


Network Working Group                                        G. Richards
Internet-Draft                         RSA, The Security Division of EMC
Intended status: Standards Track                           July 14, 2008
Expires: January 15, 2009


                         OTP Pre-authentication
                    draft-ietf-krb-wg-otp-preauth-05

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   This Internet-Draft will expire on January 15, 2009.

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.











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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Overall Design . . . . . . . . . . . . . . . . . . . . . .  3
     1.3.  Conventions Used in this Document  . . . . . . . . . . . .  4
   2.  Usage Overview . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  OTP Mechanism Support  . . . . . . . . . . . . . . . . . .  4
     2.2.  Pre-Authentication . . . . . . . . . . . . . . . . . . . .  4
     2.3.  PIN Change . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.4.  Re-Synchronization . . . . . . . . . . . . . . . . . . . .  6
   3.  Pre-Authentication Protocol Details  . . . . . . . . . . . . .  6
     3.1.  Initial Client Request . . . . . . . . . . . . . . . . . .  6
     3.2.  KDC Challenge  . . . . . . . . . . . . . . . . . . . . . .  6
     3.3.  Client Response  . . . . . . . . . . . . . . . . . . . . .  7
     3.4.  Verifying the pre-auth Data  . . . . . . . . . . . . . . .  9
     3.5.  Confirming the Reply Key Change  . . . . . . . . . . . . . 10
     3.6.  Reply Key Generation . . . . . . . . . . . . . . . . . . . 11
   4.  OTP Kerberos Message Types . . . . . . . . . . . . . . . . . . 13
     4.1.  PA-OTP-CHALLENGE . . . . . . . . . . . . . . . . . . . . . 13
     4.2.  PA-OTP-REQUEST . . . . . . . . . . . . . . . . . . . . . . 15
     4.3.  PA-OTP-CONFIRM . . . . . . . . . . . . . . . . . . . . . . 18
     4.4.  PA-OTP-PIN-CHANGE  . . . . . . . . . . . . . . . . . . . . 19
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
     6.1.  Man-in-the-Middle  . . . . . . . . . . . . . . . . . . . . 19
     6.2.  Reflection . . . . . . . . . . . . . . . . . . . . . . . . 20
     6.3.  Replay . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     6.4.  Brute Force Attack . . . . . . . . . . . . . . . . . . . . 20
     6.5.  FAST Facilities  . . . . . . . . . . . . . . . . . . . . . 20
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 21
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 21
   Appendix A.  ASN.1 Module  . . . . . . . . . . . . . . . . . . . . 22
   Appendix B.  Examples of OTP Pre-Authentication Exchanges  . . . . 24
     B.1.  Four Pass Authentication . . . . . . . . . . . . . . . . . 24
     B.2.  Two Pass Authentication  . . . . . . . . . . . . . . . . . 27
     B.3.  Pin Change . . . . . . . . . . . . . . . . . . . . . . . . 29
     B.4.  Resynchronization  . . . . . . . . . . . . . . . . . . . . 30
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 32
   Intellectual Property and Copyright Statements . . . . . . . . . . 33









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1.  Introduction

1.1.  Scope

   This document describes a FAST [ZhHa07] factor that allows One-Time
   Password (OTP) values to be used in the Kerberos V5 [RFC4120] 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 [RFC2808], counter-
   based tokens [RFC4226], challenge-response and [RFC2289] type
   systems.  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
   [ZhHa07]): 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 [HoReNeZo04] and
   allows for the use of the existing password-change extensions to
   handle PIN change as described in [RFC3244].

1.2.  Overall Design

   This proposal supports 4-pass and 2-pass variants.  In the 4-pass
   system, the client sends the KDC an initial AS-REQ and the KDC
   responds with a KRB-ERROR containing padata that includes a random
   nonce.  The client then encrypts the nonce and returns it along with
   its own random value to the KDC in a second AS-REQ.  Finally, the KDC
   returns the client's random value encrypted within the padata of the
   AS-REP.  In the 2-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.  This variant can be used in cases
   where the client can determine in advance that OTP pre-authentication
   is supported by the KDC and which OTP key should be used.

   In both systems, in order to create the message sent to the KDC, the
   client must generate the OTP value and three keys: the standard Reply
   Key, a key to encrypt the data sent to the KDC and a final key to
   decrypt the KDC's reply.  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.  [RFC2289]), the system also
   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



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   derivation.

