<pre>Network Working Group                                         T. Gondrom
Request for Comments: 4998                         Open Text Corporation
Category: Standards Track                                    R. Brandner
                                                   InterComponentWare AG
                                                             U. Pordesch
                                                 Fraunhofer Gesellschaft
                                                             August 2007


                      <span class="h1">Evidence Record Syntax (ERS)</span>

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   In many scenarios, users must be able prove the existence and
   integrity of data, including digitally signed data, in a common and
   reproducible way over a long and possibly undetermined period of
   time.  This document specifies the syntax and processing of an
   Evidence Record, a structure designed to support long-term non-
   repudiation of existence of data.




















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<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


Table of Contents

   <a href="#section-1">1</a>.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-3">3</a>
     <a href="#section-1.1">1.1</a>.  Motivation . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-3">3</a>
     <a href="#section-1.2">1.2</a>.  General Overview and Requirements  . . . . . . . . . . . .  <a href="#page-4">4</a>
     <a href="#section-1.3">1.3</a>.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-5">5</a>
     <a href="#section-1.4">1.4</a>.  Conventions Used in This Document  . . . . . . . . . . . .  <a href="#page-6">6</a>
   <a href="#section-2">2</a>.  Identification and References  . . . . . . . . . . . . . . . .  <a href="#page-7">7</a>
     <a href="#section-2.1">2.1</a>.  ASN.1 Module Definition  . . . . . . . . . . . . . . . . .  <a href="#page-7">7</a>
       <a href="#section-2.1.1">2.1.1</a>.  ASN.1 Module Definition for 1988 ASN.1 Syntax  . . . .  <a href="#page-7">7</a>
       <a href="#section-2.1.2">2.1.2</a>.  ASN.1 Module Definition for 1997-ASN.1 Syntax  . . . .  <a href="#page-7">7</a>
     <a href="#section-2.2">2.2</a>.  ASN.1 Imports and Exports  . . . . . . . . . . . . . . . .  <a href="#page-7">7</a>
       <a href="#section-2.2.1">2.2.1</a>.  Imports and Exports Conform with 1988 ASN.1  . . . . .  <a href="#page-8">8</a>
       <a href="#section-2.2.2">2.2.2</a>.  Imports and Exports Conform with 1997-ASN.1  . . . . .  <a href="#page-8">8</a>
     <a href="#section-2.3">2.3</a>.  LTANS Identification . . . . . . . . . . . . . . . . . . .  <a href="#page-9">9</a>
   <a href="#section-3">3</a>.  Evidence Record  . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-9">9</a>
     <a href="#section-3.1">3.1</a>.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . .  <a href="#page-9">9</a>
     <a href="#section-3.2">3.2</a>.  Generation . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-10">10</a>
     <a href="#section-3.3">3.3</a>.  Verification . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-11">11</a>
   <a href="#section-4">4</a>.  Archive Timestamp  . . . . . . . . . . . . . . . . . . . . . . <a href="#page-11">11</a>
     <a href="#section-4.1">4.1</a>.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-11">11</a>
     <a href="#section-4.2">4.2</a>.  Generation . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-12">12</a>
     <a href="#section-4.3">4.3</a>.  Verification . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-15">15</a>
   <a href="#section-5">5</a>.  Archive Timestamp Chain and Archive Timestamp Sequence . . . . <a href="#page-16">16</a>
     <a href="#section-5.1">5.1</a>.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-17">17</a>
     <a href="#section-5.2">5.2</a>.  Generation . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-17">17</a>
     <a href="#section-5.3">5.3</a>.  Verification . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-19">19</a>
   <a href="#section-6">6</a>.  Encryption . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-20">20</a>
     <a href="#section-6.1">6.1</a>.  Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-21">21</a>
       <a href="#section-6.1.1">6.1.1</a>.  EncryptionInfo in 1988 ASN.1 . . . . . . . . . . . . . <a href="#page-21">21</a>
       <a href="#section-6.1.2">6.1.2</a>.  EncryptionInfo in 1997-ASN.1 . . . . . . . . . . . . . <a href="#page-22">22</a>
   <a href="#section-7">7</a>.  Security Considerations  . . . . . . . . . . . . . . . . . . . <a href="#page-22">22</a>
   <a href="#section-8">8</a>.  References . . . . . . . . . . . . . . . . . . . . . . . . . . <a href="#page-23">23</a>
     <a href="#section-8.1">8.1</a>.  Normative References . . . . . . . . . . . . . . . . . . . <a href="#page-23">23</a>
     <a href="#section-8.2">8.2</a>.  Informative References . . . . . . . . . . . . . . . . . . <a href="#page-24">24</a>
   <a href="#appendix-A">Appendix A</a>.  Evidence Record Using CMS . . . . . . . . . . . . . . <a href="#page-26">26</a>
   <a href="#appendix-B">Appendix B</a>.  ASN.1-Module with 1988 Syntax . . . . . . . . . . . . <a href="#page-27">27</a>
   <a href="#appendix-C">Appendix C</a>.  ASN.1-Module with 1997 Syntax . . . . . . . . . . . . <a href="#page-29">29</a>













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<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


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

<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  Motivation</span>

   In many application areas of electronic data exchange, a non-
   repudiable proof of the existence of digital data must be possible.
   In some cases, this proof must survive the passage of long periods of
   time.  An important example is digitally signed data.  Digital
   signatures can be used to demonstrate data integrity and to perform
   source authentication.  In some cases, digitally signed data must be
   archived for 30 years or more.  However, the reliability of digital
   signatures over long periods is not absolute.  During the archival
   period, hash algorithms and public key algorithms can become weak or
   certificates can become invalid.  These events complicate the
   reliance on digitally signed data after many years by increasing the
   likelihood that forgeries can be created.  To avoid losing the
   desired security properties derived from digital signatures, it is
   necessary to prove that the digitally signed data already existed
   before such a critical event.  This can be accomplished using a
   timestamp.  However, some timestamps rely upon mechanisms that will
   be subject to the same problems.  To counter this problem, timestamps
   are renewed by simply obtaining a new timestamp that covers the
   original data and its timestamps prior to the compromise of
   mechanisms used to generate the timestamps.  This document provides a
   syntax to support the periodic renewal of timestamps.

   It is necessary to standardize the data formats and processing
   procedures for such timestamps in order to be able to verify and
   communicate preservation evidence.  A first approach was made by IETF
   within [<a href="./rfc3126" title="&quot;Electronic Signature Formats for long term electronic signatures&quot;">RFC3126</a>], where an optional Archive Timestamp Attribute was
   specified for integration in signatures according to the
   Cryptographic Messages Syntax (CMS) [<a href="./rfc3852" title="&quot;Cryptographic Message Syntax (CMS)&quot;">RFC3852</a>].

   Evidence Record Syntax (ERS) broadens and generalizes this approach
   for data of any format and takes long-term archive service
   requirements [<a href="./rfc4810" title="&quot;Long-Term Archive Service Requirements&quot;">RFC4810</a>] into account -- in particular, the handling of
   large sets of data objects.  ERS specifies a syntax for an
   EvidenceRecord, which contains a set of Archive Timestamps and some
   additional data.  This Evidence Record can be stored separately from
   the archived data, as a file, or integrated into the archived data,
   i.e., as an attribute.  ERS also specifies processes for generation
   and verification of Evidence Records.  <a href="#appendix-A">Appendix A</a> describes the
   integration and use of an EvidenceRecord in context of signed and
   enveloped messages according to the Cryptographic Message Syntax
   (CMS).  ERS does not specify a protocol for interacting with a long-
   term archive system.  The Long-term Archive Protocol specification
   being developed by the IETF LTANS WG addresses this interface.




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<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>.  General Overview and Requirements</span>

   ERS is designed to meet the requirements for data structures set
   forth in [<a href="./rfc4810" title="&quot;Long-Term Archive Service Requirements&quot;">RFC4810</a>].

