       Signature Group                   SG                 1

       Signature Priority              SPRI                1-3

       Total Payload Block Length      TPBL                1-8

       Index into Payload Block       INDEX                1-8

       Fragment Length                 FLEN                1-4

       Payload Block Fragment          FRAG              variable
                                                (base64 encoded binary)

       Signature                       SIGN             variable
                                                (base64 encoded binary)

   The fields MUST be provided in the order listed.  New SD parameters
   MUST NOT be added unless a new Version of the protocol is defined.
   (Implementations that wish to add proprietary extensions will need to
   define a separate SD ELEMENT.)  A Certificate Block is accordingly
   encoded as follows, where xxx denotes a placeholder for the
   particular values:

   [ssign-cert VER="xxx" RSID="xxx" SG="xxx" SPRI="xxx" TPBL="xxx"
   INDEX="xxx" FLEN="xxx" FRAG="xxx" SIGN="xxx"]

   Values of the fields constitute SD parameter values and are hence
   enclosed in quotes, per [<a href="./rfc5424" title="&quot;The syslog Protocol&quot;">RFC5424</a>].  The fields are separated by
   single spaces and are described below.  Each SD parameter MUST occur
   once and only once.

<span class="h5"><a class="selflink" id="section-5.3.2.1" href="#section-5.3.2.1">5.3.2.1</a>.  Version</span>

   The Version field is 4 octets in length.  This field is identical in
   format and meaning to the Version field described in <a href="#section-4.2.1">Section 4.2.1</a>.

<span class="h5"><a class="selflink" id="section-5.3.2.2" href="#section-5.3.2.2">5.3.2.2</a>.  Reboot Session ID</span>

   The Reboot Session ID is identical in format and meaning to the RSID
   field described in <a href="#section-4.2.2">Section 4.2.2</a>.




<span class="grey">Kelsey, et al.               Standards Track                   [Page 21]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-22" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


<span class="h5"><a class="selflink" id="section-5.3.2.3" href="#section-5.3.2.3">5.3.2.3</a>.  Signature Group and Signature Priority</span>

   The SIG field is identical in format and meaning to the SIG field
   described in <a href="#section-4.2.3">Section 4.2.3</a>.  The SPRI field is identical in format
   and meaning to the SPRI field described there.

   A signer SHOULD send separate Certificate Block messages for each
   Signature Group.  This ensures that each collector that is associated
   with a Signature Group will receive the necessary key material in the
   case that messages of different Signature Groups are sent to
   different collectors.  Note that the signer needs to get the same
   Payload Block to each collector, as for any given signer there is a
   one-to-one relationship between Payload Block and Reboot Session
   across all Signature Groups.  Deployments that wish to associate
   different key material (and hence different Payload Blocks) with
   different Signature Groups can use separate signers for that purpose,
   each distinguished by its own combination of HOSTNAME, APP-NAME, and
   PROCID.

<span class="h5"><a class="selflink" id="section-5.3.2.4" href="#section-5.3.2.4">5.3.2.4</a>.  Total Payload Block Length</span>

   The Total Payload Block Length is a value representing the total
   length of the Payload Block in octets, expressed as a decimal with 1
   to 8 octets with leading zeroes omitted.

<span class="h5"><a class="selflink" id="section-5.3.2.5" href="#section-5.3.2.5">5.3.2.5</a>.  Index into Payload Block</span>

   This is a decimal value between 1 and 8 octets, with leading zeroes
   omitted.  It contains the number of octets into the Payload Block at
   which this fragment starts.  The first octet of the first fragment is
   numbered "1".  (Note, it is not numbered "0".)

<span class="h5"><a class="selflink" id="section-5.3.2.6" href="#section-5.3.2.6">5.3.2.6</a>.  Fragment Length</span>

   The total length of this fragment expressed as a decimal integer with
   1 to 4 octets with leading zeroes omitted.  The fragment length must
   be at least 1.

<span class="h5"><a class="selflink" id="section-5.3.2.7" href="#section-5.3.2.7">5.3.2.7</a>.  Payload Block Fragment</span>

   The Payload Block Fragment contains a fragment of the payload block.
   Its length must match the indicated fragment length.

<span class="h5"><a class="selflink" id="section-5.3.2.8" href="#section-5.3.2.8">5.3.2.8</a>.  Signature</span>

   This is a digital signature, encoded in base64, as per [<a href="./rfc4648" title="&quot;The Base16, Base32, and Base64 Data Encodings&quot;">RFC4648</a>].
   The Version field effectively specifies the original encoding of the
   signature.  The signature is calculated over the completely formatted



<span class="grey">Kelsey, et al.               Standards Track                   [Page 22]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-23" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


   Certificate Block message, before the SIGN parameter is added (see
   <a href="#section-4.2.8">Section 4.2.8</a>).  For the OpenPGP DSA signature scheme, the value of
   the signature field contains the DSA values r and s, encoded as 2
   multiprecision integers (see [<a href="./rfc4880" title="&quot;OpenPGP Message Format&quot;">RFC4880</a>], Sections <a href="#section-5.2.2">5.2.2</a> and <a href="#section-3.2">3.2</a>),
   concatenated, and then encoded in base64 [<a href="./rfc4648" title="&quot;The Base16, Base32, and Base64 Data Encodings&quot;">RFC4648</a>].

<span class="h5"><a class="selflink" id="section-5.3.2.9" href="#section-5.3.2.9">5.3.2.9</a>.  Example</span>

   An example of a Certificate Block message is depicted below, broken
   into lines to fit publication rules.  There are no spaces at the end
   of the lines that contain the key blob and the signature.

