<pre>Internet Engineering Task Force (IETF)                       G. Richards
Request for Comments: 6560             RSA, The Security Division of EMC
Category: Standards Track                                     April 2012
ISSN: 2070-1721


               <span class="h1">One-Time Password (OTP) Pre-Authentication</span>

Abstract

   The Kerberos protocol provides a framework authenticating a client
   using the exchange of pre-authentication data.  This document
   describes the use of this framework to carry out One-Time Password
   (OTP) authentication.

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/rfc6560">http://www.rfc-editor.org/info/rfc6560</a>.

Copyright Notice

   Copyright (c) 2012 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
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   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.

   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



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   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-1.1">1.1</a>. Scope ......................................................<a href="#page-3">3</a>
      <a href="#section-1.2">1.2</a>. Overall Design .............................................<a href="#page-3">3</a>
      <a href="#section-1.3">1.3</a>. Conventions Used in This Document ..........................<a href="#page-4">4</a>
   <a href="#section-2">2</a>. Usage Overview ..................................................<a href="#page-4">4</a>
      <a href="#section-2.1">2.1</a>. OTP Mechanism Support ......................................<a href="#page-4">4</a>
      <a href="#section-2.2">2.2</a>. Pre-Authentication .........................................<a href="#page-4">4</a>
      <a href="#section-2.3">2.3</a>. PIN Change .................................................<a href="#page-5">5</a>
      <a href="#section-2.4">2.4</a>. Resynchronization ..........................................<a href="#page-6">6</a>
   <a href="#section-3">3</a>. Pre-Authentication Protocol Details .............................<a href="#page-6">6</a>
      <a href="#section-3.1">3.1</a>. Initial Client Request .....................................<a href="#page-6">6</a>
      <a href="#section-3.2">3.2</a>. KDC Challenge ..............................................<a href="#page-7">7</a>
      <a href="#section-3.3">3.3</a>. Client Response ............................................<a href="#page-9">9</a>
      <a href="#section-3.4">3.4</a>. Verifying the Pre-Authentication Data .....................<a href="#page-13">13</a>
      <a href="#section-3.5">3.5</a>. Confirming the Reply Key Change ...........................<a href="#page-15">15</a>
      <a href="#section-3.6">3.6</a>. Reply Key Generation ......................................<a href="#page-15">15</a>
   <a href="#section-4">4</a>. OTP Kerberos Message Types .....................................<a href="#page-17">17</a>
      <a href="#section-4.1">4.1</a>. PA-OTP-CHALLENGE ..........................................<a href="#page-17">17</a>
      <a href="#section-4.2">4.2</a>. PA-OTP-REQUEST ............................................<a href="#page-21">21</a>
      <a href="#section-4.3">4.3</a>. PA-OTP-PIN-CHANGE .........................................<a href="#page-25">25</a>
   <a href="#section-5">5</a>. IANA Considerations ............................................<a href="#page-26">26</a>
   <a href="#section-6">6</a>. Security Considerations ........................................<a href="#page-27">27</a>
      <a href="#section-6.1">6.1</a>. Man-in-the-Middle Attacks .................................<a href="#page-27">27</a>
      <a href="#section-6.2">6.2</a>. Reflection ................................................<a href="#page-28">28</a>
      <a href="#section-6.3">6.3</a>. Denial-of-Service Attacks .................................<a href="#page-28">28</a>
      <a href="#section-6.4">6.4</a>. Replay ....................................................<a href="#page-29">29</a>
      <a href="#section-6.5">6.5</a>. Brute-Force Attack ........................................<a href="#page-29">29</a>
      <a href="#section-6.6">6.6</a>. FAST Facilities ...........................................<a href="#page-30">30</a>
   <a href="#section-8">8</a>. Acknowledgments ................................................<a href="#page-30">30</a>
   <a href="#section-8">8</a>. References .....................................................<a href="#page-31">31</a>
      <a href="#section-8.1">8.1</a>. Normative References ......................................<a href="#page-31">31</a>
      <a href="#section-8.2">8.2</a>. Informative References ....................................<a href="#page-32">32</a>
   <a href="#appendix-A">Appendix A</a>.  ASN.1 Module  ....................................... <a href="#page-33">33</a>
   <a href="#appendix-B">Appendix B</a>.  Examples of OTP Pre-Authentication Exchanges ........ <a href="#page-36">36</a>
     <a href="#appendix-B.1">B.1</a>.  Four-Pass Authentication ................................. <a href="#page-36">36</a>
     <a href="#appendix-B.2">B.2</a>.  Two-Pass Authentication  ................................. <a href="#page-38">38</a>
     <a href="#appendix-B.3">B.3</a>.  PIN Change ............................................... <a href="#page-40">40</a>
     <a href="#appendix-B.4">B.4</a>.  Resynchronization  ....................................... <a href="#page-41">41</a>



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<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>.  Scope</span>

   This document describes a Flexible Authentication Secure Tunneling
   (FAST) [<a href="./rfc6113" title="&quot;A Generalized Framework for Kerberos Pre-Authentication&quot;">RFC6113</a>] factor that allows One-Time Password (OTP) values to
   be used in the Kerberos V5 [<a href="./rfc4120" title="&quot;The Kerberos Network Authentication Service (V5)&quot;">RFC4120</a>] pre-authentication in a manner
   that does not require use of the user's Kerberos password.  The
   system is designed to work with different types of OTP algorithms
   such as time-based OTPs [<a href="./rfc2808" title="&quot;The SecurID(r) SASL Mechanism&quot;">RFC2808</a>], counter-based tokens [<a href="./rfc4226" title="&quot;HOTP: An HMAC-Based One-Time Password Algorithm&quot;">RFC4226</a>] and
   challenge-response systems such as [<a href="./rfc2289" title="&quot;A One-Time Password System&quot;">RFC2289</a>].  It is also designed to
   work with tokens that are electronically connected to the user's
   computer via means such as a USB interface.

   This FAST factor provides the following facilities (as defined in
   [<a href="./rfc6113" title="&quot;A Generalized Framework for Kerberos Pre-Authentication&quot;">RFC6113</a>]): client-authentication, replacing-reply-key, and KDC-
   authentication.  It does not provide the strengthening-reply-key
   facility.

   This proposal is partially based upon previous work on integrating
   single-use authentication mechanisms into Kerberos [<a href="#ref-HORENEZ004" title="&quot;Integrating Single-use Authentication Mechanisms with Kerberos&quot;">HORENEZ004</a>].

<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>.  Overall Design</span>

   This proposal supports four- and two-pass variants.  In the four-pass
   system, the client sends the Key Distribution Center (KDC) an initial
   AS-REQ, and the KDC responds with a KRB-ERROR containing pre-
   authentication data that includes a random nonce.  The client then
   encrypts the nonce and returns it to the KDC in a second AS-REQ.
   Finally, the KDC returns the AS-REP.  In the two-pass variant, the
   client encrypts a timestamp rather than a nonce from the KDC, and the
   encrypted data is sent to the KDC in the initial AS-REQ.  The two-
   pass system can be used in cases where the client can determine in
   advance that OTP pre-authentication is supported by the KDC, which
   OTP key should be used and the encryption parameters required by the
   KDC.

   In both systems, in order to create the message sent to the KDC, the
   client must generate the OTP value and two keys: the classic Reply
   Key used to decrypt the KDC's reply and a key to encrypt the data
   sent to the KDC.  In most cases, the OTP value will be used in the
   key generation, but in order to support algorithms where the KDC
   cannot obtain the value (e.g., [<a href="./rfc2289" title="&quot;A One-Time Password System&quot;">RFC2289</a>]), the system supports the
   option of including the OTP value in the request along with the
   encrypted nonce.  In addition, in order to support situations where
   the KDC is unable to obtain the plaintext OTP value, the system also
   supports the use of hashed OTP values in the key derivation.




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   The pre-authentication data sent from the client to the KDC is sent
   within the encrypted data provided by the FAST pre-authentication
   data type of the AS-REQ.  The KDC then obtains the OTP value,
   generates the same keys, and verifies the pre-authentication data by
   decrypting the nonce.  If the verification succeeds, then it confirms
   knowledge of the Reply Key by using it to encrypt data in the AS-REP.

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

   This document assumes familiarity with the Kerberos
   pre-authentication framework [<a href="./rfc6113" title="&quot;A Generalized Framework for Kerberos Pre-Authentication&quot;">RFC6113</a>] and so freely uses terminology
   and notation from that document.

   The word padata is used as shorthand for pre-authentication data.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Usage Overview</span>

<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>.  OTP Mechanism Support</span>

   As described above, this document describes a generic system for
   supporting different OTP mechanisms in Kerberos pre-authentication.
   To ensure interoperability, all implementations of this specification
   SHOULD provide a mechanism (e.g., a provider interface) to add or
   remove support for a particular OTP mechanism.

<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a>.  Pre-Authentication</span>

   The approach uses pre-authentication data in AS-REQ, AS-REP, and
   KRB-ERROR messages.

   In the four-pass system, the client begins by sending an initial
   AS-REQ to the KDC that may contain pre-authentication data such as
   the standard Kerberos password data.  The KDC will then determine, in
   an implementation dependent fashion, whether OTP authentication is
   required and if it is, it will respond with a KRB-ERROR message
   containing a PA-OTP-CHALLENGE (see <a href="#section-4.1">Section 4.1</a>) in the PA-DATA.

   The PA-OTP-CHALLENGE will contain a KDC-generated nonce, a list of
   hash algorithm identifiers, and an iteration count if hashed OTP
   values are used (see <a href="#section-3.6">Section 3.6</a>) and OPTIONAL information on how the
   OTP should be generated by the client.  The client will then generate
   the OTP value and two keys: a Client Key to encrypt the KDC's nonce
   and a Reply Key used to decrypt the KDC's reply.




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   As described in <a href="./rfc6113#section-5.4.1">Section&nbsp;5.4.1 of [RFC6113]</a>, the FAST system uses an
   Armor Key to set up an encrypted tunnel for use by FAST factors.  As
   described in <a href="#section-3.6">Section 3.6</a> of this document, the Client Key and Reply
   Key will be generated from the Armor Key and the OTP value, unless
   the OTP algorithm does not allow the KDC to obtain the OTP value.  If
   hash algorithm identifiers were included in the PA-OTP-CHALLENGE,
   then the client will use the hash of the OTP value rather than the
   plaintext value in the key generation.  Both keys will have the same
   encryption type as the Armor Key.

   The generated Client Key will be used to encrypt the nonce received
   from the KDC.  The encrypted value along with optional information on
   how the OTP was generated are then sent to the KDC in a
   PA-OTP-REQUEST (see <a href="#section-4.2">Section 4.2</a>) encrypted within the armored-data of
   a PA-FX-FAST-REQUEST PA-DATA element of a second AS-REQ.

   In the two-pass system, the client sends the PA-OTP-REQUEST in the
   initial AS-REQ instead of sending it in response to a
   PA-OTP-CHALLENGE returned by the KDC.  Since no challenge is received
   from the KDC, the client includes an encrypted timestamp in the
   request rather than the encrypted KDC nonce.

   In both cases, on receipt of a PA-OTP-REQUEST, the KDC generates the
   keys in the same way as the client, and uses the generated Client Key
   to verify the pre-authentication by decrypting the encrypted data
   sent by the client (either nonce or timestamp).  If the validation
   succeeds, then the KDC will authenticate itself to the client and
   confirm that the Reply Key has been updated by using the generated
   Reply Key in the AS-REP response.