   The basis of the ERS are Archive Timestamps, which can cover a single
   data object (as an <a href="./rfc3161">RFC3161</a> compliant timestamp does) or can cover a
   group of data objects.  Groups of data objects are addressed using
   hash trees, first described by Merkle [<a href="#ref-MER1980" title="&quot;Protocols for Public Key Cryptosystems, Proceedings of the 1980 IEEE Symposium on Security and Privacy (Oakland, CA, USA)&quot;">MER1980</a>], combined with a
   timestamp.  The leaves of the hash tree are hash values of the data
   objects in a group.  A timestamp is requested only for the root hash
   of the hash tree.  The deletion of a data object in the tree does not
   influence the provability of others.  For any particular data object,
   the hash tree can be reduced to a few sets of hash values, which are
   sufficient to prove the existence of a single data object.
   Similarly, the hash tree can be reduced to prove existence of a data
   group, provided all members of the data group have the same parent
   node in the hash tree.  Archive Timestamps are comprised of an
   optional reduced hash tree and a timestamp.

   An EvidenceRecord may contain many Archive Timestamps.  For the
   generation of the initial Archive Timestamp, the data objects to be
   timestamped have to be determined.  Depending on the context, this
   could be a file or a data object group consisting of multiple files,
   such as a document and its associated digital signature.

   Before the cryptographic algorithms used within the Archive Timestamp
   become weak or timestamp certificates become invalid, Archive
   Timestamps have to be renewed by generating a new Archive Timestamp.
   (Note: Information about the weakening of the security properties of
   public key and hash algorithms, as well as the risk of compromise of
   private keys of Time Stamping Units, has to be closely watched by the
   Long-Term Archive provider or the owner of the data objects himself.
   This information should be gathered by "out-of-band" means and is out
   of scope of this document.)  ERS distinguishes two ways for renewal
   of an Archive Timestamp: Timestamp Renewal and Hash-Tree Renewal.

   Depending on the conditions, the respective type of renewal is
   required: The timestamp renewal is necessary if the private key of a
   Timestamping Unit has been compromised, or if an asymmetric algorithm
   or a hash algorithm used for the generation of the timestamps is no
   longer secure for the given key size.  If the hash algorithm used to
   build the hash trees in the Archive Timestamp loses its security
   properties, the Hash-Tree Renewal is required.

   In the case of Timestamp Renewal, the timestamp of an Archive
   Timestamp has to be hashed and timestamped by a new Archive
   Timestamp.  This mode of renewal can only be used when it is not



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<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


   necessary to access the archived data objects covered by the
   timestamp.  For example, this simple form of renewal is sufficient if
   the public key algorithm of the timestamp is going to lose its
   security or the timestamp authority certificate is about to expire.
   This is very efficient, in particular, if Archive Timestamping is
   done by an archiving system or service, which implements a central
   management of Archive Timestamps.

   Timestamp renewal is not sufficient if the hash algorithm used to
   build the hash tree of an Archive Timestamp becomes insecure.  In the
   case of Hash-Tree Renewal, all evidence data must be accessed and
   timestamped.  This includes not only the timestamps but also the
   complete Archive Timestamps and the archived data objects covered by
   the timestamps, which must be hashed and timestamped again by a new
   Archive Timestamp.

<span class="h3"><a class="selflink" id="section-1.3" href="#section-1.3">1.3</a>.  Terminology</span>

   Archived data object: A data unit that is archived and has to be
   preserved for a long time by the Long-term Archive Service.

   Archived data object group: A set of two or more of data objects,
   which for some reason belong together.  For example, a document file
   and a signature file could be an archived data object group, which
   represent signed data.

   Archive Timestamp: A timestamp and typically lists of hash values,
   which allow the verification of the existence of several data objects
   at a certain time.  (In its most simple variant, when it covers only
   one object, it may only consist of the timestamp.)

   Archive Timestamp Chain: Part of an Archive Timestamp Sequence, it is
   a time-ordered sequence of Archive Timestamps, where each Archive
   Timestamp preserves non-repudiation of the previous Archive
   Timestamp, even after the previous Archive Timestamp becomes invalid.
   Overall non-repudiation is maintained until the new Archive Timestamp
   itself becomes invalid.  The process of generating such an Archive
   Timestamp Chain is called Timestamp Renewal.

   Archive Timestamp Sequence: Part of the Evidence Record, it is a
   sequence of Archive Timestamp Chains, where each Archive Timestamp
   Chain preserves non-repudiation of the previous Archive Timestamp
   Chains, even after the hash algorithm used within the previous
   Archive Timestamp's hash tree became weak.  Non-repudiation is
   preserved until the last Archive Timestamp of the last chain becomes
   invalid.  The process of generating such an Archive Timestamp
   Sequence is called Hash-Tree Renewal.




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<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


   Evidence: Information that may be used to resolve a dispute about
   various aspects of authenticity of archived data objects.

   Evidence record: Collection of evidence compiled for one or more
   given archived data objects over time.  An evidence record includes
   all Archive Timestamps (within structures of Archive Timestamp Chains
   and Archive Timestamp Sequences) and additional verification data,
   like certificates, revocation information, trust anchors, policy
   details, role information, etc.

   Long-term Archive (LTA) Service: A service responsible for preserving
   data for long periods of time, including generation and collection of
   evidence, storage of archived data objects and evidence, etc.

   Reduced hash tree: The process of reducing a Merkle hash tree
   [<a href="#ref-MER1980" title="&quot;Protocols for Public Key Cryptosystems, Proceedings of the 1980 IEEE Symposium on Security and Privacy (Oakland, CA, USA)&quot;">MER1980</a>] to a list of lists of hash values.  This is the basis of
   storing the evidence for a single data object.

   Timestamp: A cryptographically secure confirmation generated by a
   Time Stamping Authority (TSA).  [<a href="./rfc3161" title="&quot;Internet X.509 Public Key Infrastructure Time-Stamp Protocol (TSP)&quot;">RFC3161</a>] specifies a structure for
   timestamps and a protocol for communicating with a TSA.  Besides
   this, other data structures and protocols may also be appropriate,
   e.g., such as defined in [<a href="#ref-ISO-18014-1.2002">ISO-18014-1.2002</a>], [<a href="#ref-ISO-18014-2.2002">ISO-18014-2.2002</a>],
   [<a href="#ref-ISO-18014-3.2004">ISO-18014-3.2004</a>], and [<a href="#ref-ANSI.X9-95.2005">ANSI.X9-95.2005</a>].

   An Archive Timestamp relates to a data object, if the hash value of
   this data object is part of the first hash value list of the Archive
   Timestamp.  An Archive Timestamp relates to a data object group, if
   it relates to every data object of the group and no other data
   objects.  An Archive Timestamp Chain relates to a data object / data
   object group, if its first Archive Timestamp relates to this data
   object/data object group.  An Archive Timestamp Sequence relates to a
   data object / data object group, if its first Archive Timestamp Chain
   relates to this data object/data object group.

<span class="h3"><a class="selflink" id="section-1.4" href="#section-1.4">1.4</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>].











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<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Identification and References</span>

<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>.  ASN.1 Module Definition</span>

   As many open ASN.1 compilers still support the 1988 syntax, this
   standard offers to support two versions of ASN.1 1997-ASN.1 and 1988-
   ASN.1.  (For specification of ASN.1 refer to [<a href="#ref-CCITT.X208.1988">CCITT.X208.1988</a>],
   [<a href="#ref-CCITT.X209.1988">CCITT.X209.1988</a>], [<a href="#ref-CCITT.X680.2002">CCITT.X680.2002</a>] and [<a href="#ref-CCITT.X690.2002">CCITT.X690.2002</a>].)  This
   specification defines the two ASN.1 modules, one for 1988 conform
   ASN.1 and another in 1997-ASN.1 syntax.  Depending on the syntax
   version of your compiler implementation, you can use the imports for
   the 1988 conformant ASN.1 syntax or the imports for the 1997-ASN.1
   syntax.  The appendix of this document lists the two complete
   alternative ASN.1 modules.  If there is a conflict between both
   modules, the 1988-ASN.1 module precedes.