   &lt;110&gt;1 2009-05-03T14:00:39.519307+02:00 host.example.org syslogd
   2138 - [ssign-cert VER="0111" RSID="1" SG="0" SPRI="0" TPBL="587"
   INDEX="1" FLEN="587" FRAG="2009-05-03T14:00:39.519005+02:00 K BACsLMZ
   NCV2NUAwe4RAeAnSQuvv2KS51SnHFAaWJNU2XVDYvW1LjmJgg4vKvQPo3HEOD+2hEkt1z
   cXADe03u5pmHoWy5FGiyCbglYxJkUJJrQqlTSS6vID9yhsmEnh07w3pOsxmb4qYo0uWQr
   AAenBweVMlBgV3ZA5IMA8xq8l+i8wCgkWJjCjfLar7s+0X3HVrRroyARv8EAIYoxofh9m
   N8n821BTTuQnz5hp40d6Z3UudKePu2di5Mx3GFelwnV0Qh5mSs0YkuHJg0mcXyUAoeYry
   5X6482fUxbm+gOHVmYSDtBmZEB8PTEt8Os8aedWgKEt/E4dT+Hmod4omECLteLXxtScTM
   gDXyC+bSBMjRRCaeWhHrYYdYBACCWMdTc12hRLJTn8LX99kv1I7qwgieyna8GCJv/rEgC
   ssS9E1qARM+h19KovIUOhl4VzBw3rK7v8Dlw/CJyYDd5kwSvCwjhO21LiReeS90VPYuZF
   RC1B82Sub152zOqIcAWsgd4myCCiZbWBsuJ8P0gtarFIpleNacCc6OV3i2Rg=="
   SIGN="AKAQEUiQptgpd0lKcXbuggGXH/dCdQCgdysrTBLUlbeGAQ4vwrnLOqSL7+c="]

   The message is of syslog-sign protocol version "01".  It uses SHA1 as
   hash algorithm and an OpenPGP DSA signature scheme.  Its reboot
   session ID is 1.  Its Signature Group is 0; its Signature Priority is
   0.  The Total Payload Block Length is 587 octets.  The index into the
   payload block is 1 (meaning this is the first fragment).  The length
   of the fragment is 587 (meaning that the Certificate Block message
   contains the entire Payload Block).  The Payload Block has the
   timestamp 2009-05-03T14:00:39.519005+02:00.  The Key Blob Type is
   'K', meaning that it contains a public key whose corresponding
   private key is being used to sign these messages.

   Note that the Certificate Block message in this example has a
   timestamp that is very close to the timestamp in the Payload Block.
   The fact that the timestamps are so close implies that this is the
   first Certificate Block message sent in this reboot session;
   additional Certificate Block messages can be sent later with a later
   timestamp, which will carry the same Payload Block that will still
   contain the same timestamp.








<span class="grey">Kelsey, et al.               Standards Track                   [Page 23]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-24" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Redundancy and Flexibility</span>

   As described in <a href="./rfc5424#section-8.5">Section&nbsp;8.5 of [RFC5424]</a>, a transport sender may
   discard syslog messages.  Likewise, when syslog messages are sent
   over unreliable transport, they can be lost in transit.  However, if
   a collector does not receive Signature and Certificate Blocks, many
   messages may not be able to be verified.  The signer is allowed to
   send Signature and Certificate Blocks multiple times.  Sending
   Signature and Certificate Blocks multiple times provides redundancy
   with the intent to ensure that the collector or relay does get the
   Signature Blocks and in particular the Payload Block at some point in
   time.  In the meantime, any online review of logs as described in
   <a href="#section-7.2">Section 7.2</a> is delayed until the needed blocks are received.  The
   collector MUST ignore duplicates of Signature Blocks and Certificate
   Blocks that it has already received and authenticated.  In principle,
   the signer can change its redundancy level for any reason, without
   communicating this fact to the collector.

   A signer that is also the originator of messages that it signs does
   not need to queue up other messages while sending redundant
   Certificate Block and Signature Block messages.  It MAY send
   redundant Certificate Block messages even after Signature Block
   messages and regular syslog messages have been sent.  By the same
   token, it MAY send redundant Signature Block messages even after
   newer syslog messages that are signed by a subsequent Signature Block
   have been sent, or even after a subsequent Signature Block message.

   In addition, the signer has flexibility in how many hashes to include
   within a Signature Block.  It is legitimate for an originator to send
   short Signature Blocks to allow the collector to verify messages with
   minimal delay.

<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>.  Configuration Parameters</span>

   Although the transport sender is not constrained in how it decides to
   send redundant Signature and Certificate Blocks, or even in whether
   it decides to send along multiple copies of normal syslog messages,
   we define some redundancy parameters below that may be useful in
   controlling redundant transmission from the transport sender to the
   transport receiver and that may be useful for administrators to
   configure.

<span class="h4"><a class="selflink" id="section-6.1.1" href="#section-6.1.1">6.1.1</a>.  Configuration Parameters for Certificate Blocks</span>

   Certificate Blocks are always sent at the beginning of a new reboot
   session.  One technique to ensure reliable delivery (see <a href="#section-8.5">Section 8.5</a>)
   is to send multiple copies.  This can be controlled by a
   "certInitialRepeat" parameter:



<span class="grey">Kelsey, et al.               Standards Track                   [Page 24]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-25" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


      certInitialRepeat = number of times each Certificate Block should
      be sent before the first message is sent.

   It is also useful to resend Certificate Blocks every now and then for
   long-lived reboot sessions.  This can be controlled by the
   certResendDelay and certResendCount parameters:

      certResendDelay = maximum time delay in seconds until resending
      the Certificate Block.

      certResendCount = maximum number of other syslog messages to send
      until resending the Certificate Block.