   The message from the client to the KDC is sent within the encrypted
   data provided by the FAST padata 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 returning the
   client's nonce encrypted under one of the generated keys within the
   encrypted part of the FAST padata of the AS-REP.

1.3.  Conventions Used in this Document

   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 [RFC2119].

   This document assumes familiarity with the Kerberos preauthentication
   framework [ZhHa07] and so freely uses terminology and notation from
   this document.

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


2.  Usage Overview

2.1.  OTP Mechanism Support

   As described above, this document describes a generic system for
   supporting different OTP mechanisms in Kerberos pre-authentication.
   However, to ensure interoperability, all implementations of this
   specification SHOULD provide a mechanism for OTP mechanism support to
   be added or removed.

2.2.  Pre-Authentication

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

   In the 4-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 in the PA-DATA.

   The PA-OTP-CHALLENGE will contain a KDC generated nonce, an
   encryption type, an optional list of hash algorithm identifiers, an
   optional iteration count and optional information on how the OTP



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   should be generated by the client.  The client will then generate the
   OTP value, its own nonce and two keys: a Client Key to encrypt the
   KDC's nonce and a Reply Key used to decrypt the KDC's reply.

   As described in Section 3.6, these keys will be generated from the
   Armor Key (defined in [ZhHa07]) 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.

   The generated Client Key will be used to encrypt the nonce received
   from the KDC using the specified encryption type.  The encrypted
   value, a random nonce generated by the client along with optional
   information on how the OTP was generated are then sent to the KDC in
   a PA-OTP-REQUEST element encrypted within the armored-data of a PA-
   FX-FAST-REQUEST PA-DATA element of a second AS-REQ.

   In the 2-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 generate the
   same keys as the client, and use 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 encrypting the client's nonce under the
   Reply Key and returning the encrypted value in the encData of a PA-
   OTP-CONFIRM.  The PA-OTP-CONFIRM is encrypted within the armored-data
   of a PA-FX-FAST-REPLY PA-DATA element of the AS-REP as described in
   [ZhHa07].

2.3.  PIN Change

   If, following successful validation of a PA-OTP-REQUEST in a AS-REQ,
   the KDC requires that the user changes their PIN then it will include
   a PA-OTP-PIN-CHANGE element in the armored data of the PA-FX-FAST-
   REPLY PA-DATA element of the AS-REP.  This data can be used to return
   a new PIN to the user if the KDC has updated the PIN or to indicate
   to the user that they must change their PIN.

   In the latter case, it is recommended that user PIN change be handled
   by a PIN change service supporting the ChangePasswdData in a AP-REQ
   as described in [RFC3244].  If a user PIN for OTP is required to
   change and such a service is used then the KDC MUST NOT return a TGT



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   when the user is authenticated using this PIN.  The KDC SHOULD return
   a service ticket to the PIN change service when the existing PIN is
   required to change, in order for the client to compute an AP-REQ
   according to [RFC3244].  In order to complicate stealing service
   tickets intended for the PIN change service (and the corresponding
   session keys), the lifetime of the PIN-change service tickets should
   be just long enough to complete the PIN change, regardless whether
   the exiting PIN needs to be changed or not.  A 1-minute lifetime is
   RECOMMENED.  This way the PIN change service can effectively force
   the user to present the existing PIN in order to change to use a new
   PIN.

2.4.  Re-Synchronization

   It is possible with time and event-based tokens that the OTP server
   will lose synchronization with the current token state.  If, when
   processing a PA-OTP-REQUEST, the pre-authentication validation fails
   for this reason then the KDC SHALL return a KRB-ERROR message
   containing a PA-OTP-CHALLENGE in the PA-DATA with the "nextOTP" flag
   set.  If this flag is set then the client MUST re-try the
   authentication using the OTP for the token "state" after that used in
   the failed authentication attempt.


3.  Pre-Authentication Protocol Details

3.1.  Initial Client Request

   In the 4-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 Section 3.2.

   If the client has all the necessary information, it MAY use the
   2-pass system by constructing a PA-OTP-REQUEST as described in
   Section 3.3 and including it in the initial request.