<span class="h4"><a class="selflink" id="section-2.1.1" href="#section-2.1.1">2.1.1</a>.  ASN.1 Module Definition for 1988 ASN.1 Syntax</span>

   1988 ASN.1 Module start

   ERS {iso(1) identified-organization(3) dod(6)
         internet(1) security(5) mechanisms(5)
         ltans(11) id-mod(0) id-mod-ers88(2) id-mod-ers88-v1(1) }
   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN

<span class="h4"><a class="selflink" id="section-2.1.2" href="#section-2.1.2">2.1.2</a>.  ASN.1 Module Definition for 1997-ASN.1 Syntax</span>

   ASN.1 Module start

   ERS {iso(1) identified-organization(3) dod(6)
         internet(1) security(5) mechanisms(5)
         ltans(11) id-mod(0) id-mod-ers(1) id-mod-ers-v1(1) }
   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN

<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a>.  ASN.1 Imports and Exports</span>

   The specification exports all definitions and imports various
   definitions.  Depending on the ASN.1 syntax version of your
   implementation, you can use the imports for the 1988 conform ASN.1
   syntax below or the imports for the 1997-ASN.1 syntax in
   <a href="#section-2.2.2">Section 2.2.2</a>.








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<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


<span class="h4"><a class="selflink" id="section-2.2.1" href="#section-2.2.1">2.2.1</a>.  Imports and Exports Conform with 1988 ASN.1</span>

   -- EXPORTS ALL --

   IMPORTS

    -- Imports from <a href="./rfc3852">RFC 3852</a> Cryptographic Message Syntax
   ContentInfo, Attribute
       FROM CryptographicMessageSyntax2004 -- FROM [<a href="./rfc3852" title="&quot;Cryptographic Message Syntax (CMS)&quot;">RFC3852</a>]
        { iso(1) member-body(2) us(840) rsadsi(113549)
          pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2004(24) }

     -- Imports from <a href="./rfc3280">RFC 3280</a> <a href="./rfc3280#appendix-A.1">[RFC3280], Appendix&nbsp;A.1</a>
   AlgorithmIdentifier
       FROM PKIX1Explicit88
           { iso(1) identified-organization(3) dod(6)
           internet(1) security(5) mechanisms(5) pkix(7)
           mod(0) pkix1-explicit(18) }
   ;

<span class="h4"><a class="selflink" id="section-2.2.2" href="#section-2.2.2">2.2.2</a>.  Imports and Exports Conform with 1997-ASN.1</span>

   -- EXPORTS ALL --

   IMPORTS

    -- Imports from PKCS-7
   ContentInfo
       FROM PKCS7
           {iso(1) member-body(2) us(840) rsadsi(113549)
           pkcs(1) pkcs-7(7) modules(0)}


     -- Imports from AuthenticationFramework
   AlgorithmIdentifier
       FROM AuthenticationFramework
           {joint-iso-itu-t ds(5) module(1)
           authenticationFramework(7) 4}


    -- Imports from InformationFramework
   Attribute
       FROM InformationFramework
           {joint-iso-itu-t ds(5) module(1)
           informationFramework(1) 4}
   ;





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<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


<span class="h3"><a class="selflink" id="section-2.3" href="#section-2.3">2.3</a>.  LTANS Identification</span>

   This document defines the LTANS object identifier tree root.

   LTANS Object Identifier tree root

   ltans OBJECT IDENTIFIER ::=
            { iso(1) identified-organization(3) dod(6) internet(1)
              security(5) mechanisms(5) ltans(11) }

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Evidence Record</span>

   An Evidence Record is a unit of data, which can be used to prove the
   existence of an archived data object or an archived data object group
   at a certain time.  The Evidence Record contains Archive Timestamps,
   generated during a long archival period and possibly useful data for
   validation.  It is possible to store this Evidence Record separately
   from the archived data objects or to integrate it into the data
   itself.  For data types, signed data and enveloped data of the CMS
   integration are specified in <a href="#appendix-A">Appendix A</a>.

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  Syntax</span>

   Evidence Record has the following ASN.1 Syntax:

   ASN.1 Evidence Record

   EvidenceRecord ::= SEQUENCE {
      version                   INTEGER { v1(1) } ,
      digestAlgorithms          SEQUENCE OF AlgorithmIdentifier,
      cryptoInfos               [0] CryptoInfos OPTIONAL,
      encryptionInfo            [1] EncryptionInfo OPTIONAL,
      archiveTimeStampSequence  ArchiveTimeStampSequence
      }

   CryptoInfos ::= SEQUENCE SIZE (1..MAX) OF Attribute

   The fields have the following meanings:

   The 'version' field indicates the syntax version, for compatibility
   with future revisions of this specification and to distinguish it
   from earlier non-conformant or proprietary versions of the ERS.  The
   value 1 indicates this specification.  Lower values indicate an
   earlier version of the ERS has been used.  An implementation
   conforming to this specification SHOULD reject a version value below
   1.





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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-10" ></span>
<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


   digestAlgorithms is a sequence of all the hash algorithms used to
   hash the data object over the archival period.  It is the union of
   all digestAlgorithm values from the ArchiveTimestamps contained in
   the EvidenceRecord.  The ordering of the values is not relevant.

   cryptoInfos allows the storage of data useful in the validation of
   the archiveTimeStampSequence.  This could include possible Trust
   Anchors, certificates, revocation information, or the current
   definition of the suitability of cryptographic algorithms, past and
   present (e.g., RSA 768-bit valid until 1998, RSA 1024-bit valid until
   2008, SHA1 valid until 2010).  These items may be added based on the
   policy used.  Since this data is not protected within any timestamp,
   the data should be verifiable through other mechanisms.  Such
   verification is out of scope of this document.

   encryptionInfo contains the necessary information to support
   encrypted content to be handled.  For discussion of syntax, please
   refer to <a href="#section-6.1">Section 6.1</a>.

   ArchiveTimeStampSequence is a sequence of ArchiveTimeStampChain,
   described in <a href="#section-5">Section 5</a>.

   If the archive data objects were encrypted before generating Archive
   Timestamps but a non-repudiation proof is needed for unencrypted data
   objects, the optional encryptionInfos field contains data necessary
   to unambiguously re-encrypt data objects.  If omitted, it means that
   data objects are not encrypted or that a non-repudiation proof for
   the unencrypted data is not required.  For further details, see
   <a href="#section-6">Section 6</a>.

<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>.  Generation</span>

   The generation of an EvidenceRecord can be described as follows:

   1.  Select a data object or group of data objects to archive.

   2.  Create the initial Archive Timestamp (see <a href="#section-4">Section 4</a>, "Archive
       Timestamp").

   3.  Refresh the Archive Timestamp when necessary, by Timestamp
       Renewal or Hash-Tree Renewal (see <a href="#section-5">Section 5</a>).

   The process of generation depends on whether the Archive Timestamps
   are generated, stored, and managed by a centralized instance.  In the
   case of central management, it is possible to collect many data
   objects, build hash trees, store them, and reduce them later.  In
   case of local generation, it might be easier to generate a simple
   Archive Timestamp without building hash trees.  This can be



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<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


   accomplished by omitting the reducedHashtree field from the
   ArchiveTimestamp.  In this case, the ArchiveTimestamp covers a single
   data object.  Using this approach, it is possible to "convert"
   existing timestamps into ArchiveTimestamps for renewal.

<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>.  Verification</span>

   The Verification of an EvidenceRecord overall can be described as
   follows:

   1.  Select an archived data object or group of data objects

   2.  Re-encrypt data object/data object group, if encryption field is
       used (for details, see <a href="#section-6">Section 6</a>).

   3.  Verify Archive Timestamp Sequence (details in <a href="#section-4">Section 4</a> and
       <a href="#section-5">Section 5</a>).