   In some cases, it may be desirable to allow for configuration of the
   transport sender such that Certificate Blocks are not sent at all
   after the first normal syslog message has been sent.  This could be
   expressed by setting both certResendDelay and certResendCount to "0".
   However, configuring the transport sender to send redundant
   Certificate Blocks even after the first message, in particular when
   the UDP transport [<a href="./rfc5426" title="&quot;Transmission of syslog Messages over UDP&quot;">RFC5426</a>] is used, is RECOMMENDED.

   In one set of circumstances, the receiver may receive a Certificate
   Block, some group of syslog messages, and some corresponding
   Signature Blocks.  If the receiver reboots after that, then the
   conditions of recovery will vary depending upon the transport.  For
   UDP [<a href="./rfc5426" title="&quot;Transmission of syslog Messages over UDP&quot;">RFC5426</a>], the receiver SHOULD continue to use the cached
   Certificate Block, but MUST validate the RSID value to make sure that
   it has the most current one.  If the receiver cannot validate that it
   has the most current Certificate Block, then it MUST wait for a
   retransmission of the Certificate Block, which may be controlled by
   the certResendDelay and certResendCount parameters.  It is up to the
   operators to ensure that Certificate Blocks are sent frequently
   enough to meet this set of circumstances.

   For TLS transport [<a href="./rfc5425" title="&quot;TLS Transport Mapping for syslog&quot;">RFC5425</a>], the sender MUST send a fresh Certificate
   Block when a session is established.  This will keep the sender and
   receiver synchronized with the most current Certificate Block.

   Implementations that support sending syslog messages of different
   Signature Groups to different collectors and which wish to offer very
   granular controls MAY allow the above parameters to be configured on
   a per Signature Group basis.

   The choice of reasonable values in a given deployment depends on
   several factors, including the acceptable delay that may be incurred
   from the receipt of a syslog message until the corresponding
   Signature Block is received, whether UDP or TLS transport is used,
   and the available management bandwidth.  The following might be a



<span class="grey">Kelsey, et al.               Standards Track                   [Page 25]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-26" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


   reasonable choice for a deployment in which reliability of underlying
   transport and of collector implementation are of little concern:

   certInitialRepeat=1, certResendDelay=1800 seconds,
   certResendCount=10000

   The following might be a reasonable choice for a deployment in which
   reliability of transmission over UDP transport could be an issue:

   certInitialRepeat=2, certResendDelay=300 seconds,
   certResendCount=1000

<span class="h4"><a class="selflink" id="section-6.1.2" href="#section-6.1.2">6.1.2</a>.  Configuration Parameters for Signature Blocks</span>

   Verification of log messages involves a certain delay of time that is
   caused by the lag in time between the sending of the message itself
   and the corresponding Signature Block.  The following configuration
   parameter can be useful to limit the time lag that will be incurred
   (note that the maximum message length may also force generating a
   Signature Block; see Sections <a href="#section-4.2.6">4.2.6</a> and <a href="#section-4.2.7">4.2.7</a>):

      sigMaxDelay = generate a new Signature Block if this many seconds
      have elapsed since the message with the First Message Number of
      the Signature Block was sent.

   Retransmissions of Signature Blocks are not sent immediately after
   the original transmission, but slightly later.  The following
   parameters control when those retransmissions are done:

      sigNumberResends = number of times a Signature Block is resent.
      (It is recommended to select a value of greater than "0" in
      particular when the UDP transport [<a href="./rfc5426" title="&quot;Transmission of syslog Messages over UDP&quot;">RFC5426</a>] is used.)

      sigResendDelay = send the next retransmission when this many
      seconds have elapsed since the previous sending of this Signature
      Block.

      sigResendCount = send the next retransmission when this many other
      syslog messages have been sent since the previous sending of this
      Signature Block.

   The choice of reasonable values in a given deployment depends on
   several factors, including the acceptable delay that may be incurred
   from the receipt of a syslog message until the corresponding
   Signature Block is received so that the syslog message can be
   verified, the reliability of the underlying transport, and the
   available management bandwidth.  The following might be a reasonable
   choice for a deployment where reliability of transport and collector



<span class="grey">Kelsey, et al.               Standards Track                   [Page 26]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-27" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


   are of little concern and where there is a need to have syslog
   messages generally signed within 5 minutes:

   sigMaxDelay=300 seconds, sigNumberResends=2, sigResendDelay=300
   seconds, sigResendCount=500

   The following would be a reasonable choice for a deployment that
   needs to validate syslog messages typically within 60 seconds, but no
   more than 3 minutes after receipt:

   sigMaxDelay=30 seconds, sigNumberResends=5, sigResendDelay=30
   seconds, sigResendCount=100

<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>.  Overlapping Signature Blocks</span>

   Notwithstanding the fact that the signer is not constrained in
   whether it decides to send redundant Signature Block messages,
   Signature Blocks SHOULD NOT overlap.  This facilitates their
   processing by the receiving collector.  This means that an originator
   of Signature Block messages, after having sent a first message with
   some First Message Number and a Count, SHOULD NOT send a second
   message with the same First Message Number but a different Count.  It
   also means that an originator of Signature Block messages SHOULD NOT
   send a second message whose First Message Number is greater than the
   First Message Number, but smaller than the First Message Number plus
   the Count indicated in the first message.

   That said, the possibility of Signature Blocks that overlap does
   provide additional flexibility with regard to redundancy; it provides
   an additional option that may be desirable in some deployments.
   Therefore, collectors MUST be designed in a way that they can cope
   with overlapping Signature Blocks when confronted with them.  The
   collector MUST ignore hashes of messages that it has already received
   and validated.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Efficient Verification of Logs</span>

   The logs secured with syslog-sign may be reviewed either online or
   offline.  Online review is somewhat more complicated and
   computationally expensive, but not prohibitively so.  This section
   outlines a method for online and a method for offline verification of
   logs that implementations MAY choose to implement to verify logs
   efficiently.  Implementations MAY also choose to implement a
   different method; it is ultimately up to each implementation how to
   process the messages that it receives.