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

   Most OTP tokens involve the use of a Personal Identification Number
   (PIN) in the generation of the OTP value.  This PIN value will be
   combined with the value generated by the token to produce the final
   OTP value that will be used in this protocol.

   If, following successful validation of a PA-OTP-REQUEST in an AS-REQ,
   the KDC determines that the user's PIN has expired and needs to
   change, then it SHOULD respond with a KRB-ERROR of type
   KDC_ERR_PIN_EXPIRED.  It MAY include formatting information on the
   PIN in a PA-OTP-PIN-CHANGE (see <a href="#section-4.3">Section 4.3</a>) encrypted within the
   armored-data of the PA-FX-FAST-REPLY PA-DATA element.

              KDC_ERR_PIN_EXPIRED           96






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   If the PIN change is to be handled by a PIN-change service, then it
   is assumed that authentication to that service will succeed if the
   PIN has expired.

   If the user's PIN has not expired but has been changed, then the KDC
   MAY return the new value to the client in a PA-OTP-PIN-CHANGE
   encrypted within the armored-data of the PA-FX-FAST-REPLY PA-DATA
   element of the AS-REP.  Similarly, if a PIN change is not required,
   then the KDC MAY return a PA-OTP-PIN-CHANGE to inform the client of
   the current PIN's expiration time.

<span class="h3"><a class="selflink" id="section-2.4" href="#section-2.4">2.4</a>.  Resynchronization</span>

   It is possible with time- and event-based tokens that the OTP server
   will lose synchronization with the current token state.  For example,
   event-based tokens may drift since the counter on the token is
   incremented every time the token is used, but the counter on the
   server is only incremented on an authentication.  Similarly, the
   clocks on time-based tokens may drift.

   Methods to recover from this type of situation are OTP
   algorithm-specific but may involve the client sending a sequence of
   OTP values to allow the server to further validate the correct
   position in its search window (see <a href="./rfc4226#section-7.4">Section&nbsp;7.4 of [RFC4226]</a> for an
   example).

   If, when processing a PA-OTP-REQUEST, the pre-authentication
   validation fails for this reason, then the KDC MAY return a KRB-ERROR
   message.  The KRB-ERROR message MAY contain a PA-OTP-CHALLENGE in the
   PA-DATA with a single otp-tokenInfo representing the token used in
   the initial authentication attempt but with the "nextOTP" flag set.
   If this flag is set, then the client SHOULD re-try the authentication
   using an OTP value generated using the token in the "state" after
   that used in the failed authentication attempt, for example, using
   the next time interval or counter value.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Pre-Authentication Protocol Details</span>

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

   In the four-pass mode, the client begins by sending an initial
   AS-REQ, possibly containing other pre-authentication data.  If the
   KDC determines that OTP-based pre-authentication is required and the
   request does not contain a PA-OTP-REQUEST, then it will respond as
   described in <a href="#section-3.2">Section 3.2</a>.






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   If the client has all the necessary information, it MAY use the
   two-pass system by constructing a PA-OTP-REQUEST as described in
   <a href="#section-3.3">Section 3.3</a> and including it in the initial request.

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

   If the user is required to authenticate using an OTP, then the KDC
   SHALL respond to the initial AS-REQ with a KRB-ERROR (as described in
   <a href="./rfc6113#section-2.2">Section&nbsp;2.2 of [RFC6113]</a>), with a PA-OTP-CHALLENGE contained within
   the enc-fast-rep of the armored-data of a PA-FX-FAST-REPLY encrypted
   under the current Armor Key as described in [<a href="./rfc6113" title="&quot;A Generalized Framework for Kerberos Pre-Authentication&quot;">RFC6113</a>].

   If the OTP mechanism is to be carried out as an individual mechanism,
   then the PA-OTP-CHALLENGE SHALL be carried within the padata of the
   KrbFastResponse.  Alternatively, if the OTP mechanism is required as
   part of an authentication set, then the PA-OTP-CHALLENGE SHALL be
   carried within a PA-AUTHENTICATION-SET-ELEM as described in <a href="./rfc6113#section-5.3">Section</a>
   <a href="./rfc6113#section-5.3">5.3 of [RFC6113]</a>.

   The PA-OTP-CHALLENGE SHALL contain a nonce value to be returned
   encrypted in the client's PA-OTP-REQUEST.  This nonce string MUST
   contain a randomly chosen component at least as long as the Armor Key
   length (see [<a href="./rfc4086" title="&quot;Randomness Requirements for Security&quot;">RFC4086</a>] for an in-depth discussion of randomness).  In
   order to allow it to maintain any state necessary to verify the
   returned nonce, the KDC SHOULD use the mechanism described in <a href="./rfc6113#section-5.2">Section</a>
   <a href="./rfc6113#section-5.2">5.2 of [RFC6113]</a>.

   The KDC MAY use the otp-service field to assist the client in
   locating the OTP token to be used by identifying the purpose of the
   authentication.  For example, the otp-service field could assist a
   user in identifying the token to be used when a user has multiple OTP
   tokens that are used for different purposes.  If the token is a
   connected device, then these values SHOULD be an exact octet-level
   match for the values present on the target token.

   The KDC SHALL include a sequence of one or more otp-tokenInfo
   elements containing information on the token or tokens that the user
   can use for the authentication and how the OTP value is to be
   generated using those tokens.  If a single otp-tokenInfo element is
   included, then only a single token is acceptable by the KDC, and any
   OTP value generated by the client MUST be generated according to the
   information contained within that element.  If more than one
   otp-tokenInfo element is included, then the OTP value MUST be
   generated according to the information contained within one of those
   elements.






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   The KDC MAY include the otp-vendor field in an otp-tokenInfo to
   identify the vendor of the token that can be used in the
   authentication request in order to assist the client in locating that
   token.

   If the KDC is able to obtain the OTP values for the token, then the
   OTP value SHOULD be used in the key generation as described in
   <a href="#section-3.6">Section 3.6</a>; therefore, the KDC SHOULD set the "must-encrypt-nonce"
   flag in the otp-tokenInfo.  If the KDC is unable to obtain the OTP
   values for the token, then the "must-encrypt-nonce" flag MUST NOT be
   set.  If the flag is not set, then the OTP value will be returned by
   the client in the otp-value field of the PA-OTP-REQUEST and so, if
   returning of OTP values in this way does not conform to KDC policy,
   then the KDC SHOULD NOT include the otp-tokenInfo for that token in
   the PA-OTP-CHALLENGE.

   If the KDC requires that hashed OTPs be used in the key generation as
   described in <a href="#section-3.6">Section 3.6</a> (for example, it is only able to obtain
   hashed OTP values for the token), then it MUST include the supported
   hash algorithms in order of preference in the supportedHashAlg of the
   otp-KeyInfo and the minimum value of the iteration count in the
   iterationCount element.

   Since the OTP mechanism described in this document is replacing the
   Reply Key, the classic shared-key system cannot be relied upon to
   allow the client to verify the KDC.  Therefore, as described in
   <a href="./rfc6113#section-3.4">Section&nbsp;3.4 of [RFC6113]</a>, some other mechanism must be provided to
   support this.  If the OTP value is used in the Reply Key generation,
   then the client and KDC have a shared key and KDC-authentication is
   provided by the KDC using the Reply Key generated from the OTP value.
   However, if the OTP value is sent in the otp-value element of the
   PA-OTP-REQUEST, then there is no such shared key and the OTP
   mechanism does not provide KDC-authentication.  Therefore, if the OTP
   mechanism is not being used in an environment where
   KDC-authentication is being provided by other means (e.g., by the use
   of a host-key-based Armor Key), then the KDC MUST NOT include any
   otp-tokenInfo elements in the PA-OTP-CHALLENGE that do not have the
   "must-encrypt-nonce" flag set.

   If the OTP for a token is to be generated using a server-generated
   challenge, then the value of the challenge SHALL be included in the
   otp-challenge field of the otp-tokenInfo for that token.  If the
   token is a connected device and the OTP is to be generated by
   combining the challenge with the token's current state (e.g., time),
   then the "combine" flag SHALL be set within the otp-tokenInfo
   containing the challenge.





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   If the KDC can determine which OTP token key (the seed value on the
   token used to generate the OTP) is to be used, then the otp-tokenID
   field MAY be included in the otp-tokenInfo to pass that value to the
   client.

   The otp-algID field MAY be included in an otp-tokenInfo to identify
   the algorithm that should be used in the OTP calculation for that
   token.  For example, it could be used when a user has been issued
   with multiple tokens that support different algorithms.

   If the KDC can determine that an OTP token that can be used by the
   user does not require the client to collect a PIN, then it SHOULD set
   the "do-not-collect-pin" flag in the otp-tokenInfo representing that
   token.  If the KDC can determine that the token requires the client
   to collect a PIN, then it SHOULD set the "collect-pin" flag.  If the
   KDC is unable to determine whether or not the client should collect a
   PIN, then the "collect-pin" and "do-not-collect-pin" flags MUST NOT
   be set.

   If the KDC requires the PIN of an OTP token to be returned to it
   separately, then it SHOULD set the "separate-pin-required" flag in
   the otp-KeyInfo representing that token.

   If the KDC requires that the OTPs generated by the token have a Luhn
   check digit appended, as defined in [<a href="#ref-ISOIEC7812" title="&quot;ISO/IEC 7812-1:2006 Identification cards -- Identification of issuers -- Part 1: Numbering system&quot;">ISOIEC7812</a>], then it MUST set
   the "check-digit" flag.  This flag only applies if the format of the
   OTP is decimal; therefore, the otp-format field, if present, MUST
   have the value of "decimal".

   Finally, in order to support connected tokens that can generate OTP
   values of varying lengths or formats, the KDC MAY include the desired
   otp-length and format of the OTP in the otp-length and otp-format
   fields of an otp-tokenInfo.

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

   The client response SHALL be sent to the KDC as a PA-OTP-REQUEST
   included within the enc-fast-req of the armored-data within a
   PA-FX-FAST-REQUEST encrypted under the current Armor Key as described
   in [<a href="./rfc6113" title="&quot;A Generalized Framework for Kerberos Pre-Authentication&quot;">RFC6113</a>].

   In order to generate its response, the client MUST generate an OTP
   value.  If the PA-OTP-CHALLENGE contained one or more otp-tokenInfo
   elements, then the OTP value MUST be based on the information
   contained within one of those elements.






<span class="grey">Richards                     Standards Track                    [Page 9]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-10" ></span>
<span class="grey"><a href="./rfc6560">RFC 6560</a>                 OTP Pre-Authentication               April 2012</span>


   The otp-service, otp-vendor, otp-tokenID, otp-length, otp-format, and
   otp-algID elements of the PA-OTP-CHALLENGE are provided by the KDC to
   assist the client in locating the correct token to use, but the use
   of the above fields will depend on the type of token.

   If the token is a disconnected device, then the values of otp-service
   and otp-vendor MAY be displayed to the user in order to help the user
   select the correct token, and the values of otp-algID, otp-tokenID,
   otp-length, and otp-format MAY be ignored.

   If the token is a connected device, then these values, if present,
   SHOULD be used by the client to locate the correct token.  When the
   token is connected, clients MUST support matching based on a binary
   comparison of the otp-vendor and otp-service strings when comparing
   the values against those present on the token.  Clients MAY have
   other comparisons including normalization insensitive comparisons to
   try and find the right token.  The values of otp-vendor and
   otp-service MAY be displayed to prompt the user if the correct token
   is not found.