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

   An Archive Timestamp is a timestamp and a set of lists of hash
   values.  The lists of hash values are generated by reduction of an
   ordered Merkle hash tree [<a href="#ref-MER1980" title="&quot;Protocols for Public Key Cryptosystems, Proceedings of the 1980 IEEE Symposium on Security and Privacy (Oakland, CA, USA)&quot;">MER1980</a>].  The leaves of this hash tree are
   the hash values of the data objects to be timestamped.  Every inner
   node of the tree contains one hash value, which is generated by
   hashing the concatenation of the children nodes.  The root hash
   value, which represents unambiguously all data objects, is
   timestamped.

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Syntax</span>

   An Archive Timestamp has the following ASN.1 Syntax:

   ASN.1 Archive Timestamp

   ArchiveTimeStamp ::= SEQUENCE {
     digestAlgorithm [0] AlgorithmIdentifier OPTIONAL,
     attributes      [1] Attributes OPTIONAL,
     reducedHashtree [2] SEQUENCE OF PartialHashtree OPTIONAL,
     timeStamp       ContentInfo}

   PartialHashtree ::= SEQUENCE OF OCTET STRING

   Attributes ::= SET SIZE (1..MAX) OF Attribute

   The fields of type ArchiveTimeStamp have the following meanings:





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<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


   digestAlgorithm identifies the digest algorithm and any associated
   parameters used within the reduced hash tree.  If the optional field
   digestAlgorithm is not present, the digest algorithm of the timestamp
   MUST be used.  Which means, if timestamps according to [<a href="./rfc3161" title="&quot;Internet X.509 Public Key Infrastructure Time-Stamp Protocol (TSP)&quot;">RFC3161</a>] are
   used in this case, the content of this field is identical to
   hashAlgorithm of messageImprint field of TSTInfo.

   attributes contains information an LTA might want to provide to
   document individual renewal steps and the creation of the individual
   ArchiveTimeStamps, e.g., applied policies.  As the structure of the
   ArchiveTimeStamp may be protected by hash and timestamps, the
   ordering is relevant, which is why a SET is used instead of a
   SEQUENCE.

   reducedHashtree contains lists of hash values, organized in
   PartialHashtrees for easier understanding.  They can be derived by
   reducing a hash tree to the nodes necessary to verify a single data
   object.  Hash values are represented as octet strings.  If the
   optional field reducedHashtree is not present, the ArchiveTimestamp
   simply contains an ordinary timestamp.

   timeStamp should contain the timestamp as defined in <a href="#section-1.3">Section 1.3</a>.
   (e.g., as defined with TimeStampToken in [<a href="./rfc3161" title="&quot;Internet X.509 Public Key Infrastructure Time-Stamp Protocol (TSP)&quot;">RFC3161</a>]).  Other types of
   timestamp MAY be used, if they contain time data, timestamped data,
   and a cryptographically secure confirmation from the TSA of these
   data.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Generation</span>

   The lists of hash values of an Archive Timestamp can be generated by
   building and reducing a Merkle hash tree [<a href="#ref-MER1980" title="&quot;Protocols for Public Key Cryptosystems, Proceedings of the 1980 IEEE Symposium on Security and Privacy (Oakland, CA, USA)&quot;">MER1980</a>].

   Such a hash tree can be built as follows:

   1.  Collect data objects to be timestamped.

   2.  Choose a secure hash algorithm H and generate hash values for the
       data objects.  These values will be the leaves of the hash tree.

   3.  For each data group containing more than one document, its
       respective document hashes are binary sorted in ascending order,
       concatenated, and hashed.  The hash values are the complete
       output from the hash algorithm, i.e., leading zeros are not
       removed, with the most significant bit first.

   4.  If there is more than one hash value, place them in groups and
       sort each group in binary ascending order.  Concatenate these
       values and generate new hash values, which are inner nodes of



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<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


       this tree.  (If additional hash values are needed, e.g., so that
       all nodes have the same number of children, any data may be
       hashed using H and used.)  Repeat this step until there is only
       one hash value, which is the root node of the hash tree.

   5.  Obtain a timestamp for this root hash value.  The hash algorithm
       in the timestamp request MUST be the same as the hash algorithm
       of the hash tree, or the digestAlgorithm field of the
       ArchiveTimeStamp MUST be present and specify the hash algorithm
       of the hash tree.

   An example of a constructed hash tree for 3 data groups, where data
   groups 1 and 3 only contain one document, and data group 2 contains 3
   documents:

                 +------+
                 | h123 |
                 +------+
               /         \
              /           \
           +----+      +----+
           | h12|      | h3 |
           +----+      +----+
           /     \
          /       \
       +----+  +-------+
       | h1 |  | h2abc |
       +----+  +-------+
               /   |   \
              /    |    \
             /     |     \
            /      |      \
        +----+  +----+  +----+
        | h2a|  | h2b|  | h2c|
        +----+  +----+  +----+

   Figure 1: Hash tree

     h1 = H(d1) where d1 is the only data object in data group 1
     h3 = H(d3) where d3 is the only data object in data group 3
     h12 = H( binary sorted and concatenated (h1, h2abc))
     h123 = H( binary sorted and concatenated (h12, h3))
     h2a = H(first data object of data object group 2)
     h2b = H(second data object of data object group 2)
     h2c = H(third data object of data object group 2)
     h2abc = H( binary sorted and concatenated (h2a, h2b, h2c))





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<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


   The hash tree can be reduced to lists of hash values, necessary to
   have a proof of existence for a single data object:

   1.  Generate hash value h of the data object, using hash algorithm H
       of the hash tree.

   2.  Select all hash values, which have the same father node as h.
       Generate the first list of hash values by arranging these hashes,
       in binary ascending order.  This will be stored in the structure
       of the PartialHashtree.  Repeat this step for the father node of
       all hashes until the root hash is reached.  The father nodes
       themselves are not saved in the hash lists -- they are
       computable.

   3.  The list of all partialHashtrees finally is the reducedHashtree.
       (All of the specified hash values under the same father node,
       except the father node of the nodes below, are grouped in a
       PartialHashtree.  The sequence list of all Partialhashtrees is
       the reducedHashtree.)

   4.  Finally, add the timestamp and the info about the hash algorithm
       to get an Archive Timestamp.

   Assuming that the sorted binary ordering of the hashes in Figure 1
   is: h2abc &lt; h1, then the reduced hash tree for data group 1 (d1) is:

       +--------------------------------+
       | +-----------------+ +--------+ |
       | | +------+ +----+ | | +----+ | |
       | | | h2abc| | h1 | | | | h3 | | |
       | | +------+ +----+ | | +----+ | |
       | +-----------------+ +--------+ |
       +--------------------------------+

   Figure 2: Reduced hash tree for data group 1

      The pseudo ASN1 for this reduced hash tree rht1 would look like:
        rht1 = SEQ(pht1, pht2)

      with the PartialHashtrees pht1 and pht2
        pht1 = SEQ (h2abc, h1)
        pht2 = SEQ (h3)









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<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


   Assuming the same hash tree as in Figure 1, the reduced hash tree for
   all data objects in data group 2 is identical.

    +-------------------------------------------------+
    | +----------------------+  +--------+ +--------+ |
    | | +----+ +----+ +----+ |  | +----+ | | +----+ | |
    | | | h2b| | h2c| | h2a| |  | | h1 | | | | h3 | | |
    | | +----+ +----+ +----+ |  | +----+ | | +----+ | |
    | +----------------------+  +--------+ +--------+ |
    +-------------------------------------------------+

   Figure 3: Reduced hash tree for data object group 2

      The pseudo ASN1 for this reduced hash tree would look like:
        rht2 = SEQ(pht3, pht4, pht5)

      with the PartialHashtrees pht3, pht4, and pht5
       pht3 = SEQ (h2b, h2c, h2a)
       pht4 = SEQ (h1)
       pht5 = SEQ (h3)

   Note there are no restrictions on the quantity or length of hash-
   value lists.  Also note that it is profitable but not required to
   build hash trees and reduce them.  An Archive Timestamp may consist
   only of one list of hash-values and a timestamp or only a timestamp
   with no hash value lists.