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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-28" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>.  Offline Review of Logs</span>

   When the collector stores logs to be reviewed later, they can be
   authenticated offline just before they are reviewed.  Reviewing these
   logs offline is simple and relatively inexpensive in terms of
   resources used, so long as there is enough space available on the
   reviewing machine.

   To do so, we first go through the stored log file.  Each message in
   the log file is classified as a normal message, a Signature Block
   message, or a Certificate Block message.  Signature Blocks and
   Certificate Blocks are then separated by signer (as identified by
   HOSTNAME, APP-NAME, PROCID), Reboot Session ID, and Signature Group,
   and stored in their own files.  Normal messages are stored in a keyed
   file, indexed on their hash values.  They are not separated by
   signer, as their (HOSTNAME, APP-NAME, PROCID) identifies the
   application that generated the message.  The application that
   generated the message does not have to coincide with the signer.

   For each signer, Reboot Session ID, and Signature Group, we then:

   a.  Sort the Certificate Block file by INDEX value, and check to see
       whether we have a set of Certificate Blocks that can reconstruct
       the Payload Block.  If so, we reconstruct the Payload Block,
       verify any key-identifying information, and then use this to
       verify the signatures on the Certificate Blocks we have received.
       When this is done, we have verified the reboot session and key
       used for the rest of the process.

   b.  Sort the Signature Block file by First Message Number.  We now
       create an authenticated log file, which consists of some header
       information and then a (sequence of message number, message text
       pairs).  We next go through the Signature Block file.  We
       initialize a cursor for the last message number processed with
       the number 0.  For each Signature Block in the file, we do the
       following:

       1.  Verify the signature on the Signature Block.

       2.  If the value of the First Message Number of the Signature
           Block is less than or equal to the last message number
           processed, skip the first (last message number processed
           minus First Message Number plus 1) hashes.

       3.  For each remaining hashed message in the Signature Block:

           a.  Look up the hash value in the keyed message file.




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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-29" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


           b.  If the message is found, write (message number, message
               text) to the authenticated log file.

       4.  Set the last message number processed to the value of the
           First Message Number plus the Count of the Signature Block
           minus 1.

       5.  Skip all other Signature Blocks with the same First Message
           Number unless one with a larger Count is encountered.

       The resulting authenticated log file contains all messages that
       have been authenticated.  In addition, it implicitly indicates
       all gaps in the authenticated messages (specifically in the case
       when all messages of the same Signature Group are sent to the
       same collector), because their message numbers are missing.

   One can see that, assuming sufficient space for building the keyed
   file, this whole process is linear in the number of messages
   (generally two seeks, one to write and the other to read, per normal
   message received), and O(N lg N) in the number of Signature Blocks.
   This estimate comes with two caveats: first, the Signature Blocks
   arrive very nearly in sorted order, and so can probably be sorted
   more cheaply on average than O(N lg N) steps.  Second, the signature
   verification on each Signature Block almost certainly is more
   expensive than the sorting step in practice.  We have not discussed
   error-recovery, which may be necessary for the Certificate Blocks.
   In practice, a simple error-recovery strategy is probably enough: if
   the Payload Block is not valid, then we can just try alternate
   instances of each Certificate Block, if such are available, until we
   get the Payload Block right.

   It is easy for an attacker to flood us with plausible-looking
   messages, Signature Blocks, and Certificate Blocks.

<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>.  Online Review of Logs</span>

   Some collector implementations may need to monitor log messages in
   close to real time.  This can be done with syslog-sign, though it is
   somewhat more complex than offline verification.  This is done as
   follows:

   a.  We have an authenticated message file, into which we write
       (message number, message text) pairs that have been
       authenticated.  We will assume that we are handling only one
       signer, Signature Group, and Reboot Session ID at any given time.
       (For the concurrent support of multiple signers, Signature
       Groups, and Reboot Session IDs, the same procedure is applied
       analogously to each.  Signature Block messages and Certificate



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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-30" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


       Block messages clearly indicate their respective signer,
       Signature Group, and Reboot Session ID.)

   b.  We have two data structures: A "Waiting for Signature" queue in
       which (arrival sequence, hash of message) pairs are kept in
       sorted order, and a "Waiting for Message" queue in which (message
       number, hash of message) pairs are kept in sorted order.  In
       addition, we have a hash table that stores (message text, count)
       pairs indexed by hash value.  In the hash table, count may be any
       number greater than zero; when count is zero, the entry in the
       hash table is cleared.

       Note: The "Waiting for Signature" queue gets used in the normal
       case, when the signature arrives after the message itself.  It
       holds messages that have been received but whose signature has
       yet to arrive.  The "Waiting for Message" queue gets used in the
       case that messages are lost or misordered (either in the network
       or in relays).  It holds signatures that have been received but
       whose corresponding messages have yet to arrive.  Since a single
       Signature Block can cover only a limited number of messages (due
       to size restrictions), and massive reordering/delaying is rare,
       it is expected that both queues would be relatively small.

   c.  We must receive all the Certificate Blocks before any other
       processing can really be done.  (This is why they are sent
       first.)  Once that is done, any additional Certificate Block
       message that arrives is discarded.  Any syslog messages or
       Signature Block messages that arrive before all Certificate
       Blocks have been received need to be buffered.  Once all
       Certificate Blocks have been received, the messages in the buffer
       can be retrieved and processed as if they were just arriving.

   d.  Whenever a normal message arrives, we first check if its hash
       value is found in the "Waiting for Message" queue.  If it is, we
       write the message number (from the "Waiting for Message" queue)
       and the message into the authenticated message file and remove
       the entry from the queue.