   If the "nextOTP" flag is set in the otp-tokenInfo from the
   PA-OTP-CHALLENGE, then the OTP value MUST be generated from the next
   token state rather than that used in the previous PA-OTP-REQUEST for
   that token.  The "nextOTP" flag MUST also be set in the new
   PA-OTP-REQUEST.

   If the "collect-pin" flag is set, then the token requires a PIN to be
   collected by the client.  If the "do-not-collect-pin" flag is set in
   the otp-tokenInfo from the PA-OTP-CHALLENGE, then the token
   represented by the otp-tokenInfo does not require a PIN to be
   collected by the client as part of the OTP value.  If neither of the
   "collect-pin" nor "do-not-collect-pin" flags are set, then PIN
   requirements of the token are unspecified.  If both flags are set,
   then the client SHALL regard the request as invalid.

   If the "separate-pin-required" flag is set, then any PIN collected by
   the client MUST be included as a UTF-8 string in the otp-pin of the
   PA-OTP-REQUEST.

   If the token is a connected device, then how the PIN is used to
   generate the OTP value will depend on the type of device.  However,
   if the token is a disconnected device, then it will depend on the
   "separate-pin-required" flag.  If the flag is not set, then the OTP
   value MUST be generated by appending the PIN with the value from the
   token entered by the user and, if the flag is set, then the OTP value
   MUST be the value from the token.





<span class="grey">Richards                     Standards Track                   [Page 10]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-11" ></span>
<span class="grey"><a href="./rfc6560">RFC 6560</a>                 OTP Pre-Authentication               April 2012</span>


   The clients SHOULD NOT normalize the PIN value or any OTP value
   collected from the user or returned by a connected token in any way.

   If the "check-digit" flag is set, then any OTP values SHOULD be
   decimal and have a Luhn check digit appended [<a href="#ref-ISOIEC7812" title="&quot;ISO/IEC 7812-1:2006 Identification cards -- Identification of issuers -- Part 1: Numbering system&quot;">ISOIEC7812</a>].  If the
   token is disconnected, then the Client MAY ignore this flag; if the
   token is connected, then the Client MUST enforce it.  The Client MUST
   regard the request as invalid, if otp-format is present and set to
   any value other than "decimal".

   If an otp-challenge is present in the otp-tokenInfo selected by the
   client from the PA-OTP-CHALLENGE, then the OTP value for the token
   MUST be generated based on a challenge, if the token is capable of
   accepting a challenge.  The client MAY ignore the provided challenge
   if and only if the token is not capable of including a challenge in
   the OTP calculation.

   If the "combine" flag is not set in the otp-tokenInfo of the
   PA-OTP-CHALLENGE, then the OTP SHALL be calculated based only the
   challenge and not the internal state (e.g., time or counter) of the
   token.  If the "combine" flag is set, then the OTP SHALL be
   calculated using both the internal state and the provided challenge,
   if that value is obtainable by the client.  If the flag is set but
   otp-challenge is not present, then the client SHALL regard the
   request as invalid.

   If token is a connected device, then the use of the challenge will
   depend on the type of device but will involve passing the challenge
   and the value of the "combine" flag in a token-specific manner to the
   token, along with a PIN if collected and the values of otp-length and
   otp-format if specified, in order to obtain the OTP value.  If the
   token is disconnected, then the challenge MUST be displayed to the
   user and the value of the "combine" flag MAY be ignored by the
   client.

   If the OTP value was generated using a challenge that was not sent by
   the KDC, then the challenge SHALL be included in the otp-challenge of
   the PA-OTP-REQUEST.  If the OTP was generated by combining a
   challenge (either received from the KDC or generated by the client)
   with the token state, then the "combine" flag SHALL be set in the
   PA-OTP-REQUEST.










<span class="grey">Richards                     Standards Track                   [Page 11]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
<span class="grey"><a href="./rfc6560">RFC 6560</a>                 OTP Pre-Authentication               April 2012</span>


   If the "must-encrypt-nonce" flag is set in the otp-tokenInfo, then
   the OTP value MUST be used to generate the Client Key and Reply Key
   as described in <a href="#section-3.6">Section 3.6</a> and MUST NOT be included in the otp-value
   field of the PA-OTP-REQUEST.  If the flag is not set, then the OTP
   value MUST be included in the otp-value field of the PA-OTP-REQUEST
   and MUST NOT be used in the key derivation.  In this case, the Client
   Key and Reply Key SHALL be the same as the Armor Key as described in
   <a href="#section-3.6">Section 3.6</a>; so, if the returning of OTP values in this way does not
   conform to local policy on the client (for example, if
   KDC-Authentication is required and is not being provided by other
   means), then it SHOULD NOT use the token for authentication.

   If the supportedHashAlg and iterationCount elements are included in
   the otp-tokenInfo, then the client MUST use hashed OTP values in the
   generation of the Reply Key and Client Key as described in <a href="#section-3.6">Section</a>
   <a href="#section-3.6">3.6</a>.  The client MUST select the first algorithm from the list that
   it supports and the AlgorithmIdentifer [<a href="./rfc5280" title="&quot;Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile&quot;">RFC5280</a>] selected MUST be
   placed in the hashAlg element of the PA-OTP-REQUEST.  However, if
   none of the algorithm identifiers conform to local policy
   restrictions, then the authentication attempt MUST NOT proceed using
   that token.  If the value of iterationCount does not conform to local
   policy on the client, then the client MAY use a larger value, but
   MUST NOT use a lower value.  The value of the iteration count used by
   the client MUST be returned in the PA-OTP-REQUEST sent to the KDC.

   If hashed OTP values are used, then the nonce generated by the client
   MUST be as long as the longest key length of the symmetric key types
   that it supports and MUST be chosen randomly (see [<a href="./rfc4086" title="&quot;Randomness Requirements for Security&quot;">RFC4086</a>]).  The
   nonce MUST be included in the PA-OTP-REQUEST, along with the hash
   algorithm and iteration count used in the nonce, hashAlg, and
   iterationCount fields of the PA-OTP-REQUEST.  These fields MUST NOT
   be included if hashed OTP values were not used.  It is RECOMMENDED
   that the iteration count used by the client be chosen in such a way
   that it is computationally infeasible/unattractive for an attacker to
   brute-force search for the given OTP.

   The PA-OTP-REQUEST returned by the client SHOULD include information
   on the generated OTP value reported by the OTP token when available
   to the client.  The otp-time and otp-counter fields of the
   PA-OTP-REQUEST SHOULD be used to return the time and counter values
   used by the token if available to the client.  The otp-format field
   MAY be used to report the format of the generated OTP.  This field
   SHOULD be used if a token can generate OTP values in multiple
   formats.  The otp-algID field SHOULD be used by the client to report
   the algorithm used in the OTP calculation, and the otp-tokenID SHOULD
   be used to report the identifier of the OTP token key used if the
   information is known to the client.




<span class="grey">Richards                     Standards Track                   [Page 12]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-13" ></span>
<span class="grey"><a href="./rfc6560">RFC 6560</a>                 OTP Pre-Authentication               April 2012</span>


   If the PA-OTP-REQUEST is being sent in response to a PA-OTP-CHALLENGE
   that contained an otp-vendor field in the selected otp-tokenInfo,
   then the otp-vendor field of the PA-OTP-REQUEST MUST be set to the
   same value.  If no otp-vendor field was provided by the KDC, then the
   field SHOULD be set to the vendor identifier of the token if known to
   the client.

   The generated Client Key is used by the client to encrypt data to be
   included in the encData of the PA-OTP-REQUEST to allow the KDC to
   authenticate the user.  The key usage for this encryption is
   KEY_USAGE_OTP_REQUEST.

   o  If the PA-OTP-REQUEST is being generated in response to a
      PA-OTP-CHALLENGE returned by the KDC, then the client SHALL
      encrypt a PA-OTP-ENC-REQUEST containing the value of nonce from
      the PA-OTP-CHALLENGE using the same encryption type as the Armor
      Key.

   o  If the PA-OTP-REQUEST is not in response to a PA-OTP-CHALLENGE,
      then the client SHALL encrypt a PA-ENC-TS-ENC containing the
      current time as in the encrypted timestamp pre-authentication
      mechanism [<a href="./rfc4120" title="&quot;The Kerberos Network Authentication Service (V5)&quot;">RFC4120</a>].

   If the client is working in two-pass mode and so, is not responding
   to an initial KDC challenge, then the values of the iteration count
   and hash algorithms cannot be obtained from that challenge.  The
   client SHOULD use any values obtained from a previous
   PA-OTP-CHALLENGE or, if no values are available, it MAY use initial
   configured values.

<span class="h3"><a class="selflink" id="section-3.4" href="#section-3.4">3.4</a>.  Verifying the Pre-Authentication Data</span>

   The KDC validates the pre-authentication data by generating the
   Client Key and Reply Key in the same way as the client and using the
   generated Client Key to decrypt the value of encData from the
   PA-OTP-REQUEST.  The generated Reply Key is used to encrypt data in
   the AS-REP.

   If the otp-value field is included in the PA-OTP-REQUEST, then the
   KDC MUST use that value unless the OTP method is required to support
   KDC-authentication (see <a href="#section-3.2">Section 3.2</a>).  If the otp-value is not
   included in the PA-OTP-REQUEST, then the KDC will need to generate or
   obtain the OTP value.

   If the otp-pin field is present in the PA-OTP-REQUEST, then the PIN
   value has to be value provided by the client.  The KDC SHOULD
   SASLPrep (Stringprep Profile for User Names and Passwords) [<a href="./rfc4013" title="&quot;SASLprep: Stringprep Profile for User Names and Passwords&quot;">RFC4013</a>]
   the value in lookup mode before comparison.



<span class="grey">Richards                     Standards Track                   [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc6560">RFC 6560</a>                 OTP Pre-Authentication               April 2012</span>


   It should be noted that it is anticipated that, as improved string
   comparison technologies are standardized, the processing done by the
   KDC will change, but efforts will be made to maintain as much
   compatibility with SASLprep as possible.

   If the otp-challenge field is present, then the OTP was calculated
   using that challenge.  If the "combine" flag is also set, then the
   OTP was calculated using the challenge and the token's current state.

   It is RECOMMENDED that the KDC act upon the values of otp-time,
   otp-counter, otp-format, otp-algID, and otp-tokenID if they are
   present in the PA-OTP-REQUEST.  If the KDC receives a request
   containing these values, but cannot act upon them, then they MAY be
   ignored.