   The data (e.g. certificates, Certificate Revocation Lists (CRLs), or
   Online Certificate Status Protocol (OCSP) responses) needed to verify
   the timestamp MUST be preserved, and SHOULD be stored in the
   timestamp itself unless this causes unnecessary duplication.  A
   timestamp according to [<a href="./rfc3161" title="&quot;Internet X.509 Public Key Infrastructure Time-Stamp Protocol (TSP)&quot;">RFC3161</a>] is a CMS object in which
   certificates can be stored in the certificates field and CRLs can be
   stored in the crls field of signed data.  OCSP responses can be
   stored as unsigned attributes [<a href="./rfc3126" title="&quot;Electronic Signature Formats for long term electronic signatures&quot;">RFC3126</a>].  Note [<a href="#ref-ANSI.X9-95.2005">ANSI.X9-95.2005</a>],
   [<a href="#ref-ISO-18014-2.2002">ISO-18014-2.2002</a>], and [<a href="#ref-ISO-18014-3.2004">ISO-18014-3.2004</a>], which specify verifiable
   timestamps that do not depend on certificates, CRLs, or OCSP
   responses.

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  Verification</span>

   An Archive Timestamp shall prove that a data object existed at a
   certain time, given by timestamp.  This can be verified as follows:

   1.  Calculate hash value h of the data object with hash algorithm H
       given in field digestAlgorithm of the Archive Timestamp.





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<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


   2.  Search for hash value h in the first list (partialHashtree) of
       reducedHashtree.  If not present, terminate verification process
       with negative result.

   3.  Concatenate the hash values of the actual list (partialHashtree)
       of hash values in binary ascending order and calculate the hash
       value h' with algorithm H.  This hash value h' MUST become a
       member of the next higher list of hash values (from the next
       partialHashtree).  Continue step 3 until a root hash value is
       calculated.

   4.  Check timestamp.  In case of a timestamp according to [<a href="./rfc3161" title="&quot;Internet X.509 Public Key Infrastructure Time-Stamp Protocol (TSP)&quot;">RFC3161</a>],
       the root hash value must correspond to hashedMessage, and
       digestAlgorithm must correspond to hashAlgorithm field, both in
       messageImprint field of timeStampToken.  In case of other
       timestamp formats, the hash value and digestAlgorithm must also
       correspond to their equivalent fields if they exist.

   If a proof is necessary for more than one data object, steps 1 and 2
   have to be done for all data objects to be proved.  If an additional
   proof is necessary that the Archive Timestamp relates to a data
   object group (e.g., a document and all its signatures), it can be
   verified additionally, that only the hash values of the given data
   objects are in the first hash-value list.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Archive Timestamp Chain and Archive Timestamp Sequence</span>

   An Archive Timestamp proves the existence of single data objects or
   data object group at a certain time.  However, this first Archive
   Timestamp in the first ArchiveTimeStampChain can become invalid, if
   hash algorithms or public key algorithms used in its hash tree or
   timestamp become weak or if the validity period of the timestamp
   authority certificate expires or is revoked.

   Prior to such an event, the existence of the Archive Timestamp or
   archive timestamped data has to be reassured.  This can be done by
   creating a new Archive Timestamp.  Depending on whether the timestamp
   becomes invalid or the hash algorithm of the hash tree becomes weak,
   two kinds of Archive Timestamp renewal are possible:

   o  Timestamp Renewal: A new Archive Timestamp is generated, which
      covers the timestamp of the old one.  One or more Archive
      Timestamps generated by Timestamp Renewal yield an Archive
      Timestamp Chain for a data object or data object group.







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<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


   o  Hash-Tree Renewal: A new Archive Timestamp is generated, which
      covers all the old Archive Timestamps as well as the data objects.
      A new Archive Timestamp Chain is started.  One or more Archive
      Timestamp Chains for a data object or data object group yield an
      Archive Timestamp Sequence.

   After the renewal, always only the last (i.e., most recent)
   ArchiveTimeStamp and the algorithms and timestamps used by it must be
   watched regarding expiration and loss of security.

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Syntax</span>

   ArchiveTimeStampChain and ArchiveTimeStampSequence have the following
   ASN.1 Syntax:

   ASN.1 ArchiveTimeStampChain and ArchiveTimeStampSequence

   ArchiveTimeStampChain    ::= SEQUENCE OF ArchiveTimeStamp

   ArchiveTimeStampSequence ::= SEQUENCE OF
                                ArchiveTimeStampChain

   ArchiveTimeStampChain and ArchiveTimeStampSequence MUST be ordered
   ascending by time of timestamp.  Within an ArchiveTimeStampChain, all
   reducedHashtrees of the contained ArchiveTimeStamps MUST use the same
   Hash-Algorithm.

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

   The initial Archive Timestamp relates to a data object or a data
   object group.  Before cryptographic algorithms that are used within
   the most recent Archive Timestamp (which is, at the beginning, the
   initial one) become weak or their timestamp certificates become
   invalid, Archive Timestamps have to be renewed by generating a new
   Archive Timestamp.

   In the case of Timestamp Renewal, the content of the timeStamp field
   of the old Archive Timestamp has to be hashed and timestamped by a
   new Archive Timestamp.  The new Archive Timestamp MAY not contain a
   reducedHashtree field, if the timestamp only simply covers the
   previous timestamp.  However, generally one can collect a number of
   old Archive Timestamps and build the new hash tree with the hash
   values of the content of their timeStamp fields.

   The new Archive Timestamp MUST be added to the ArchiveTimestampChain.
   This hash tree of the new Archive Timestamp MUST use the same hash
   algorithm as the old one, which is specified in the digestAlgorithm




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<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


   field of the Archive Timestamp or, if this value is not set (as it is
   optional), within the timestamp itself.

   In the case of Hash-Tree Renewal, the Archive Timestamp and the
   archived data objects covered by the Archive Timestamp must be hashed
   and timestamped again, as described below:

   1.  Select a secure hash algorithm H.

   2.  Select data objects d(i) referred to by initial Archive Timestamp
       (objects that are still present and not deleted).  Generate hash
       values h(i) = H((d(i)).  If data groups with more than one
       document are present, then one will have more than one hash for a
       group, i.e., h(i_a), h(i_b).., h(i_n)

   3.  atsc(i) is the encoded ArchiveTimeStampSequence, the
       concatenation of all previous Archive Timestamp Chains (in
       chronological order) related to data object d(i).  Generate hash
       value ha(i) = H(atsc(i)).
       Note: The ArchiveTimeStampChains used are DER encoded, i.e., they
       contain sequence and length tags.

   4.  Concatenate each h(i) with ha(i) and generate hash values
       h(i)' = H (h(i)+ ha(i)).  For multi-document groups, this is:
       h(i_a)' = H (h(i_a)+ ha(i))
       h(i_b)' = H (h(i_b)+ ha(i)), etc.

   5.  Build a new Archive Time Stamp for each h(i)'.  (Hash-tree
       generation and reduction is defined in <a href="#section-4.2">Section 4.2</a>; note that
       each h(i)' will be treated in <a href="#section-4.2">Section 4.2</a> as the document hash.
       The first hash value list in the reduced hash tree should only
       contain h(i)'.  For a multi-document group, the first hash value
       list will contain the new hashes for all the documents in this
       group, i.e., h(i_a)', h(i_b)'.., h(i_n)')

   6.  Create new ArchiveTimeStampChain containing the new Archive
       Timestamp and append this ArchiveTimeStampChain to the
       ArchiveTimeStampSequence.