       Otherwise, we add (arrival sequence, hash of message) to the
       "Waiting for Signature" queue.  If our hash table already has an
       entry for the message's hash value, we increment its count by
       one; otherwise, we create a new entry with Count = 1.

       If the "Waiting for Signature" message queue is full, we remove
       the oldest message from the queue.  That message could not be
       validated close enough to real time.  In order to update the hash
       table accordingly, we use that entry's hash to index the hash
       table.  If that entry has count 1, we delete the entry from the



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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-31" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


       hash table; otherwise, we decrement its count.  By removing the
       message from the "Waiting for Signature" message queue without
       having actually received the message's signature, we make it
       impossible to authenticate the message should its signature
       arrive later.  Implementers therefore need to ensure that queues
       are dimensioned sufficiently large to not expose the collector
       against Denial-of-Service (DoS) attacks that attempt to flood the
       collector with unsigned messages.

   e.  Whenever a Signature Block message arrives, we check its
       originator, (i.e., the signer) by way of HOSTNAME, APP-NAME, and
       PROCID, as well as its Signature Group and Reboot Session ID to
       ensure it matches our Certificate Blocks.  We then check to see
       whether the First Message Number value is too old to still be of
       interest, or if another Signature Block with that First Message
       Number and the same Count or a greater Count has already been
       received.  If so, we discard the Signature Block.  We then check
       the signature.  Again, we discard the Signature Block if the
       signature is not valid.

       Otherwise, we proceed with processing the hashes in the Signature
       Block.  A Signature Block contains a sequence of hashes, each of
       which is associated with a message number, starting with the
       First Message Number for the first hash and incrementing by one
       for each subsequent hash.  For each hash, we first check to see
       whether the message hash is in the hash table.  If this is the
       case, it means that we have received the signature for a message
       that was received earlier, and we do the following:

       1.  We check if a message with the same message number is already
           in the authenticated message file.  If that is the case, the
           signed hash is a duplicate and we discard it.

       2.  Otherwise (the signed hash is not a duplicate), we write the
           (message number, message text) into the authenticated message
           file.  We also update the hash table accordingly, using that
           entry's hash to index the hash table.  If that entry has
           Count 1, we delete the entry from the hash table; otherwise,
           we decrement its count.

       Otherwise (the message hash is not in the hash table), we write
       the (message number, message hash) to the "Waiting for Message"
       queue.

       If the "Waiting for Message" queue is full, we remove the oldest
       entry.  In that case, a message that was signed by the signer
       could not be validated by the receiver, either because the
       message was lost or because the signature arrived way ahead of



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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-32" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


       the actual message.  By removing the entry from the "Waiting for
       Message" queue without having actually received the message, we
       make it impossible to authenticate the a legitimate message
       should that message still arrive later.  Implementers need to
       ensure queues are dimensioned sufficiently large so that the
       chances of such a scenario actually occurring is minimized.

   f.  The result of this is a sequence of messages in the authenticated
       message file.  Each message in the message file has been
       authenticated.  The sequence is labeled with numbers showing the
       order in which the messages were originally transmitted.

   One can see that this whole process is roughly linear in the number
   of messages, and also in the number of Signature Blocks received.
   The process is susceptible to flooding attacks; an attacker can send
   enough normal messages that the messages roll off their queue before
   their Signature Blocks can be processed.

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Security Considerations</span>

   Normal syslog event messages are unsigned and have most of the
   security attributes described in <a href="./rfc5424#section-8">Section&nbsp;8 of [RFC5424]</a>.  This
   document also describes Certificate Blocks and Signature Blocks,
   which are signed syslog messages.  The Signature Blocks contain
   signature information for previously sent syslog event messages.  All
   of this information can be used to authenticate syslog messages and
   to minimize or obviate many of the security concerns described in
   [<a href="./rfc5424" title="&quot;The syslog Protocol&quot;">RFC5424</a>].

   The model for syslog-sign is a direct trust system where the
   certificate transferred is its own trust anchor.  If a transport
   sender sends a stream of syslog messages that is signed using a
   certificate, the operator or application will transfer to the
   transport receiver the certificate that was used when signing.  There
   is no need for a certificate chain.

<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>.  Cryptographic Constraints</span>

   As with any technology involving cryptography, it is advisable to
   check the current literature to determine whether any algorithms used
   here have been found to be vulnerable to attack.

   This specification uses Public Key Cryptography technologies.  The
   proper party or parties have to control the private key portion of a
   public-private key pair.  Any party that controls a private key can
   sign anything it pleases.





<span class="grey">Kelsey, et al.               Standards Track                   [Page 32]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-33" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


   Certain operations in this specification involve the use of random
   numbers.  An appropriate entropy source SHOULD be used to generate
   these numbers.  See [<a href="./rfc4086" title="&quot;Randomness Recommendations for Security&quot;">RFC4086</a>] and [<a href="#ref-NIST800.90" title="&quot;NIST Special Publication 800-90: Recommendation for Random Number Generation using Deterministic Random Bit Generators&quot;">NIST800.90</a>].

<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>.  Packet Parameters</span>

   As a signer, it is advisable to avoid message lengths exceeding 2048
   octets.  Various problems might result if a signer were to send
   messages with a length greater than 2048 octets, because relays MAY
   truncate messages with lengths greater than 2048 octets, which would
   make it impossible for collectors to validate a hash of the packet.
   To increase the chance of interoperability, it tends to be best to be
   conservative with what you send but liberal in what you are able to
   receive.