   The KDC generates the Client Key and Reply Key as described in
   <a href="#section-3.6">Section 3.6</a> from the OTP value using the nonce, hash algorithm, and
   iteration count if present in the PA-OTP-REQUEST.  The KDC MUST fail
   the request with KDC_ERR_INVALID_HASH_ALG, if the KDC requires hashed
   OTP values and the hashAlg field was not present in the
   PA-OTP-REQUEST or if the value of this field does not conform to
   local KDC policy.  Similarly, the KDC MUST fail the request with
   KDC_ERR_INVALID_ITERATION_COUNT, if the value of the iterationCount
   included in the PA-OTP-REQUEST does not conform to local KDC policy
   or is less than that specified in the PA-OTP-CHALLENGE.  In addition,
   the KDC MUST fail the authentication request with
   KDC_ERR_PIN_REQUIRED, if it requires a separate PIN to the OTP value
   and an otp-pin was not included in the PA-OTP-REQUEST.  The above
   error codes are defined as follows:

              KDC_ERR_INVALID_HASH_ALG           94
              KDC_ERR_INVALID_ITERATION_COUNT    95
              KDC_ERR_PIN_REQUIRED               97

   The generated Client Key is then used to decrypt the encData from the
   PA-OTP-REQUEST.  If the client response was sent as a result of a
   PA-OTP-CHALLENGE, then the decrypted data will be a
   PA-OTP-ENC-REQUEST and the client authentication MUST fail with
   KDC_ERR_PREAUTH_FAILED if the nonce value from the PA-OTP-ENC-REQUEST
   is not the same as the nonce value sent in the PA-OTP-CHALLENGE.  If
   the response was not sent as a result of a PA-OTP-CHALLENGE, then the
   decrypted value will be a PA-ENC-TS-ENC, and the authentication
   process will be the same as with classic encrypted timestamp
   pre-authentication [<a href="./rfc4120" title="&quot;The Kerberos Network Authentication Service (V5)&quot;">RFC4120</a>].

   The KDC MUST fail the request with KDC_ERR_ETYPE_NOSUPP, if the
   encryption type used by the client in the encData does not conform to
   KDC policy.



<span class="grey">Richards                     Standards Track                   [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc6560">RFC 6560</a>                 OTP Pre-Authentication               April 2012</span>


   If authentication fails due to the hash algorithm, iteration count,
   or encryption type used by the client, then the KDC SHOULD return a
   PA-OTP-CHALLENGE with the required values in the error response.  If
   the authentication fails due to the token state on the server is no
   longer being synchronized with the token used, then the KDC MAY
   return a PA-OTP-CHALLENGE with the "nextOTP" flag set as described in
   <a href="#section-2.4">Section 2.4</a>.

   If, during the authentication process, the KDC determines that the
   user's PIN has been changed, then it SHOULD include a
   PA-OTP-PIN-CHANGE in the response, as described in <a href="#section-2.3">Section 2.3</a>,
   containing the new PIN value.  The KDC MAY also include the new PIN's
   expiration time and the expiration time of the OTP account within the
   last-req field of the PA-OTP-PIN-CHANGE.  (These fields can be used
   by the KDC to handle cases where the account related to the user's
   OTP token has a different expiration time to the user's Kerberos
   account.)  If the KDC determines that the user's PIN or OTP account
   are about to expire, it MAY return a PA-OTP-PIN-CHANGE with that
   information.  Finally, if the KDC determines that the user's PIN has
   expired, then it SHOULD return a KRB-ERROR of type
   KDC_ERR_PIN_EXPIRED as described in <a href="#section-2.3">Section 2.3</a>

<span class="h3"><a class="selflink" id="section-3.5" href="#section-3.5">3.5</a>.  Confirming the Reply Key Change</span>

   If the pre-authentication data was successfully verified, then, in
   order to support mutual authentication, the KDC SHALL respond to the
   client's PA-OTP-REQUEST by using the generated Reply Key to encrypt
   the data in the AS-REP.  The client then uses its generated Reply Key
   to decrypt the encrypted data and MUST NOT continue with the
   authentication process, if decryption is not successful.

<span class="h3"><a class="selflink" id="section-3.6" href="#section-3.6">3.6</a>.  Reply Key Generation</span>

   In order to authenticate the user, the client and KDC need to
   generate two encryption keys:

   o  The Client Key to be used by the client to encrypt and by the KDC
      to decrypt the encData in the PA-OTP-REQUEST.

   o  The Reply Key to be used in the standard manner by the KDC to
      encrypt data in the AS-REP.

      The method used to generate the two keys will depend on the OTP
      algorithm.

   o  If the OTP value is included in the otp-value of the PA-OTP-
      REQUEST, then the two keys SHALL be the same as the Armor Key
      (defined in [<a href="./rfc6113" title="&quot;A Generalized Framework for Kerberos Pre-Authentication&quot;">RFC6113</a>]).



<span class="grey">Richards                     Standards Track                   [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc6560">RFC 6560</a>                 OTP Pre-Authentication               April 2012</span>


   o  If the OTP value is not included in the otp-value of the
      PA-OTP-REQUEST, then the two keys SHALL be derived from the Armor
      Key and the OTP value as described below.

   If the OTP value is not included in the PA-OTP-REQUEST, then the
   Reply Key and Client Key SHALL be generated using the KRB-FX-CF2
   algorithm from [<a href="./rfc6113" title="&quot;A Generalized Framework for Kerberos Pre-Authentication&quot;">RFC6113</a>] as follows:

              Client Key = KRB-FX-CF2(K1, K2, O1, O2)
              Reply Key = KRB-FX-CF2(K1, K2, O3, O4)

   The octet string parameters, O1, O2, O3, and O4 shall be the ASCII
   string "OTPComb1", "OTPComb2", "OTPComb3", and "OTPComb4" as shown
   below:

              {0x4f, 0x54, 0x50, 0x43, 0x6f, 0x6d, 0x62, 0x31}
              {0x4f, 0x54, 0x50, 0x43, 0x6f, 0x6d, 0x62, 0x32}
              {0x4f, 0x54, 0x50, 0x43, 0x6f, 0x6d, 0x62, 0x33}
              {0x4f, 0x54, 0x50, 0x43, 0x6f, 0x6d, 0x62, 0x34}

   The first input key, K1, SHALL be the Armor Key and so, as described
   in <a href="./rfc6113#section-5.1">Section&nbsp;5.1 of [RFC6113]</a>, the enctypes of the generated Client Key
   and Reply Key will be the same as the enctype of Armor Key.  The
   second input key, K2, shall be derived from the OTP value using
   string-to-key (defined in [<a href="./rfc3961" title="&quot;Encryption and Checksum Specifications for Kerberos 5&quot;">RFC3961</a>]) as described below.

   If the hash of the OTP value is to be used, then K2 SHALL be derived
   as follows:

   o  An initial hash value, H, is generated:

            H = hash(realm|nonce|OTP)

   Where:

      *  "|" denotes concatenation.
      *  hash is the hash algorithm selected by the client.
      *  realm is the name of the server's realm as carried in the realm
         field of the AS-REQ (not including the tag and length from the
         DER encoding).
      *  nonce is the value of the random nonce value generated by the
         client and carried in the nonce field of the PA-OTP-REQUEST
         (not including the tag and length from the DER encoding).
      *  If the OTP format is decimal, hexadecimal, or alphanumeric,
         then OTP is the value of the OTP generated as described in
         <a href="#section-3.3">Section 3.3</a> with SASLprep [<a href="./rfc4013" title="&quot;SASLprep: Stringprep Profile for User Names and Passwords&quot;">RFC4013</a>] applied in lookup mode;
         otherwise, it is the unnormalized OTP value.




<span class="grey">Richards                     Standards Track                   [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc6560">RFC 6560</a>                 OTP Pre-Authentication               April 2012</span>


   o  The initial hash value is then hashed iterationCount-1 times to
      produce a final hash value, H' (where iterationCount is the value
      from the PA-OTP-REQUEST).

            H' = hash(hash(...(iterationCount-1 times)...(H)))

   o  The value of K2 is then derived from the Base64 [<a href="./rfc2045" title="&quot;Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies&quot;">RFC2045</a>] encoding
      of this final hash value.

            K2 = string-to-key(Base64(H')|"Krb-preAuth")

   If the hash value is not used, then K2 SHALL be derived from the
   base64 encoding of the OTP value.

            K2 = string-to-key(Base64(OTP)|"Krb-preAuth")

   The enctype used for string-to-key SHALL be that of the Armor Key and
   the salt and any additional parameters for string-to-key MAY be
   provided by the KDC in the PA-OTP-CHALLENGE.  If the salt and
   string-to-key parameters are not provided, then the default values
   defined for the particular enctype SHALL be used.

   If the strengthen-key is present in KrbFastResponse, then it is
   combined with the Reply Key to generate the final AS-REQ as described
   in [<a href="./rfc6113" title="&quot;A Generalized Framework for Kerberos Pre-Authentication&quot;">RFC6113</a>].  The strengthen-key does not influence the Client Key.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  OTP Kerberos Message Types</span>

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

   The padata-type PA-OTP-CHALLENGE is returned by the KDC to the client
   in the enc-fast-rep of a PA-FX-FAST-REPLY in the PA-DATA of a
   KRB-ERROR when OTP pre-authentication is required.  The corresponding
   padata-value field contains the Distinguished Encoding Rules (DER)
   [<a href="#ref-X.680" title="&quot;Recommendation X.680 (2002) | ISO/IEC 8824-1:2002, Information technology - Abstract Syntax Notation One (ASN.1): Specification of basic notation.&quot;">X.680</a>] and [<a href="#ref-X.690" title="&quot;Recommendation X.690 (2008) | ISO/IEC 8825-1:2008, X.690 : Information technology - ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)&quot;">X.690</a>] encoding of a PA-OTP-CHALLENGE containing a
   server-generated nonce and information for the client on how to
   generate the OTP.

            PA-OTP-CHALLENGE     141

            PA-OTP-CHALLENGE ::= SEQUENCE {
              nonce            [0] OCTET STRING,
              otp-service      [1] UTF8String               OPTIONAL,
              otp-tokenInfo    [2] SEQUENCE (SIZE(1..MAX)) OF
                                                       OTP-TOKENINFO,
              salt             [3] KerberosString           OPTIONAL,
              s2kparams        [4] OCTET STRING             OPTIONAL,
              ...



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            }

            OTP-TOKENINFO ::= SEQUENCE {
              flags            [0] OTPFlags,
              otp-vendor       [1] UTF8String               OPTIONAL,
              otp-challenge    [2] OCTET STRING (SIZE(1..MAX))
                                                            OPTIONAL,
              otp-length       [3] Int32                    OPTIONAL,
              otp-format       [4] OTPFormat                OPTIONAL,
              otp-tokenID      [5] OCTET STRING             OPTIONAL,
              otp-algID        [6] AnyURI                   OPTIONAL,
              supportedHashAlg [7] SEQUENCE OF AlgorithmIdentifier
                                                            OPTIONAL,
              iterationCount   [8] Int32                    OPTIONAL,
              ...
            }

            OTPFormat ::= INTEGER {
              decimal(0),
              hexadecimal(1),
              alphanumeric(2),
              binary(3),
              base64(4)
            }

            OTPFlags ::= KerberosFlags
            -- reserved(0),
            -- nextOTP(1),
            -- combine(2),
            -- collect-pin(3),
            -- do-not-collect-pin(4),
            -- must-encrypt-nonce (5),
            -- separate-pin-required (6),
            -- check-digit (7)

   nonce
      A KDC-supplied nonce value to be encrypted by the client in the
      PA-OTP-REQUEST.  This nonce string MUST contain a randomly chosen
      component at least as long as the Armor Key length.

   otp-service
      Use of this field is OPTIONAL, but MAY be used by the KDC to
      assist the client to locate the appropriate OTP tokens to be used.
      For example, this field could be used when a user has multiple OTP
      tokens for different purposes.