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<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


                 +-------+
                 | h123' |
                 +-------+
               /         \
              /           \
           +-----+      +----+
           | h12'|      | h3'|
           +-----+      +----+
           /     \
          /       \
       +----+  +--------+
       | h1'|  | h2abc' |
       +----+  +--------+
               /   |   \
              /    |    \
             /     |     \
            /      |      \
        +----+  +----+  +----+
        |h2a'|  |h2b'|  |h2c'|
        +----+  +----+  +----+

   Figure 4: Hash tree from Hash-Tree Renewal

     Let H be the new secure hash algorithm
     ha(1), ha(2), ha(3) are as defined in step 4 above
     h1' = H( binary sorted and concatenated (H(d1), ha(1)))
       d1 is the original document from data group 1
     h3' = H( binary sorted and concatenated (H(d3), ha(3)))
       d3 is the original document from data group 3

     h2a = H(first data object of data object group 2)
      ...
     h2c = H(third data object of data object group 2)
     h2a' = H( binary sorted and concatenated (h2a, ha(2)))
      ...
     h2c' = H( binary sorted and concatenated (h2c, ha(2)))

     h2abc = H( binary sorted and concatenated (h2a', h2b', h2c'))

   ArchiveTimeStamps that are not necessary for verification should not
   be added to an ArchiveTimeStampChain or ArchiveTimeStampSequence.

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

   To get a non-repudiation proof that a data object existed at a
   certain time, the Archive Timestamp Chains and their relations to
   each other and to the data objects have to be proved:




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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


   1.  Verify that the first Archive Timestamp of the first
       ArchiveTimestampChain (the initial Archive Timestamp) contains
       the hash value of the data object.

   2.  Verify each ArchiveTimestampChain.  The first hash value list of
       each ArchiveTimeStamp MUST contain the hash value of the
       timestamp of the Archive Timestamp before.  Each Archive
       Timestamp MUST be valid relative to the time of the following
       Archive Timestamp.  All Archive Timestamps within a chain MUST
       use the same hash algorithm and this algorithm MUST be secure at
       the time of the first Archive Timestamp of the following
       ArchiveTimeStampChain.

   3.  Verify that the first hash value list (partialHashtree) of the
       first Archive Timestamp of all other ArchiveTimeStampChains
       contains a hash value of the concatenation of the data object
       hash and the hash value of all older ArchiveTimeStampChain.
       Verify that this Archive Timestamp was generated before the last
       Archive Timestamp of the ArchiveTimeStampChain became invalid.

   In order to complete the non-repudiation proof for the data objects,
   the last Archive Timestamp has to be valid at the time the
   verification is performed.

   If the proof is necessary for more than one data object, steps 1 and
   3 have to be done for all these data objects.  To prove the Archive
   Timestamp Sequence relates to a data object group, verify that each
   first Archive Timestamp of the first ArchiveTimeStampChain of the
   ArchiveTimeStampSequence of each data object does not contain other
   hash values in its first hash value list (than the hash values of the
   other data objects).

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

   If service providers are used to archive data and generate Archive
   Timestamps, it might be desirable or required that clients only send
   encrypted data to be archived.  However, this means that evidence
   records refer to encrypted data objects.  ERS directly protects the
   integrity of the bit-stream and this freezes the bit structure at the
   time of archiving.  This precludes changing of the encryption scheme
   during the archival period, e.g., if the encryption scheme is no
   longer secure, a change is not possible without losing the integrity
   proof of the EvidenceRecord.  In such cases, the services of a data
   transformation (and by this also possible re-encryption) done by a
   notary service might be a possible solution.  To avoid problems when
   using the evidence records in the future, additional special
   precautions have to be taken:




<span class="grey">Gondrom, 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="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


   o  Evidence generated to prove the existence of encrypted data cannot
      always be relied upon to prove the existence of unencrypted data.
      It may be possible to choose an algorithm or a key for decryption
      that is not the algorithm or key used for encryption.  In this
      case, the evidence record would not be a non-repudiation proof for
      the unencrypted data.  Therefore, only encryption methods should
      be used that make it possible to prove that archive-timestamped
      encrypted data objects unambiguously represent unencrypted data
      objects.  All data necessary to prove unambiguous representation
      should be included in the archived data objects.  (Note: In
      addition, the long-term security of the encryption schemes should
      be analyzed to determine if it could be used to create collision
      attacks.)

   o  When a relying party uses an evidence record to prove the
      existence of encrypted data objects, it may be desirable for
      clients to only store the unencrypted data objects and to delete
      the encrypted copy.  In order to use the evidence record, it must
      then be possible to unambiguously re-encrypt the unencrypted data
      to get exactly the data that was originally archived.  Therefore,
      additional data necessary to re-encrypt data objects should be
      inserted into the evidence record by the client, i.e., the LTA
      never sees these values.

   An extensible structure is defined to store the necessary parameters
   of the encryption methods.  The use of the specified
   encryptionInfoType and encryptionInfoValue may be heavily dependent
   on the mechanisms and has to be defined in other specifications.

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

   The EncryptionInfo field in EvidenceRecord has the following syntax
   depending on the version of ASN.1.

<span class="h4"><a class="selflink" id="section-6.1.1" href="#section-6.1.1">6.1.1</a>.  EncryptionInfo in 1988 ASN.1</span>

   1988 ASN.1 EncryptionInfo

   EncryptionInfo       ::=     SEQUENCE {
       encryptionInfoType     OBJECT IDENTIFIER,
       encryptionInfoValue    ANY DEFINED BY encryptionInfoType
   }









<span class="grey">Gondrom, 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="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


<span class="h4"><a class="selflink" id="section-6.1.2" href="#section-6.1.2">6.1.2</a>.  EncryptionInfo in 1997-ASN.1</span>

   1997-ASN.1 EncryptionInfo

   EncryptionInfo       ::=     SEQUENCE {
       encryptionInfoType   ENCINFO-TYPE.&amp;id
                                      ({SupportedEncryptionAlgorithms}),
       encryptionInfoValue  ENCINFO-TYPE.&amp;Type
                  ({SupportedEncryptionAlgorithms}{@encryptionInfoType})
   }

   ENCINFO-TYPE ::= TYPE-IDENTIFIER

   SupportedEncryptionAlgorithms ENCINFO-TYPE ::= {...}

   encryptionInfo contains information necessary for the unambiguous
   re-encryption of unencrypted content so that it exactly matches with
   the encrypted data objects protected by the EvidenceRecord.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Security Considerations</span>

   Secure Algorithms

   Cryptographic algorithms and parameters that are used within Archive
   Timestamps must be secure at the time of generation.  This concerns
   the hash algorithm used in the hash lists of Archive Timestamp as
   well as hash algorithms and public key algorithms of the timestamps.
   Publications regarding security suitability of cryptographic
   algorithms ([<a href="#ref-NIST.800-57-Part1.2006">NIST.800-57-Part1.2006</a>] and [<a href="#ref-ETSI-TS102176-1-2005">ETSI-TS102176-1-2005</a>]) have
   to be considered by verifying components.  A generic solution for
   automatic interpretation of security suitability policies in
   electronic form is desirable but not the subject of this
   specification.

   Redundancy

   Retrospectively, certain parts of an Archive Timestamp may turn out
   to have lost their security suitability before this has been publicly
   known.  For example, retrospectively, it may turn out that algorithms
   have lost their security suitability, and that even TSAs are
   untrustworthy.  This can result in Archive Timestamps using those
   losing their probative force.  Many TSAs are using the same signature
   algorithms.  While the compromise of a private key will only affect
   the security of one specific TSA, the retrospective loss of security
   of a signature algorithm will have impact on a potentially large
   number of TSAs at once.  To counter such risks, it is recommended to





<span class="grey">Gondrom, 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="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


   generate and manage at least two redundant Evidence Records with
   ArchiveTimeStampSequences using different hash algorithms and
   different TSAs using different signature algorithms.

   To best achieve and manage this redundancy, it is recommended to
   manage the Archive Timestamps in a central LTA.

   Secure Timestamps

   Archive Timestamping depends upon the security of normal time
   stamping.  Security requirements for Time Stamping Authorities stated
   in security policies have to be met.  Renewed Archive Timestamps
   should have the same or higher quality as the initial Archive
   Timestamp.  Archive Timestamps used for signature renewal of signed
   data, should have the same or higher quality than the maximum quality
   of the signatures.