   Signers need to rigidly adhere to the <a href="./rfc5424">RFC 5424</a> format when sending
   messages.  If a collector receives a message that is not formatted
   properly, then it might drop it, or it may modify it while receiving
   it.  (See <a href="./rfc5424#appendix-A.2">Appendix&nbsp;A.2 of [RFC5424]</a>.)  If that were to happen, the
   hash of the sent message would not match the hash of the received
   message.

   Collectors are not to malfunction in the case that they receive
   malformed syslog messages or messages containing characters other
   than those specified in this document.  In other words, they are to
   ignore such messages and continue working.

<span class="h3"><a class="selflink" id="section-8.3" href="#section-8.3">8.3</a>.  Message Authenticity</span>

   Syslog does not strongly associate the message with the message
   originator.  That association is established by the collector upon
   verification of the Signature Block.  Before a Signature Block is
   used to ascertain the authenticity of an event message, it might be
   received, stored, and reviewed by a person or automated parser.  It
   is advisable not to assume a message is authentic until after a
   message has been validated by checking the contents of the Signature
   Block.

   With the Signature Block checking, an attacker may only forge
   messages if he or she can compromise the private key of the true
   originator.

<span class="h3"><a class="selflink" id="section-8.4" href="#section-8.4">8.4</a>.  Replaying</span>

   Event messages might be recorded and replayed by an attacker.  Using
   the information contained in the Signature Blocks, a reviewer can
   determine whether the received messages are the ones originally sent
   by an originator.  The reviewer can also identify messages that have



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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-34" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


   been replayed.  Using a method for the verification of logs such as
   the one outlined in <a href="#section-7">Section 7</a>, a replayed message can be detected by
   checking prior to writing a message to the authenticated log file
   whether the message is already contained in it.

<span class="h3"><a class="selflink" id="section-8.5" href="#section-8.5">8.5</a>.  Reliable Delivery</span>

   Event messages sent over UDP might be lost in transit.  [<a href="./rfc5425" title="&quot;TLS Transport Mapping for syslog&quot;">RFC5425</a>] can
   be used for the reliable delivery of syslog messages; however, it
   does not protect against loss of syslog messages at the application
   layer, for example, if the TCP connection or TLS session has been
   closed by the transport receiver for some reason.  A reviewer can
   identify any messages sent by the originator but not received by the
   collector by reviewing the Signature Block information.  In addition,
   the information in subsequent Signature Blocks allows a reviewer to
   determine whether any Signature Block messages were lost in transit.

<span class="h3"><a class="selflink" id="section-8.6" href="#section-8.6">8.6</a>.  Sequenced Delivery</span>

   Syslog messages delivered over UDP might not only be lost, but also
   arrive out of sequence.  A reviewer can determine the original order
   of syslog messages and identify which messages were delivered out of
   order by examining the information in the Signature Block along with
   any timestamp information in the message.

<span class="h3"><a class="selflink" id="section-8.7" href="#section-8.7">8.7</a>.  Message Integrity</span>

   Syslog messages might be damaged in transit.  A review of the
   information in the Signature Block determines whether the received
   message was the intended message sent by the originator.  A damaged
   Signature Block or Certificate Block is evident because the collector
   will not be able to validate that it was signed by the signer.

<span class="h3"><a class="selflink" id="section-8.8" href="#section-8.8">8.8</a>.  Message Observation</span>

   Unless TLS is used as a secure transport [<a href="./rfc5425" title="&quot;TLS Transport Mapping for syslog&quot;">RFC5425</a>], event messages,
   Certificate Blocks, and Signature Blocks are all sent in plaintext.
   This allows network administrators to read the message when sniffing
   the wire.  However, this also allows an attacker to see the contents
   of event messages and perhaps to use that information for malicious
   purposes.

<span class="h3"><a class="selflink" id="section-8.9" href="#section-8.9">8.9</a>.  Man-in-the-Middle Attacks</span>

   It is conceivable that an attacker might intercept Certificate Block
   messages and insert its own Certificate information.  In that case,
   the attacker would be able to receive event messages from the actual
   originator and then relay modified messages, insert new messages, or



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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-35" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


   delete messages.  It would then be able to construct a Signature
   Block and sign it with its own private key.  Network administrators
   need to verify that the key contained in the Payload Block is indeed
   the key being used on the actual signer.  If that is the case, then
   this MITM attack will not succeed.  Methods for establishing a chain
   of trust are also described in [<a href="./rfc5425" title="&quot;TLS Transport Mapping for syslog&quot;">RFC5425</a>].

<span class="h3"><a class="selflink" id="section-8.10" href="#section-8.10">8.10</a>.  Denial of Service</span>

   An attacker might send invalid Signature Block messages to overwhelm
   the collector's processing capability and consume all available
   resources.  For this reason, it can be appropriate to simply receive
   the Signature Block messages and process them only as time permits.

   An attacker might also just overwhelm a collector by sending more
   messages to it than it can handle.  Implementers are advised to
   consider features that minimize this threat, such as only accepting
   syslog messages from known IP addresses.

<span class="h3"><a class="selflink" id="section-8.11" href="#section-8.11">8.11</a>.  Covert Channels</span>

   Nothing in this protocol attempts to eliminate covert channels.  In
   fact, just about every aspect of syslog messages lends itself to the
   conveyance of covert signals.  For example, a collusionist could send
   odd and even PRI values to indicate Morse Code dashes and dots.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  IANA Considerations</span>

<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>.  Structured Data and Syslog Messages</span>

   With regard to [<a href="./rfc5424" title="&quot;The syslog Protocol&quot;">RFC5424</a>], IANA has added the following values (with
   each parameter listed as mandatory) to the registry titled "syslog
   Structured Data ID Values":


















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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-36" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


          Structured Data ID  Structured Data Parameter
          ------------------  -------------------------
          ssign
                              VER
                              RSID
                              SG
                              SPRI
                              GBC
                              FMN
                              CNT
                              HB
                              SIGN

          ssign-cert
                              VER
                              RSID
                              SG
                              SPRI
                              TPBL
                              INDEX
                              FLEN
                              FRAG
                              SIGN

   In addition, several fields are controlled by the IANA in both the
   Signature Block and the Certificate Block, as outlined in the
   following sections.