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   otp-tokenInfo
      This element MUST be included, and it is a sequence of one or more
      OTP-TOKENINFO objects containing information on the token or
      tokens that the user can use for the authentication and how the
      OTP value is to be generated using those tokens.  If a single
      OTP-TOKENINFO object is included, then only a single token is
      acceptable by the KDC and any OTP value generated by the client
      MUST be generated according to the information contained within
      that element.  If more than one OTP-TOKENINFO object is included,
      then the OTP value MUST be generated according to the information
      contained within one of those objects.

      flags
         If the "nextOTP" flag is set, then the OTP SHALL be based on
         the next token "state" rather than the one used in the previous
         authentication.  As an example, for a time-based token, this
         means the next time slot and for an event-based token, this
         could mean the next counter value.  If the "nextOTP" flag is
         set, then there MUST only be a single otp-tokenInfo element in
         the PA-OTP-CHALLENGE.

         The "combine" flag controls how the challenge included in
         otp-challenge shall be used.  If the flag is set, then OTP
         SHALL be calculated using the challenge from otp-challenge and
         the internal token state (e.g., time or counter).  If the
         "combine" flag is not set, then the OTP SHALL be calculated
         based only on the challenge.  If the flag is set and otp-
         challenge is not present, then the request SHALL be regarded as
         invalid.

         If the "do-not-collect-pin" flag is set, then the token
         represented by the current otp-tokenInfo does not require a PIN
         to be collected as part of the OTP.  If the "collect-pin" flag
         is set, then the token requires a PIN.  If neither flag is set,
         then whether or not a PIN is required is unspecified.  The
         flags are mutually exclusive and so both flags MUST NOT be set,
         or the client MUST regard the request as invalid.

         If the "must-encrypt-nonce" flag is set, then the OTP value
         MUST NOT be included in the otp-value field of the
         PA-OTP-REQUEST, but instead the OTP value MUST be used in the
         generation of the Reply Key and Client Key as described in
         <a href="#section-3.6">Section 3.6</a>.

         If the "separate-pin-required" flag is set, then the PIN
         collected by the client SHOULD NOT be used in the generation of
         the OTP value and SHOULD be returned in the otp-pin field of
         the PA-OTP-REQUEST.



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         The "check-digit" flag controls whether or not the OTP values
         generated by the token need to include a Luhn check digit
         [<a href="#ref-ISOIEC7812" title="&quot;ISO/IEC 7812-1:2006 Identification cards -- Identification of issuers -- Part 1: Numbering system&quot;">ISOIEC7812</a>].  If the token is disconnected, then the Client
         MAY ignore this flag; if this flag is set and the token is
         connected, then the OTP MUST be a decimal with a check digit
         appended.

      otp-vendor
         Use of this field is OPTIONAL, but MAY be used by the KDC to
         identify the vendor of the OTP token to be used.

      otp-challenge
         The otp-challenge is used by the KDC to send a challenge value
         for use in the OTP calculation.  The challenge is an OPTIONAL
         octet string that SHOULD be uniquely generated for each request
         in which it is present.  When the challenge is not present, the
         OTP will be calculated on the current token state only.  The
         client MAY ignore a provided challenge if and only if the OTP
         token the client is interacting with is not capable of
         including a challenge in the OTP calculation.  In this case,
         KDC policies will determine whether or not to accept a provided
         OTP value.

      otp-length
         Use of this field is OPTIONAL, but MAY be used by the KDC to
         specify the desired length of the generated OTP.  For example,
         this field could be used when a token is capable of producing
         OTP values of different lengths.  If the format of the OTP is
         'decimal', 'hexidecimal', or 'alphanumeric', then this value
         indicates the desired length in digits/characters; if the OTP
         format is 'binary', then this value indicates the desired
         length in octets; and if the OTP format is 'base64', then this
         value indicates the desired length of the unencoded OTP value
         in octets.

      otp-format
         Use of this field is OPTIONAL, but MAY be used by the KDC to
         specify the desired format of the generated OTP value.  For
         example, this field could be used when a token is capable of
         producing OTP values of different formats.

      otp-tokenID
         Use of this field is OPTIONAL, but MAY be used by the KDC to
         identify which token key should be used for the authentication.
         For example, this field could be used when a user has been
         issued multiple token keys by the same server.





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      otp-algID
         Use of this field is OPTIONAL, but MAY be used by the KDC to
         identify the algorithm to use when generating the OTP.  The
         value of this field MUST be a URI [<a href="./rfc3986" title="&quot;Uniform Resource Identifier (URI): Generic Syntax&quot;">RFC3986</a>] and SHOULD be
         obtained from the Portable Symmetric Key Container (PSKC)
         algorithm registry [<a href="./rfc6030" title="&quot;Portable Symmetric Key Container (PSKC)&quot;">RFC6030</a>].

      supportedHashAlg
         If present, then a hash of the OTP value MUST be used in the
         key derivation rather than the plain text value.  Each
         AlgorithmIdentifier identifies a hash algorithm that is
         supported by the KDC in decreasing order of preference.  The
         client MUST select the first algorithm from the list that it
         supports.  Support for SHA-256 by both the client and KDC is
         REQUIRED.  The AlgorithmIdentifier selected by the client MUST
         be placed in the hashAlg element of the PA-OTP-REQUEST.

      iterationCount
         The minimum value of the iteration count to be used by the
         client when hashing the OTP value.  This value MUST be present
         if supportedHashAlg is present and otherwise MUST NOT be
         present.  If the value of this element does not conform to
         local policy on the client, then the client MAY use a larger
         value but MUST NOT use a lower value.  The value of the
         iteration count used by the client MUST be returned in the
         PA-OTP-REQUEST sent to the KDC.

   salt
      The salt value to be used in string-to-key when used to calculate
      the keys as described in <a href="#section-3.6">Section 3.6</a>.

   s2kparams
      Any additional parameters required by string-to-key as described
      in <a href="#section-3.6">Section 3.6</a>.

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

   The padata-type PA-OTP-REQUEST is sent by the client to the KDC in
   the KrbFastReq padata of a PA-FX-FAST-REQUEST that is included in the
   PA-DATA of an AS-REQ.  The corresponding padata-value field contains
   the DER encoding of a PA-OTP-REQUEST.

   The message contains pre-authentication data encrypted by the client
   using the generated Client Key and optional information on how the
   OTP was generated.  It may also, optionally, contain the generated
   OTP value.





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            PA-OTP-REQUEST     142

            PA-OTP-REQUEST ::= SEQUENCE {
              flags          [0]  OTPFlags,
              nonce          [1]  OCTET STRING                OPTIONAL,
              encData        [2]  EncryptedData,
                                 -- PA-OTP-ENC-REQUEST or PA-ENC-TS-ENC
                                 -- Key usage of KEY_USAGE_OTP_REQUEST
              hashAlg        [3]  AlgorithmIdentifier         OPTIONAL,
              iterationCount [4]  Int32                       OPTIONAL,
              otp-value      [5]  OCTET STRING                OPTIONAL,
              otp-pin        [6]  UTF8String                  OPTIONAL,
              otp-challenge  [7]  OCTET STRING (SIZE(1..MAX)) OPTIONAL,
              otp-time       [8]  KerberosTime                OPTIONAL,
              otp-counter    [9]  OCTET STRING                OPTIONAL,
              otp-format     [10] OTPFormat                   OPTIONAL,
              otp-tokenID    [11] OCTET STRING                OPTIONAL,
              otp-algID      [12] AnyURI                      OPTIONAL,
              otp-vendor     [13] UTF8String                  OPTIONAL,
              ...
            }

            KEY_USAGE_OTP_REQUEST  45

            PA-OTP-ENC-REQUEST ::= SEQUENCE {
               nonce     [0] OCTET STRING,
               ...
            }

   flags
      This field MUST be present.

      If the "nextOTP" flag is set, then the OTP was calculated based on
      the next token "state" rather than the current one.  This flag
      MUST be set if and only if it was set in a corresponding
      PA-OTP-CHALLENGE.

      If the "combine" flag is set, then the OTP was calculated based on
      a challenge and the token state.

      No other OTPFlag bits are applicable and MUST be ignored by the
      KDC.

   nonce
      This field MUST be present if a hashed OTP value was used as input
      to string-to-key (see <a href="#section-3.6">Section 3.6</a>) and MUST NOT be present
      otherwise.  If present, it MUST contain the nonce value generated
      by the client and used in the generation of hashed OTP values as



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      described in <a href="#section-3.6">Section 3.6</a>.  This nonce string MUST be as long as
      the longest key length of the symmetric key types that the client
      supports and MUST be chosen randomly.

   encData
      This field MUST be present and MUST contain the pre-authentication
      data encrypted under the Client Key with a key usage of
      KEY_USAGE_OTP_REQUEST.  If the PA-OTP-REQUEST is sent as a result
      of a PA-OTP-CHALLENGE, then this MUST contain a PA-OTP-ENC-REQUEST
      with the nonce from the PA-OTP-CHALLENGE.  If no challenge was
      received, then this MUST contain a PA-ENC-TS-ENC.

   hashAlg
      This field MUST be present if a hashed OTP value was used as input
      to string-to-key (see <a href="#section-3.6">Section 3.6</a>) and MUST NOT be present
      otherwise.  If present, it MUST contain the AlgorithmIdentifier of
      the hash algorithm used.  If the PA-OTP-REQUEST is sent as a
      result of a PA-OTP-CHALLENGE, then the AlgorithmIdentifer MUST be
      the first one supported by the client from the supportedHashAlg of
      the PA-OTP-CHALLENGE.

   iterationCount
      This field MUST be present if a hashed OTP value was used as input
      to string-to-key (see <a href="#section-3.6">Section 3.6</a>) and MUST NOT be present
      otherwise.  If present, it MUST contain the iteration count used
      when hashing the OTP value.  If the PA-OTP-REQUEST is sent as a
      result of a PA-OTP-CHALLENGE, then the value MUST NOT be less that
      specified in the PA-OTP-CHALLENGE.

   otp-value
      The generated OTP value.  This value MUST NOT be present if the
      "must-encrypt-nonce" flag was set in the PA-OTP-CHALLENGE.

   otp-pin
      The OTP PIN value entered by the user.  This value MUST NOT be
      present unless the "separate-pin-required" flag was set in the
      PA-OTP-CHALLENGE.

   otp-challenge
      Value used by the client in the OTP calculation.  It MUST be sent
      to the KDC if and only if the value would otherwise be unknown to
      the KDC (for example, the token- or client-modified or generated
      challenge).