   Secure Encryption

   For non-repudiation proof, it does not matter whether encryption has
   been broken or not.  Nevertheless, users should keep secret their
   private keys and randoms used for encryption and disclose them only
   if needed, e.g., in a lawsuit to a judge or expert.  They should use
   encryption algorithms and parameters that are prospected to be
   unbreakable as long as confidentiality of the archived data is
   important.

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

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

   [<a id="ref-CCITT.X208.1988">CCITT.X208.1988</a>]
              International Telephone and Telegraph Consultative
              Committee, "Specification of Abstract Syntax Notation One
              (ASN.1)", CCITT Recommendation X.208, November 1988.

   [<a id="ref-CCITT.X209.1988">CCITT.X209.1988</a>]
              International Telephone and Telegraph Consultative
              Committee, "Specification of Basic Encoding Rules for
              Abstract Syntax Notation One (ASN.1)",
              CCITT Recommendation X.209, 1988.

   [<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-RFC3161">RFC3161</a>]  Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
              "Internet X.509 Public Key Infrastructure Time-Stamp
              Protocol (TSP)", <a href="./rfc3161">RFC 3161</a>, August 2001.



<span class="grey">Gondrom, 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="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


   [<a id="ref-RFC3280">RFC3280</a>]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", <a href="./rfc3280">RFC 3280</a>,
              April 2002.

   [<a id="ref-RFC3852">RFC3852</a>]  Housley, R., "Cryptographic Message Syntax (CMS)",
              <a href="./rfc3852">RFC 3852</a>, July 2004.

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

   [<a id="ref-ANSI.X9-95.2005">ANSI.X9-95.2005</a>]
              American National Standard for Financial Services,
              "Trusted Timestamp Management and Security", ANSI X9.95,
              June 2005.

   [<a id="ref-CCITT.X680.2002">CCITT.X680.2002</a>]
              International Telephone and Telegraph Consultative
              Committee, "Abstract Syntax Notation One (ASN.1):
              Specification of basic notation", CCITT Recommendation
              X.680, July 2002.

   [<a id="ref-CCITT.X690.2002">CCITT.X690.2002</a>]
              International Telephone and Telegraph Consultative
              Committee, "ASN.1 encoding rules:  Specification of basic
              encoding Rules (BER), Canonical encoding rules (CER) and
              Distinguished encoding rules (DER)", CCITT Recommendation
              X.690, July 2002.

   [<a id="ref-ETSI-TS102176-1-2005">ETSI-TS102176-1-2005</a>]
              European Telecommunication Standards Institute (ETSI),
              Electronic Signatures and Infrastructures (ESI);,
              "Algorithms and Parameters for Secure Electronic
              Signatures; Part 1: Hash functions and asymmetric
              algorithms", ETSI  TS 102 176-1 V1.2.1, July 2005.

   [<a id="ref-ISO-18014-1.2002">ISO-18014-1.2002</a>]
              ISO/IEC JTC 1/SC 27, "Time stamping services - Part 1:
              Framework", ISO ISO-18014-1, February 2002.

   [<a id="ref-ISO-18014-2.2002">ISO-18014-2.2002</a>]
              ISO/IEC JTC 1/SC 27, "Time stamping services - Part 2:
              Mechanisms producing independent tokens", ISO ISO-18014-2,
              December 2002.

   [<a id="ref-ISO-18014-3.2004">ISO-18014-3.2004</a>]
              ISO/IEC JTC 1/SC 27, "Time stamping services - Part 3:
              Mechanisms producing linked tokens", ISO ISO-18014-3,
              February 2004.



<span class="grey">Gondrom, 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="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


   [<a id="ref-MER1980">MER1980</a>]  Merkle, R., "Protocols for Public Key Cryptosystems,
              Proceedings of the 1980 IEEE Symposium on Security and
              Privacy (Oakland, CA, USA)", pages 122-134, April 1980.

   [<a id="ref-NIST.800-57-Part1.2006">NIST.800-57-Part1.2006</a>]
              National Institute of Standards and Technology,
              "Recommendation for Key Management - Part 1: General
              (Revised)", NIST 800-57 Part1, May 2006.

   [<a id="ref-RFC3126">RFC3126</a>]  Pinkas, D., Ross, J., and N. Pope, "Electronic Signature
              Formats for long term electronic signatures", <a href="./rfc3126">RFC 3126</a>,
              September 2001.

   [<a id="ref-RFC4810">RFC4810</a>]  Wallace, C., Pordesch, U., and R. Brandner, "Long-Term
              Archive Service Requirements", <a href="./rfc4810">RFC 4810</a>, March 2007.




































<span class="grey">Gondrom, 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="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>.  Evidence Record Using CMS</span>

   An Evidence Record can be added to signed data or enveloped data in
   order to transfer them in a conclusive way.  For CMS, a sensible
   place to store such an Evidence Record is an unsigned attribute
   (signed message) or an unprotected attribute (enveloped message).

   One advantage of storing the Evidence Record within the CMS structure
   is that all data can be transferred in one conclusive file and is
   directly connected.  The documents, the signatures, and their
   Evidence Records can be bundled and managed together.  The
   description in the appendix contains the normative specification of
   how to integrate ERS in CMS structures.

   The Evidence Record also contains information about the selection
   method that was used for the generation of the data objects to be
   timestamped.  In the case of CMS, two selection methods can be
   distinguished:

   1.  The CMS Object as a whole including contentInfo is selected as
       data object and archive timestamped.  This means that a hash
       value of the CMS object MUST be located in the first list of hash
       values of Archive Timestamps.

   2.  The CMS Object and the signed or encrypted content are included
       in the Archive Timestamp as separated objects.  In this case, the
       hash value of the CMS Object as well as the hash value of the
       content have to be stored in the first list of hash values as a
       group of data objects.

   However, other selection methods could also be applied, for instance,
   as in [<a href="./rfc3126" title="&quot;Electronic Signature Formats for long term electronic signatures&quot;">RFC3126</a>].

   In the case of the two selection methods defined above, the Evidence
   Record has to be added to the first signature of the CMS Object of
   signed data.  Depending on the selection method, the following Object
   Identifiers are defined for the Evidence Record:

   ASN.1 Internal EvidenceRecord Attribute

   id-aa-er-internal  OBJECT IDENTIFIER ::= { iso(1) member-body(2)
      us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) id-aa(2) 49 }

   ASN.1 External EvidenceRecord Attribute

   id-aa-er-external  OBJECT IDENTIFIER ::= { iso(1) member-body(2)
      us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) id-aa(2) 50 }




<span class="grey">Gondrom, 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="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


   The attributes SHOULD only occur once.  If they appear several times,
   they have to be stored within the first signature in chronological
   order.

   If the CMS object doesn't have the EvidenceRecord Attributes -- which
   indicates that the EvidenceRecord has been provided externally -- the
   archive timestamped data object has to be generated over the complete
   CMS object within the existing coding.

   In case of verification, if only one EvidenceRecord is contained in
   the CMS object, the hash value must be generated over the CMS object
   without the one EvidenceRecord.  This means that the attribute has to
   be removed before verification.  The length of fields containing tags
   has to be adapted.  Apart from that, the existing coding must not be
   modified.

   If several Archive Timestamps occur, the data object has to be
   generated as follows:

   o  During verification of the first (in chronological order)
      EvidenceRecord, all EvidenceRecord have to be removed in order to
      generate the data object.

   o  During verification of the nth one EvidenceRecord, the first n-1
      attributes should remain within the CMS object.

   o  The verification of the nth one EvidenceRecord must result in a
      point of time when the document must have existed with the first n
      attributes.  The verification of the n+1th attribute must prove
      that this requirement has been met.