<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>.  Version Field</span>

   IANA has created three registries, each associated with a different
   subfield of the Version field of Signature Blocks and Certificate
   Blocks, described in Sections <a href="#section-4.2.1">4.2.1</a> and <a href="#section-5.3.2.1">5.3.2.1</a>, respectively.

   The first registry that IANA has created is titled "syslog-sign
   Protocol Version Values".  It is for the values of the Protocol
   Version subfield.  The Protocol Version subfield constitutes the
   first two octets in the Version field.  New values shall be assigned
   by the IANA using the "IETF Review" policy defined in [<a href="./rfc5226" title="">RFC5226</a>].
   Assigned numbers are to be increased by 1, up to a maximum value of
   "50".  Protocol Version numbers of "51" through "99" are vendor
   specific; values in this range are not to be assigned by the IANA.









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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-37" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


   IANA has registered the Protocol Version values shown below.

         Value                    Protocol Version
         -----                    ----------------
         00                       Reserved
         01                       Defined in <a href="./rfc5848">RFC 5848</a>

   The second registry that IANA has created is titled "syslog-sign Hash
   Algorithm Values".  It is for the values of the Hash Algorithm
   subfield.  The Hash Algorithm subfield constitutes the third octet in
   the Version field Signature Blocks and Certificate Blocks.  New
   values shall be assigned by the IANA using the "IETF Review" policy
   defined in [<a href="./rfc5226" title="">RFC5226</a>].  Assigned values are to be increased
   sequentially, first up to a maximum value of "9", then from "a" to
   "z", then from "A" to "Z".  The values are registered relative to the
   Protocol Version.  This means that the same Hash Algorithm value can
   be reserved for different Protocol Versions, possibly referring to a
   different hash algorithm each time.  This makes it possible to deal
   with future scenarios in which the single octet representation
   becomes a limitation, as more Hash Algorithms can be supported by
   defining additional Protocol Versions that implementations might
   support concurrently.

   IANA has registered the Hash Algorithm values shown below.

         Value     Protocol Version     Hash Algorithm
         -----     ----------------     --------------
         0         01                   Reserved
         1         01                   SHA1
         2         01                   SHA256

   The third registry that IANA has created is titled "syslog-sign
   Signature Scheme Values".  It is for the values of the Signature
   Scheme subfield.  The Signature Scheme subfield constitutes the
   fourth octet in the Version field of Signature Blocks and Certificate
   Blocks.  New values shall be assigned by the IANA using the "IETF
   Review" policy defined in [<a href="./rfc5226" title="">RFC5226</a>].  Assigned values are to be
   increased by 1, up to a maximum value of "9".  This means that the
   same Signature Scheme value can be reserved for different Protocol
   Versions, possibly in each case referring to a different Signature
   Scheme each time.  This makes it possible to deal with future
   scenarios in which the single octet representation becomes a
   limitation, as more Signature Schemes can be supported by defining
   additional Protocol Versions that implementations might support
   concurrently.






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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-38" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


   IANA has registered the Signature Scheme values shown below.

         Value     Protocol Version    Signature Scheme
         -----     ----------------    ----------------
         0         01                  Reserved
         1         01                  OpenPGP DSA

<span class="h3"><a class="selflink" id="section-9.3" href="#section-9.3">9.3</a>.  SG Field</span>

   IANA has created a registry titled "syslog-sign SG Field Values".  It
   is for values of the SG Field as defined in <a href="#section-4.2.3">Section 4.2.3</a>.  New
   values shall be assigned by the IANA using the "IETF Review" policy
   defined in [<a href="./rfc5226" title="">RFC5226</a>].  Assigned values are to be incremented by 1, up
   to a maximum value of "7".  Values "8" and "9" shall be left as
   vendor specific and shall not be assigned by the IANA.

   IANA has registered the SG Field values shown below.

         Value     Meaning
         -----     -------
         0         There is only one Signature Group.
         1         Each PRI value is associated with its own Signature
                   Group.
         2         Each Signature Group contains a range of PRI
                   values.
         3         Signature Groups are not assigned with any of the
                   above relationships to PRI values of the syslog
                   messages they sign.

<span class="h3"><a class="selflink" id="section-9.4" href="#section-9.4">9.4</a>.  Key Blob Type</span>

   IANA has created a registry titled "syslog-sign Key Blob Type
   Values".  It is to register one-character identifiers for the Key
   Blob Type, per <a href="#section-5.2">Section 5.2</a>.  New values shall be assigned by the IANA
   using the "IETF Review" policy defined in [<a href="./rfc5226" title="">RFC5226</a>].  Uppercase
   letters may be assigned as values.  Lowercase letters are left as
   vendor specific and shall not be assigned by the IANA.

   IANA has registered the Key Blob Type values shown below.