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<span class="grey"><a href="./rfc6560">RFC 6560</a>                 OTP Pre-Authentication               April 2012</span>


   otp-time
      This field MAY be included by the client to carry the time value
      as reported by the OTP token.  Use of this element is OPTIONAL,
      but it MAY be used by a client to simplify the OTP calculations
      carried out by the KDC.  It is RECOMMENDED that the KDC act upon
      this value if it is present in the request and it is capable of
      using it in the generation of the OTP value.

   otp-counter
      This field MAY be included by the client to carry the token
      counter value, as reported by the OTP token.  Use of this element
      is OPTIONAL, but it MAY be used by a client to simplify the OTP
      calculations carried out by the KDC.  It is RECOMMENDED that the
      KDC act upon this value if it is present in the request and it is
      capable of using it in the generation of the OTP value.

   otp-format
      This field MAY be used by the client to send the format of the
      generated OTP as reported by the OTP token.  Use of this element
      is OPTIONAL, but it MAY be used by the client to simplify the OTP
      calculations carried out by the KDC.  It is RECOMMENDED that the
      KDC act upon this value, if it is present in the request and it is
      capable of using it in the generation of the OTP value.

   otp-tokenID
      This field MAY be used by the client to send the identifier of the
      token key used, as reported by the OTP token.  Use of this field
      is OPTIONAL, but MAY be used by the client to simplify the
      authentication process by identifying a particular token key
      associated with the user.  It is RECOMMENDED that the KDC act upon
      this value, if it is present in the request and it is capable of
      using it in the generation of the OTP value.

   otp-algID
      This field MAY be used by the client to send the identifier of the
      OTP algorithm used, as reported by the OTP token.  Use of this
      element is OPTIONAL, but it MAY be used by the client to simplify
      the OTP calculations carried out by the KDC.  It is RECOMMENDED
      that the KDC act upon this value, if it is present in the request
      and it is capable of using it in the generation of the OTP value.
      The value of this field MUST be a URI [<a href="./rfc3986" title="&quot;Uniform Resource Identifier (URI): Generic Syntax&quot;">RFC3986</a>] and SHOULD be
      obtained from the PSKC algorithm registry [<a href="./rfc6030" title="&quot;Portable Symmetric Key Container (PSKC)&quot;">RFC6030</a>].









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   otp-vendor
      If the PA-OTP-REQUEST is being sent in response to a
      PA-OTP-CHALLENGE that contained an otp-vendor field in the
      selected otp-tokenInfo, then this field MUST be set to the same
      value; otherwise, this field SHOULD be set to the vendor
      identifier of the token, if known to the client.  It is
      RECOMMENDED that the KDC act upon this value if it is present in
      the request and it is capable of using it in the generation of the
      OTP value.

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  PA-OTP-PIN-CHANGE</span>

   The padata-type PA-OTP-PIN-CHANGE is returned by the KDC in the
   enc-fast-rep of a PA-FX-FAST-REPLY in the AS-REP if the user must
   change their PIN, if the user's PIN has been changed, or to notify
   the user of the PIN's expiry time.

   The corresponding padata-value field contains the DER encoding of a
   PA-OTP-PIN-CHANGE.

            PA-OTP-PIN-CHANGE     144

            PA-OTP-PIN-CHANGE ::= SEQUENCE {
              flags     [0] PinFlags,
              pin       [1] UTF8String OPTIONAL,
              minLength [2] INTEGER    OPTIONAL,
              maxLength [3] INTEGER    OPTIONAL,
              last-req  [4] LastReq    OPTIONAL,
              format    [5] OTPFormat  OPTIONAL,
              ...
            }

            PinFlags ::= KerberosFlags
              -- reserved(0),
              -- systemSetPin(1),
              -- mandatory(2)

   flags
      The "systemSetPin" flag is used to indicate the type of PIN change
      that is taking place.  If the flag is set, then the user's PIN has
      been changed for the user by the system.  If the flag is not set,
      then the user's PIN needs to be changed by the user.

      If the "systemSetPin" flag is not set and the "mandatory" flag is
      set, then user PIN change is required before the next
      authentication using the current OTP token.  If the "mandatory"
      flag is not set, then the user PIN change is optional.  If the




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      "systemSetPin" flag is set, then the "mandatory" flag has no
      meaning and SHOULD be ignored by the client.

   pin
      The pin field is used to carry a new PIN value.  If the
      "systemSetPin" flag is set, then the pin field is used to carry
      the new PIN value set for the user and MUST be present.  If the
      "systemSetPin" flag is not set, then use of this field is OPTIONAL
      and MAY be used to carry a system-generated PIN that MAY be used
      by the user when changing the PIN.

   minLength and maxLength
      Use of the minLength and maxLength fields is OPTIONAL.  If the
      "systemSetPin" flag is not set, then these fields MAY be included
      to pass restrictions on the size of the user-selected PIN.

   last-req
      Use of the last-req field (as defined in <a href="./rfc4120#section-5.4.2">Section&nbsp;5.4.2 of
      [RFC4120]</a>)) is OPTIONAL, but MAY be included with an lr-type of 6
      to carry PIN expiration information.

      *  If the "systemSetPin" flag is set, then the expiration time
         MUST be that of the new system-set PIN.

      *  If the "systemSetPin" flag is not set, then the expiration time
         MUST be that of the current PIN of the token used in the
         authentication.

      The element MAY also be included with an lr-type of 7 to indicate
      when the OTP account will expire.

   format
      The format element MAY be included by the KDC to carry format
      restrictions on the new PIN.

      *  If the "systemSetPin" flag is set, then the element MUST
         describe the format of the new system-generated PIN.

      *  If the "systemSetPin" flag is not set, then the element MUST
         describe restrictions on any new user-generated PIN.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  IANA Considerations</span>

   The OTP algorithm identifier URIs used as otp-algID values in the
   PA-OTP-CHALLENGE described in <a href="#section-4.1">Section 4.1</a> and the PA-OTP-REQUEST
   described in <a href="#section-4.2">Section 4.2</a> have been registered in the "Algorithm URI
   Registry and Related PSKC Profiles" registry [<a href="./rfc6030" title="&quot;Portable Symmetric Key Container (PSKC)&quot;">RFC6030</a>].




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   The following pre-authentication types are defined in this document:

            PA-OTP-CHALLENGE           141
            PA-OTP-REQUEST             142
            PA-OTP-PIN-CHANGE          144

   Note that PA-OTP-CONFIRM (143) has been marked as OBSOLETE per this
   document.

   These values are currently registered in a registry created by
   [<a href="./rfc6113" title="&quot;A Generalized Framework for Kerberos Pre-Authentication&quot;">RFC6113</a>], but the entries have been updated to refer to this
   document.

   The following error codes and key usage values are defined in this
   document:

            KDC_ERR_INVALID_HASH_ALG           94
            KDC_ERR_INVALID_ITERATION_COUNT    95
            KDC_ERR_PIN_EXPIRED                96
            KDC_ERR_PIN_REQUIRED               97
            KEY_USAGE_OTP_REQUEST              45

   These values are currently not managed by IANA and have not been
   accounted for.  There is currently work in progress [<a href="#ref-LHA10" title="&quot;Kerberos number registry to IANA&quot;">LHA10</a>] to define
   IANA registries and a registration process for these values.

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

<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>.  Man-in-the-Middle Attacks</span>

   In the system described in this document, the OTP pre-authentication
   protocol is tunneled within the FAST Armor channel provided by the
   pre-authentication framework.  As described in [<a href="#ref-ASNINY02" title="&quot;Man-in-the-Middle in Tunneled Authentication Protocols&quot;">ASNINY02</a>], tunneled
   protocols are potentially vulnerable to man-in-the-middle (MITM)
   attacks if the outer tunnel is compromised, and it is generally
   considered good practice in such cases to bind the inner encryption
   to the outer tunnel.

   In order to mitigate against such attacks, the proposed system uses
   the outer Armor Key in the derivation of the inner Client and Reply
   keys and so achieves crypto-binding to the outer channel.

   As described in <a href="./rfc6113#section-5.4">Section&nbsp;5.4 of [RFC6113]</a>, FAST can use an anonymous
   Ticket-Granting Ticket (TGT) obtained using anonymous Public Key
   Cryptography for Initial Authentication in Kerberos (PKINIT)
   [<a href="./rfc6112" title="&quot;Anonymity Support for Kerberos&quot;">RFC6112</a>] [<a href="./rfc4556" title="&quot;Public Key Cryptography for Initial Authentication in Kerberos (PKINIT)&quot;">RFC4556</a>] as the Armor Key.  However, the current anonymous
   PKINIT proposal is open to MITM attacks since the attacker




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   can choose a session key such that the session key between the MITM
   and the real KDC is the same as the session key between the client
   and the MITM.

   As described in <a href="#section-3.6">Section 3.6</a>, if the OTP value is not being sent to
   the KDC, then the Armor Key is used along with the OTP value in the
   generation of the Client Key and Reply Key.  If the Armor Key is
   known, then the only entropy remaining in the key generation is
   provided by the OTP value.  If the OTP algorithm requires that the
   OTP value be sent to the KDC, then it is sent encrypted within the
   tunnel provided by the FAST Armor and so, is exposed to the attacker
   if the attacker has the Armor Key.

   Therefore, unless the identity of the KDC has been verified,
   anonymous PKINIT SHALL NOT be used with OTP algorithms that require
   the OTP value to be sent to the KDC.  In addition, the security
   considerations should be carefully considered before anonymous PKINIT
   is used with other algorithms such as those with short OTP values.

   Careful consideration should also be made if host key armor is used
   to provide the KDC-authentication facility with OTP algorithms where
   the OTP value is sent within the otp-value field of the
   PA-OTP-REQUEST since compromised host keys would allow an attacker to
   impersonate the KDC.

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

   The four-pass system described above is a challenge-response
   protocol, and such protocols are potentially vulnerable to reflection
   attacks.  No such attacks are known at this point, but to help
   mitigate against such attacks, the system uses different keys to
   encrypt the client and server nonces.

<span class="h3"><a class="selflink" id="section-6.3" href="#section-6.3">6.3</a>.  Denial-of-Service Attacks</span>

   The protocol supports the use of an iteration count in the generation
   of the Client and Reply keys, and the client can send the number of
   iterations used as part of the PA-OTP-REQUEST.  This could open the
   KDC up to a denial-of-service attack if a large value for the
   iteration count was specified by the attacker.  It is therefore,
   particularly important that, as described in <a href="#section-3.4">Section 3.4</a>, the KDC
   reject any authentication requests where the iteration count is above
   a maximum value specified by local policy.








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<span class="h3"><a class="selflink" id="section-6.4" href="#section-6.4">6.4</a>.  Replay</span>

   In the four-pass version of this protocol, the client encrypts a
   KDC-generated nonce, so replay can be detected by the KDC.  The
   two-pass version of the protocol does not involve a server nonce; the
   client instead encrypts a timestamp, and therefore is not protected
   from replay in this way, but it will instead require some other
   mechanism, such as an OTP-server-based system or a timestamp-based
   replay cache on the KDC.

   As described in <a href="./rfc6113#section-5.2">Section&nbsp;5.2 of [RFC6113]</a>, a client cannot be certain
   that it will use the same KDC for all messages in a conversation.
   Therefore, the client cannot assume that the PA-OTP-REQUEST will be
   sent to the same KDC that issued the PA-OTP-CHALLENGE.  In order to
   support this, a KDC implementing this protocol requires a means of
   sharing session state.  However, doing this does introduce the
   possibility of a replay attack where the same response is sent to
   multiple KDCs.

   In the case of time- or event-based tokens or server-generated
   challenges, protection against replay may be provided by the OTP
   server being used if that server is capable of keeping track of the
   last used value.  This protection therefore relies upon the
   assumption that the OTP server being used in this protocol is either
   not redundant or involves a commit protocol to synchronize between
   replicas.  If this does not hold for an OTP server being used, then
   the system may be vulnerable to replay attacks.

   However, OTP servers may not be able to detect replay of OTPs
   generated using only a client-generated challenge; since, the KDC
   would not be able to detect replay in two-pass mode, it is
   recommended that the use of OTPs generated from only a
   client-generated challenge (that is, not in combination with some
   other factor such as time) should not be supported in two-pass mode.