<span class="h2"><a class="selflink" id="appendix-B" href="#appendix-B">Appendix B</a>.  ASN.1-Module with 1988 Syntax</span>

   ASN.1-Module

   ERS {iso(1) identified-organization(3) dod(6)
         internet(1) security(5) mechanisms(5)
         ltans(11) id-mod(0) id-mod-ers88(2) id-mod-ers88-v1(1) }
   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN

   -- EXPORTS ALL --

   IMPORTS

    -- Imports from <a href="./rfc3852">RFC 3852</a> Cryptographic Message Syntax
   ContentInfo, Attribute




<span class="grey">Gondrom, et al.             Standards Track                    [Page 27]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-28" ></span>
<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


       FROM CryptographicMessageSyntax2004 -- FROM [<a href="./rfc3852" title="&quot;Cryptographic Message Syntax (CMS)&quot;">RFC3852</a>]
        { iso(1) member-body(2) us(840) rsadsi(113549)
          pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2004(24) }

     -- Imports from <a href="./rfc3280">RFC 3280</a> <a href="./rfc3280#appendix-A.1">[RFC3280], Appendix&nbsp;A.1</a>
   AlgorithmIdentifier
       FROM PKIX1Explicit88
           { iso(1) identified-organization(3) dod(6)
           internet(1) security(5) mechanisms(5) pkix(7)
           mod(0) pkix1-explicit(18) }
   ;

   ltans OBJECT IDENTIFIER ::=
            { iso(1) identified-organization(3) dod(6) internet(1)
              security(5) mechanisms(5) ltans(11) }

   EvidenceRecord ::= SEQUENCE {
      version                   INTEGER { v1(1) } ,
      digestAlgorithms          SEQUENCE OF AlgorithmIdentifier,
      cryptoInfos               [0] CryptoInfos OPTIONAL,
      encryptionInfo            [1] EncryptionInfo OPTIONAL,
      archiveTimeStampSequence  ArchiveTimeStampSequence
      }

   CryptoInfos ::= SEQUENCE SIZE (1..MAX) OF Attribute

   ArchiveTimeStamp ::= SEQUENCE {
     digestAlgorithm [0] AlgorithmIdentifier OPTIONAL,
     attributes      [1] Attributes OPTIONAL,
     reducedHashtree [2] SEQUENCE OF PartialHashtree OPTIONAL,
     timeStamp       ContentInfo}

   PartialHashtree ::= SEQUENCE OF OCTET STRING

   Attributes ::= SET SIZE (1..MAX) OF Attribute

   ArchiveTimeStampChain    ::= SEQUENCE OF ArchiveTimeStamp

   ArchiveTimeStampSequence ::= SEQUENCE OF
                                ArchiveTimeStampChain

   EncryptionInfo       ::=     SEQUENCE {









<span class="grey">Gondrom, et al.             Standards Track                    [Page 28]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-29" ></span>
<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


       encryptionInfoType     OBJECT IDENTIFIER,
       encryptionInfoValue    ANY DEFINED BY encryptionInfoType}

   id-aa-er-internal  OBJECT IDENTIFIER ::= { iso(1) member-body(2)
      us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) id-aa(2) 49 }

   id-aa-er-external  OBJECT IDENTIFIER ::= { iso(1) member-body(2)
      us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) id-aa(2) 50 }

   END

<span class="h2"><a class="selflink" id="appendix-C" href="#appendix-C">Appendix C</a>.  ASN.1-Module with 1997 Syntax</span>

   ASN.1-Module

   ERS {iso(1) identified-organization(3) dod(6)
         internet(1) security(5) mechanisms(5)
         ltans(11) id-mod(0) id-mod-ers(1) id-mod-ers-v1(1) }
   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN

   -- EXPORTS ALL --

   IMPORTS

    -- Imports from PKCS-7
   ContentInfo
       FROM PKCS7
           {iso(1) member-body(2) us(840) rsadsi(113549)
           pkcs(1) pkcs-7(7) modules(0)}

     -- Imports from AuthenticationFramework
   AlgorithmIdentifier
       FROM AuthenticationFramework
           {joint-iso-itu-t ds(5) module(1)
           authenticationFramework(7) 4}


    -- Imports from InformationFramework
   Attribute
       FROM InformationFramework
           {joint-iso-itu-t ds(5) module(1)
           informationFramework(1) 4}
   ;

   ltans OBJECT IDENTIFIER ::=
            { iso(1) identified-organization(3) dod(6) internet(1)
              security(5) mechanisms(5) ltans(11) }



<span class="grey">Gondrom, et al.             Standards Track                    [Page 29]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-30" ></span>
<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


   EvidenceRecord ::= SEQUENCE {
      version                   INTEGER { v1(1) } ,
      digestAlgorithms          SEQUENCE OF AlgorithmIdentifier,
      cryptoInfos               [0] CryptoInfos OPTIONAL,
      encryptionInfo            [1] EncryptionInfo OPTIONAL,
      archiveTimeStampSequence  ArchiveTimeStampSequence
      }

   CryptoInfos ::= SEQUENCE SIZE (1..MAX) OF Attribute
           (WITH COMPONENTS {
              ...,
              valuesWithContext   ABSENT
            })

   ArchiveTimeStamp ::= SEQUENCE {
     digestAlgorithm [0] AlgorithmIdentifier OPTIONAL,
     attributes      [1] Attributes OPTIONAL,
     reducedHashtree [2] SEQUENCE OF PartialHashtree OPTIONAL,
     timeStamp       ContentInfo}

   PartialHashtree ::= SEQUENCE OF OCTET STRING

   Attributes ::= SET SIZE (1..MAX) OF Attribute
           (WITH COMPONENTS {
              ...,
              valuesWithContext   ABSENT
            })

   ArchiveTimeStampChain    ::= SEQUENCE OF ArchiveTimeStamp

   ArchiveTimeStampSequence ::= SEQUENCE OF
                                ArchiveTimeStampChain

   EncryptionInfo       ::=     SEQUENCE {
       encryptionInfoType   ENCINFO-TYPE.&amp;id
                                      ({SupportedEncryptionAlgorithms}),
       encryptionInfoValue  ENCINFO-TYPE.&amp;Type
                  ({SupportedEncryptionAlgorithms}{@encryptionInfoType})
   }

   ENCINFO-TYPE ::= TYPE-IDENTIFIER

   SupportedEncryptionAlgorithms ENCINFO-TYPE ::= {...}

   id-aa-er-internal  OBJECT IDENTIFIER ::= { iso(1) member-body(2)
      us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) id-aa(2) 49 }

   id-aa-er-external  OBJECT IDENTIFIER ::= { iso(1) member-body(2)



<span class="grey">Gondrom, et al.             Standards Track                    [Page 30]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-31" ></span>
<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


      us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) id-aa(2) 50 }

   END

Authors' Addresses

   Tobias Gondrom
   Open Text Corporation
   Werner-von-Siemens-Ring 20
   Grasbrunn, Munich  D-85630
   Germany

   Phone: +49 (0) 89 4629-1816
   Fax:   +49 (0) 89 4629-33-1816
   EMail: tobias.gondrom@opentext.com


   Ralf Brandner
   InterComponentWare AG
   Industriestra?e 41
   Walldorf  D-69119
   Germany

   EMail: ralf.brandner@intercomponentware.com


   Ulrich Pordesch
   Fraunhofer Gesellschaft
   Rheinstra?e 75
   Darmstadt  D-64295
   Germany

   EMail: ulrich.pordesch@zv.fraunhofer.de


















<span class="grey">Gondrom, et al.             Standards Track                    [Page 31]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-32" ></span>
<span class="grey"><a href="./rfc4998">RFC 4998</a>                          ERS                        August 2007</span>


Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a>, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
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   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
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   on the procedures with respect to rights in RFC documents can be
   found in <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and <a href="https://www.rfc-editor.org/bcp/bcp79">BCP 79</a>.

   Copies of IPR disclosures made to the IETF Secretariat and any
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   specification can be obtained from the IETF on-line IPR repository at
   <a href="http://www.ietf.org/ipr">http://www.ietf.org/ipr</a>.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







Gondrom, et al.             Standards Track                    [Page 32]
</pre>