         Value     Key Blob Type
         -----     -------------
         C         a PKIX certificate
         P         an OpenPGP certificate
         K         the public key whose corresponding private key is
                   used to sign the messages
         N         no key information sent, key is pre-distributed
         U         installation-specific key exchange information



<span class="grey">Kelsey, et al.               Standards Track                   [Page 38]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-39" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  Acknowledgements</span>

   The authors wish to thank the current Chairs of the Syslog Working
   Group, David Harrington and Chris Lonvick, and the other members of
   the Working Group, in particular Alex Brown, Chris Calabrese, Steve
   Chang, Pasi Eronen, Carson Gaspar, Rainer Gerhards, Drew Gross,
   Albert Mietus, Darrin New, Marshall Rose, Andrew Ross, Martin
   Schuette, Holt Sorenson, Rodney Thayer, and the many Counterpane
   Internet Security engineering and operations people who commented on
   various versions of this proposal.

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  References</span>

<span class="h3"><a class="selflink" id="section-11.1" href="#section-11.1">11.1</a>.  Normative References</span>

   [<a id="ref-FIPS.186-2.2000">FIPS.186-2.2000</a>]  National Institute of Standards and Technology,
                      "Digital Signature Standard", FIPS PUB 186-2,
                      January 2000, &lt;<a href="http://csrc.nist.gov/publications/fips/archive/fips186-2/fips186-2.pdf">http://csrc.nist.gov/publications/</a>
                      <a href="http://csrc.nist.gov/publications/fips/archive/fips186-2/fips186-2.pdf">fips/archive/fips186-2/fips186-2.pdf</a>&gt;.

   [<a id="ref-FIPS.180-2.2002">FIPS.180-2.2002</a>]  National Institute of Standards and Technology,
                      "Secure Hash Standard", FIPS PUB 180-2,
                      August 2002, &lt;<a href="http://csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf">http://csrc.nist.gov/publications/</a>
                      <a href="http://csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf">fips/fips180-2/fips180-2.pdf</a>&gt;.

   [<a id="ref-RFC2119">RFC2119</a>]          Bradner, S., "Key words for use in RFCs to
                      Indicate Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>,
                      March 1997.

   [<a id="ref-RFC4648">RFC4648</a>]          Josefsson, S., "The Base16, Base32, and Base64
                      Data Encodings", <a href="./rfc4648">RFC 4648</a>, October 2006.

   [<a id="ref-RFC4880">RFC4880</a>]          Callas, J., Donnerhacke, L., Finney, H., Shaw, D.,
                      and R. Thayer, "OpenPGP Message Format", <a href="./rfc4880">RFC 4880</a>,
                      November 2007.

   [<a id="ref-RFC5226">RFC5226</a>]          Narten, T. and H. Alvestrand, "Guidelines for
                      Writing an IANA Considerations Section in RFCs",
                      <a href="https://www.rfc-editor.org/bcp/bcp26">BCP 26</a>, <a href="./rfc5226">RFC 5226</a>, May 2008.

   [<a id="ref-RFC5280">RFC5280</a>]          Cooper, D., Santesson, S., Farrell, S., Boeyen,
                      S., Housley, R., and W. Polk, "Internet X.509
                      Public Key Infrastructure Certificate and
                      Certificate Revocation List (CRL) Profile",
                      <a href="./rfc5280">RFC 5280</a>, May 2008.

   [<a id="ref-RFC5424">RFC5424</a>]          Gerhards, R., "The syslog Protocol", <a href="./rfc5424">RFC 5424</a>,
                      March 2009.



<span class="grey">Kelsey, et al.               Standards Track                   [Page 39]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-40" ></span>
<span class="grey"><a href="./rfc5848">RFC 5848</a>                 Signed Syslog Messages                 May 2010</span>


   [<a id="ref-RFC5425">RFC5425</a>]          Miao, F., Yuzhi, M., and J. Salowey, "TLS
                      Transport Mapping for syslog", <a href="./rfc5425">RFC 5425</a>,
                      March 2009.

   [<a id="ref-RFC5426">RFC5426</a>]          Okmianski, A., "Transmission of syslog Messages
                      over UDP", <a href="./rfc5426">RFC 5426</a>, March 2009.

<span class="h3"><a class="selflink" id="section-11.2" href="#section-11.2">11.2</a>.  Informative References</span>

   [<a id="ref-NIST800.90">NIST800.90</a>]       National Institute of Standards and Technology,
                      "NIST Special Publication 800-90: Recommendation
                      for Random Number Generation using Deterministic
                      Random Bit Generators", June 2006, &lt;<a href="http://csrc.nist.gov/publications/nistpubs/800-90/SP800-90revised_March2007.pdf">http://</a>
                      <a href="http://csrc.nist.gov/publications/nistpubs/800-90/SP800-90revised_March2007.pdf">csrc.nist.gov/publications/nistpubs/800-90/</a>
                      <a href="http://csrc.nist.gov/publications/nistpubs/800-90/SP800-90revised_March2007.pdf">SP800-90revised_March2007.pdf</a>&gt;.

   [<a id="ref-RFC3339">RFC3339</a>]          Klyne, G. and C. Newman, "Date and Time on the
                      Internet: Timestamps", <a href="./rfc3339">RFC 3339</a>, July 2002.

   [<a id="ref-RFC3414">RFC3414</a>]          Blumenthal, U. and B. Wijnen, "User-based Security
                      Model (USM) for version 3 of the Simple Network
                      Management Protocol (SNMPv3)", <a href="./rfc3414">RFC 3414</a>,
                      December 2002.

   [<a id="ref-RFC4086">RFC4086</a>]          Eastlake, D., Schiller, J., and S. Crocker,
                      "Randomness Recommendations for Security",
                      <a href="./rfc4086">RFC 4086</a>, June 2005.

Authors' Addresses

   John Kelsey
   NIST

   EMail: john.kelsey@nist.gov


   Jon Callas
   PGP Corporation

   EMail: jon@callas.org


   Alexander Clemm
   Cisco Systems

   EMail: alex@cisco.com





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