<span class="h3"><a class="selflink" id="section-6.5" href="#section-6.5">6.5</a>.  Brute-Force Attack</span>

   A compromised or hostile KDC may be able to obtain the OTP value used
   by the client via a brute-force attack.  If the OTP value is short,
   then the KDC could iterate over the possible OTP values until a
   Client Key is generated that can decrypt the encData sent in the
   PA-OTP-REQUEST.









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   As described in <a href="#section-3.6">Section 3.6</a>, an iteration count can be used in the
   generation of the Client Key and the value of the iteration count can
   be controlled by local client policy.  Use of this iteration count
   can make it computationally infeasible/unattractive for an attacker
   to brute-force search for the given OTP within the lifetime of that
   OTP.

   If PINs contain international characters, similar looking or similar
   functioning characters may be mapped together.  For example, the
   combined and decomposed forms of accented characters will typically
   be treated the same.  Users who attempt to exploit artifacts of
   international characters to improve the strength of their PINs may
   experience false positives in the sense that PINs they intended to be
   distinct are not actually distinct.  This decision was made in order
   to improve usability across the widest variety of input methods.
   Users can choose other methods to add strength to PINs.

<span class="h3"><a class="selflink" id="section-6.6" href="#section-6.6">6.6</a>.  FAST Facilities</span>

   The secret used to generate the OTP is known only to the client and
   the KDC, so successful decryption of the encrypted nonce by the KDC
   authenticates the user.  If the OTP value is used in the Reply Key
   generation, then successful decryption of the encrypted nonce by the
   client proves that the expected KDC replied.  The Reply Key is
   replaced by either a key generated from the OTP and Armor Key or by
   the Armor Key.  This FAST factor therefore, provides the following
   facilities: client-authentication, replacing-reply-key, and,
   depending on the OTP algorithm, KDC-authentication.

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

   Many significant contributions were made to this document by RSA
   employees, but special thanks go to Magnus Nystrom, John Linn,
   Richard Zhang, Piers Bowness, Robert Philpott, Robert Polansky, and
   Boris Khoutorski.

   Many valuable suggestions were also made by members of the Kerberos
   Working Group, but special thanks go to Larry Zhu, Jeffrey Hutzelman,
   Tim Alsop, Henry Hotz, Nicolas Williams, Sam Hartman, Frank Cusak,
   Simon Josefsson, Greg Hudson, and Linus Nordberg.

   I would also like to thank Tim Alsop and Srinivas Cheruku of
   CyberSafe for many valuable review comments.








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<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-ISOIEC7812">ISOIEC7812</a>] ISO, "ISO/IEC 7812-1:2006 Identification cards --
                Identification of issuers -- Part 1: Numbering system",
                October 2006, &lt;<a href="http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=39698">http://www.iso.org/iso/iso_catalogue/</a>
                <a href="http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=39698">catalogue_tc/catalogue_detail.htm?csnumber=39698</a>&gt;.

   [<a id="ref-RFC2045">RFC2045</a>]    Freed, N. and N. Borenstein, "Multipurpose Internet Mail
                Extensions (MIME) Part One: Format of Internet Message
                Bodies", <a href="./rfc2045">RFC 2045</a>, November 1996.

   [<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-RFC3961">RFC3961</a>]    Raeburn, K., "Encryption and Checksum Specifications for
                Kerberos 5", <a href="./rfc3961">RFC 3961</a>, February 2005.

   [<a id="ref-RFC3986">RFC3986</a>]    Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
                Resource Identifier (URI): Generic Syntax", STD 66, <a href="./rfc3986">RFC</a>
                <a href="./rfc3986">3986</a>, January 2005.

   [<a id="ref-RFC4013">RFC4013</a>]    Zeilenga, K., "SASLprep: Stringprep Profile for User
                Names and Passwords", <a href="./rfc4013">RFC 4013</a>, February 2005.

   [<a id="ref-RFC4086">RFC4086</a>]    Eastlake 3rd, D., Schiller, J., and S. Crocker,
                "Randomness Requirements for Security", <a href="https://www.rfc-editor.org/bcp/bcp106">BCP 106</a>, <a href="./rfc4086">RFC</a>
                <a href="./rfc4086">4086</a>, June 2005.

   [<a id="ref-RFC4120">RFC4120</a>]    Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
                Kerberos Network Authentication Service (V5)", <a href="./rfc4120">RFC 4120</a>,
                July 2005.

   [<a id="ref-RFC4556">RFC4556</a>]    Zhu, L. and B. Tung, "Public Key Cryptography for
                Initial Authentication in Kerberos (PKINIT)", <a href="./rfc4556">RFC 4556</a>,
                June 2006.

   [<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-RFC6112">RFC6112</a>]    Zhu, L., Leach, P., and S. Hartman, "Anonymity Support
                for Kerberos", <a href="./rfc6112">RFC 6112</a>, April 2011.






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   [<a id="ref-RFC6113">RFC6113</a>]    Hartman, S. and L. Zhu, "A Generalized Framework for
                Kerberos Pre-Authentication", <a href="./rfc6113">RFC 6113</a>, April 2011.

   [<a id="ref-X.680">X.680</a>]      ITU-T, "Recommendation X.680 (2002) | ISO/IEC
                8824-1:2002, Information technology - Abstract Syntax
                Notation One (ASN.1): Specification of basic notation.",
                July 2002.

   [<a id="ref-X.690">X.690</a>]      ITU-T, "Recommendation X.690 (2008) | ISO/IEC
                8825-1:2008, X.690 : Information technology - ASN.1
                encoding rules: Specification of Basic Encoding Rules
                (BER), Canonical Encoding Rules (CER) and Distinguished
                Encoding Rules (DER)", December 2008.

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

   [<a id="ref-ASNINY02">ASNINY02</a>]   Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle
                in Tunneled Authentication Protocols", Cryptology ePrint
                Archive Report 2002/163, November 2002.

   [<a id="ref-HORENEZ004">HORENEZ004</a>] Horstein, K., Renard, K., Neuman, C., and G. Zorn,
                "Integrating Single-use Authentication Mechanisms with
                Kerberos", Work in Progress, July 2004.

   [<a id="ref-LHA10">LHA10</a>]      Hornquist Astrand, L., "Kerberos number registry to
                IANA", Work in Progress, March 2010.

   [<a id="ref-RFC2289">RFC2289</a>]    Haller, N., Metz, C., Nesser, P., and M. Straw, "A
                One-Time Password System", STD 61, <a href="./rfc2289">RFC 2289</a>, February
                1998.

   [<a id="ref-RFC2808">RFC2808</a>]    Nystrom, M., "The SecurID(r) SASL Mechanism", <a href="./rfc2808">RFC 2808</a>,
                April 2000.

   [<a id="ref-RFC4226">RFC4226</a>]    M'Raihi, D., Bellare, M., Hoornaert, F., Naccache, D.,
                and O. Ranen, "HOTP: An HMAC-Based One-Time Password
                Algorithm", <a href="./rfc4226">RFC 4226</a>, December 2005.

   [<a id="ref-RFC6030">RFC6030</a>]    Hoyer, P., Pei, M., and S. Machani, "Portable Symmetric
                Key Container (PSKC)", <a href="./rfc6030">RFC 6030</a>, October 2010.











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

   OTPKerberos
   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN

   IMPORTS

          KerberosTime, KerberosFlags, EncryptionKey, Int32,
          EncryptedData, LastReq, KerberosString
          FROM KerberosV5Spec2 {iso(1) identified-organization(3)
                                dod(6) internet(1) security(5)
                                kerberosV5(2) modules(4) krb5spec2(2)}
                                -- as defined in <a href="./rfc4120">RFC 4120</a>.
          AlgorithmIdentifier
          FROM PKIX1Explicit88 { iso (1) identified-organization (3)
                                 dod (6) internet (1)
                                 security (5) mechanisms (5) pkix (7)
                                 id-mod (0) id-pkix1-explicit (18) };
                                 -- As defined in <a href="./rfc5280">RFC 5280</a>.

          PA-OTP-CHALLENGE ::= SEQUENCE {
            nonce            [0] OCTET STRING,
            otp-service      [1] UTF8String               OPTIONAL,
            otp-tokenInfo    [2] SEQUENCE (SIZE(1..MAX)) OF
                                                     OTP-TOKENINFO,
            salt             [3] KerberosString           OPTIONAL,
            s2kparams        [4] OCTET STRING             OPTIONAL,
            ...
          }

          OTP-TOKENINFO ::= SEQUENCE {
            flags            [0] OTPFlags,
            otp-vendor       [1] UTF8String               OPTIONAL,
            otp-challenge    [2] OCTET STRING (SIZE(1..MAX))
                                                          OPTIONAL,
            otp-length       [3] Int32                    OPTIONAL,
            otp-format       [4] OTPFormat                OPTIONAL,
            otp-tokenID      [5] OCTET STRING             OPTIONAL,
            otp-algID        [6] AnyURI                   OPTIONAL,
            supportedHashAlg [7] SEQUENCE OF AlgorithmIdentifier
                                                          OPTIONAL,
            iterationCount   [8] Int32                    OPTIONAL,
            ...
          }

          OTPFormat ::= INTEGER {
            decimal(0),



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            hexadecimal(1),
            alphanumeric(2),
            binary(3),
            base64(4)
          }

          OTPFlags ::= KerberosFlags
          -- reserved(0),
          -- nextOTP(1),
          -- combine(2),
          -- collect-pin(3),
          -- do-not-collect-pin(4),
          -- must-encrypt-nonce (5),
          -- separate-pin-required (6),
          -- check-digit (7)

          PA-OTP-REQUEST ::= SEQUENCE {
            flags          [0]  OTPFlags,
            nonce          [1]  OCTET STRING                OPTIONAL,
            encData        [2]  EncryptedData,
                               -- PA-OTP-ENC-REQUEST or PA-ENC-TS-ENC
                               -- Key usage of KEY_USAGE_OTP_REQUEST
            hashAlg        [3]  AlgorithmIdentifier         OPTIONAL,
            iterationCount [4]  Int32                       OPTIONAL,
            otp-value      [5]  OCTET STRING                OPTIONAL,
            otp-pin        [6]  UTF8String                  OPTIONAL,
            otp-challenge  [7]  OCTET STRING (SIZE(1..MAX)) OPTIONAL,
            otp-time       [8]  KerberosTime                OPTIONAL,
            otp-counter    [9]  OCTET STRING                OPTIONAL,
            otp-format     [10] OTPFormat                   OPTIONAL,
            otp-tokenID    [11] OCTET STRING                OPTIONAL,
            otp-algID      [12] AnyURI                      OPTIONAL,
            otp-vendor     [13] UTF8String                  OPTIONAL,
            ...
          }

          PA-OTP-ENC-REQUEST ::= SEQUENCE {
            nonce     [0] OCTET STRING,
            ...
          }

          PA-OTP-PIN-CHANGE ::= SEQUENCE {
            flags     [0] PinFlags,
            pin       [1] UTF8String OPTIONAL,
            minLength [2] INTEGER    OPTIONAL,
            maxLength [3] INTEGER    OPTIONAL,
            last-req  [4] LastReq    OPTIONAL,
            format    [5] OTPFormat  OPTIONAL,



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            ...
          }

          PinFlags ::= KerberosFlags
          -- reserved(0),
          -- systemSetPin(1),
          -- mandatory(2)

          AnyURI ::= UTF8String
             (CONSTRAINED BY {
             -- MUST be a valid URI in accordance with IETF <a href="./rfc2396">RFC 2396</a>
             })

      END





































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<span class="h2"><a class="selflink" id="appendix-B" href="#appendix-B">Appendix B</a>.  Examples of OTP Pre-Authentication Exchanges</span>

   This section is non-normative.

<span class="h3"><a class="selflink" id="appendix-B.1" href="#appendix-B.1">B.1</a>.  Four-Pass Authentication</span>

   In this mode, the client sends an initial AS-REQ to the KDC that does
   not contain a PA-OTP-REQUEST and the KDC responds with a KRB-ERROR
   containing a PA-OTP-CHALLENGE.

   In this example, the user has been issued with a connected,
   time-based token, and the KDC requires hashed OTP values in the key
   generation with SHA-384 as the preferred hash algorithm and a minimum
   of 1024 iterations.  The local policy on the client supports SHA-256
   and requires 100,000 iterations of the hash of the OTP value.

   The basic sequence of steps involved is as follows:

   1.   The client obtains the user name of the user.

   2.   The client sends an initial AS-REQ to the KDC that does not
        contain a PA-OTP-REQUEST.

   3.   The KDC determines that the user identified by the AS-REQ
        requires OTP authentication.

   4.   The KDC constructs a PA-OTP-CHALLENGE as follows:

        nonce
           A randomly generated value.

        otp-service
           A string that can be used by the client to assist the user in
           locating the correct token.

        otp-tokenInfo
           Information about how the OTP should be generated from the
           token.

        flags
           must-encrypt-nonce and collect-pin bits set

        supportedHashAlg
           AlgorithmIdentifiers for SHA-384, SHA-256, and SHA-1

        iterationCount
           1024




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   5.   The KDC returns a KRB-ERROR with an error code of
        KDC_ERR_PREAUTH_REQUIRED and the PA-OTP-CHALLENGE in the e-data.

   6.   The client displays the value of otp-service and prompts the
        user to connect the token.

   7.   The client collects a PIN from the user.

   8.   The client obtains the current OTP value from the token using
        the PIN and records the time as reported by the token.

   9.   The client generates the Client Key and Reply Key as described
        in <a href="#section-3.6">Section 3.6</a> using SHA-256 from the list of algorithms sent by
        the KDC, the iteration count of 100,000 as required by local
        policy, and a randomly generated nonce.

   10.  The client constructs a PA-OTP-REQUEST as follows:

        flags
           0

        nonce
           The randomly generated value.

        encData
           An EncryptedData containing a PA-OTP-ENC-REQUEST encrypted
           under the Client Key with a key usage of
           KEY_USAGE_OTP_REQUEST and the encryption type of the Armor
           Key.  The PA-OTP-ENC-REQUEST contains the nonce from the
           PA-OTP-CHALLENGE.

        hashAlg
           SHA-256

        iterationCount
           100,000

        otp-time
           The time used in the OTP calculation as reported by the OTP
           token.

   11.  The client encrypts the PA-OTP-REQUEST within the enc-fast-req
        of a PA-FX-FAST-REQUEST.

   12.  The client sends an AS-REQ to the KDC containing the
        PA-FX-FAST-REQUEST within the padata.





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   13.  The KDC validates the padata in the PA-OTP-REQUEST by performing
        the following steps:

        *  Generates the Client Key and Reply Key from the OTP value for
           the user identified in the AS-REQ, using an iteration count
           of 100,000, a hash algorithm of SHA-256, and the nonce as
           specified in the PA-OTP-REQUEST.

        *  Uses the generated Client Key to decrypt the
           PA-OTP-ENC-REQUEST in the encData of the PA-OTP-REQUEST.

        *  Authenticates the user by comparing the nonce value from the
           decrypted PA-OTP-ENC-REQUEST to that sent in the
           corresponding PA-OTP-CHALLENGE.

   14.  The KDC constructs a TGT for the user.

   15.  The KDC returns an AS-REP to the client, encrypted using the
        Reply Key, containing the TGT and padata with the
        PA-FX-FAST-REPLY.

   16.

        The client authenticates the KDC and verifies the Reply Key
        change.  The client uses the generated Reply Key to decrypt the
        encrypted data in the AS-REP.

<span class="h3"><a class="selflink" id="appendix-B.2" href="#appendix-B.2">B.2</a>.  Two-Pass Authentication</span>

   In this mode, the client includes a PA-OTP-REQUEST within a
   PA-FX-FAST-REQUEST padata of the initial AS-REQ sent to the KDC.

   In this example, the user has been issued a hand-held token, so, none
   of the OTP generation parameters (otp-length, etc.) are included in
   the PA-OTP-REQUEST.  The KDC does not require hashed OTP values in
   the key generation.

   It is assumed that the client has been configured with the following
   information or has obtained it from a previous PA-OTP-CHALLENGE.

   o  The OTP value must not be carried in the otp-value.

   o  The hashed OTP values are not required.

   The basic sequence of steps involved is as follows:

   1.   The client obtains the user name and OTP value from the user.




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   2.   The client generates the Client Key and Reply Key using unhashed
        OTP values as described in <a href="#section-3.6">Section 3.6</a>.

   3.   The client constructs a PA-OTP-REQUEST as follows:

        flags
           0

        encData
           An EncryptedData containing a PA-ENC-TS-ENC encrypted under
           the Client Key with a key usage of KEY_USAGE_OTP_REQUEST and
           an encryption type of the Armor Key.  The PA-ENC-TS-ENC
           contains the current client time.

   4.   The client encrypts the PA-OTP-REQUEST within the enc-fast-req
        of a PA-FX-FAST-REQUEST.

   5.   The client sends an AS-REQ to the KDC containing the
        PA-FX-FAST-REQUEST within the padata.

   6.   The KDC validates the padata by performing the following steps:

        *  Generates the Client Key and Reply Key from the unhashed OTP
           value for the user identified in the AS-REQ.

        *  Uses the generated Client Key to decrypt the PA-ENC-TS-ENC in
           the encData of the PA-OTP-REQUEST.

        *  Authenticates the user using the timestamp in the standard
           manner.

   7.   The KDC constructs a TGT for the user.

   8.   The KDC returns an AS-REP to the client, encrypted using the
        Reply Key, containing the TGT and padata with the
        PA-FX-FAST-REPLY.

   9.   The client authenticates the KDC and verifies the key change.
        The client uses the generated Reply Key to decrypt the encrypted
        data in the AS-REP.











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<span class="h3"><a class="selflink" id="appendix-B.3" href="#appendix-B.3">B.3</a>.  PIN Change</span>

   This exchange follows from the point where the KDC receives the
   PA-OTP-REQUEST from the client in the examples in <a href="#appendix-B.1">Appendix B.1</a> and
   <a href="#appendix-B.2">Appendix B.2</a>.  During the validation of the pre-authentication data
   (whether encrypted nonce or encrypted timestamp), the KDC determines
   that the user's PIN has expired and so, must be changed.  The KDC
   therefore, includes a PA-OTP-PIN-CHANGE in the AS-REP.

   In this example, the KDC does not generate PIN values for the user
   but requires that the user generate a new PIN that is between 4 and 8
   characters in length.  The actual PIN change is handled by a PIN
   change service.

   The basic sequence of steps involved is as follows:

   1.   The client constructs and sends a PA-OTP-REQUEST to the KDC as
        described in the previous examples.

   2.   The KDC validates the pre-authentication data and authenticates
        the user as in the previous examples but determines that the
        user's PIN has expired.

   3.   The KDC constructs a PA-OTP-PIN-CHANGE as follows:

        flags
           0
        minLength
           4
        maxLength
           8

   4.   The KDC encrypts the PA-OTP-PIN-CHANGE within the enc-fast-rep
        of a PA-FX-FAST-REPLY.

   5.   The KDC returns a KRB-ERROR to the client of type
        KDC_ERR_PIN_EXPIRED with padata containing the PA-FX-FAST-REPLY.

   6.   The client authenticates to the PIN change service and changes
        the user's PIN.

   7.   The client sends a second AS-REQ to the KDC containing a PA-OTP-
        REQUEST constructed using the new PIN.

   8.   The KDC responds with an AS-REP containing a TGT.






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<span class="h3"><a class="selflink" id="appendix-B.4" href="#appendix-B.4">B.4</a>.  Resynchronization</span>

   This exchange follows from the point where the KDC receives the
   PA-OTP-REQUEST from the client.  During the validation of the
   pre-authentication data (whether encrypted nonce or encrypted
   timestamp), the KDC determines that the local record of the token's
   state needs to be resynchronized with the token.  The KDC therefore,
   includes a KRB-ERROR containing a PA-OTP-CHALLENGE with the "nextOTP"
   flag set.

   The sequence of steps below follows is a variation of the four pass
   examples shown in <a href="#appendix-B.1">Appendix B.1</a> but the same process would also work
   in the two-pass case.

   1.   The client constructs and sends a PA-OTP-REQUEST to the KDC as
        described in the previous examples.

   2.   The KDC validates the pre-authentication data and authenticates
        the user as in the previous examples, but determines that user's
        token requires resynchronizing.

   3.   KDC constructs a PA-OTP-CHALLENGE as follows:

        nonce
           A randomly generated value.

        otp-service
           Set to a string that can be used by the client to assist the
           user in locating the correct token.

        otp-tokenInfo
           Information about how the OTP should be generated from the
           token.

        flags
           must-encrypt-nonce, collect-pin, and nextOTP bits set

        supportedHashAlg
           AlgorithmIdentifiers for SHA-256 and SHA-1

        iterationCount
           1024

   4.   KDC returns a KRB-ERROR with an error code of
        KDC_ERR_PREAUTH_REQUIRED and the PA-OTP-CHALLENGE in the e-data.

   5.   The client obtains the next OTP value from the token and records
        the time as reported by the token.



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<span class="grey"><a href="./rfc6560">RFC 6560</a>                 OTP Pre-Authentication               April 2012</span>


   6.   The client generates the Client Key and Reply Key as described
        in <a href="#section-3.6">Section 3.6</a> using SHA-256 from the list of algorithms sent by
        the KDC, the iteration count of 100,000 as required by local
        policy, and a randomly generated nonce.

   7.   The client constructs a PA-OTP-REQUEST as follows:

        flags
           nextOTP bit set

        nonce
           The randomly generated value.

        encData
           An EncryptedData containing a PA-OTP-ENC-REQUEST encrypted
           under the Client Key with a key usage of
           KEY_USAGE_OTP_REQUEST and the encryption type of the Armor
           Key.  The PA-OTP-ENC-REQUEST contains the nonce from the
           PA-OTP-CHALLENGE.

        hashAlg
           SHA-256

        iterationCount
           100,000

        otp-time
           The time used in the OTP calculation as reported by the OTP
           token.

   8.   The client encrypts the PA-OTP-REQUEST within the enc-fast-req
        of a PA-FX-FAST-REQUEST.

   9.   The client sends an AS-REQ to the KDC containing the
        PA-FX-FAST-REQUEST within the padata.

   10.  The authentication process now proceeds as with the classic
        sequence.













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<span class="grey"><a href="./rfc6560">RFC 6560</a>                 OTP Pre-Authentication               April 2012</span>


Author's Address

   Gareth Richards
   RSA, The Security Division of EMC
   RSA House
   Western Road
   Bracknell, Berkshire  RG12 1RT
   UK

   EMail: gareth.richards@rsa.com









































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</pre>