3.2.  KDC Challenge

   If the user is required to authenticate using an OTP then the KDC
   SHALL respond to the initial AS-REQ with a KRB-ERROR containing:

   o  An error code of KDC_ERR_PREAUTH_REQUIRED

   o  An e-data field containing PA-DATA with a PA-OTP-CHALLENGE.

   The PA-OTP-CHALLENGE SHALL contain a random nonce value to be



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   returned encrypted in the client response and the encryption type to
   be used by the client.

   If the OTP is to be generated using an server generated challenge
   then the value of the challenge SHALL be included in the otp-
   challenge field.  If 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.

   The KDC MAY use the otp-service to identify the service provided by
   the KDC in order to assist the client in locating the OTP token to be
   used.  For example, this field could be used when a client has
   multiple OTP tokens from different servers to identify the KDC.
   Similarly, if the KDC can determine which OTP token key is the be
   used, then the otp-keyID field MAY be used to pass that value to the
   client.

   The otp-algID field MAY be used to identify the algorithm that should
   be used in the OTP calculation.  For example, it could be used when a
   user has been issued with multiple tokens of different types.

   In order to support connected tokens that can generate OTP values of
   varying length, the KDC MAY include the desired length of the OTP in
   the otp-length field.

   In order to support cases where the KDC cannot obtain plaintext
   values for the OTPs, the challenge MAY also contain a sequence of one
   way hash function algorithm identifiers and a minimum value of the
   iteration count to be used by the client when hashing the OTP value.

3.3.  Client Response

   The client response SHALL be sent to the KDC as a PA-OTP-REQUEST
   included within the enc-fast-req of a PA-FX-FAST-REQUEST encrypted
   under the current Armor Key as described in [ZhHa07].

   In order to generate its response, the client must generate an OTP
   value.  The OTP value MUST be based on the parameters in the KDC
   challenge if present and the response SHOULD include any information
   on the generated OTP value reported by the OTP token

   If the "nextOTP" flag is set in the PA-OTP-CHALLENGE, then the client
   MUST generate the OTP value in the next token state that that used in
   the previous PA-OTP-REQUEST.  The "nextOTP" flag must also be set in
   the PA-OTP-REQUEST.

   The otp-time and otp-counter fields MAY be used to return the time
   and counter values used by the token.  The otp-format field MAY be



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   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 MAY be used by the client to report the algorithm
   used in the OTP calculation and the otp-keyID MAY be used to report
   the identifier of the OTP token key used.

   If an otp-challenge is present in the PA-OTP-CHALLENGE then the OTP
   value 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 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 the flag is set but otp-challenge is not present then the client
   SHALL regard the request as invalid.

   If the OTP value was generated by a challenge not sent by the KDC
   then the challenge SHALL be included in the otp-challenge of the PA-
   OTP-RESPONSE.  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-
   RESPONSE.

   The client MUST derive the Client Key and Reply Key as described in
   Section 3.6.  In order to support OTP algorithms where the KDC cannot
   obtain the OTP value, the client MAY include the generated value in
   the otp-value field of the response.  However, the client MUST NOT
   include the OTP value in the response unless it is allowed by the
   algorithm profile.  If it is included then the OTP value MUST NOT be
   used in the key derivation.

   If the client used hashed OTP values in the key derivation process
   then it MUST include the hash algorithm and iteration count used in
   the 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 within the
   lifetime of that OTP.

   If the PA-OTP-REQUEST is being sent in response to a PA-OTP-CHALLENGE
   that contained hash algorithm identifiers and the OTP value is to be
   used in the key derivation then the client MUST use hashed OTP values
   and MUST select the first algorithm from the list that it supports.
   However, if the algorithm identifiers do not conform to local policy
   restrictions then the authentication attempt MUST NOT proceed.  If



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   the iteration count specified in the PA-OTP-CHALLENGE does not
   conform to local policy then the client MAY use a higher value but
   MUST NOT use a lower value.  That is, the value in the KDC challenge
   is a minimum value.

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

   o  If the response is being generated in response to a KDC challenge
      then client encrypts a PA-OTP-ENC-REQUEST containing the value of
      nonce from the corresponding challenge using the encryption type
      specified in the challenge.

   o  If the response is not in response to a KDC challenge then the
      client encrypts a PA-ENC-TS-ENC containing the current time as in
      the encrypted timestamp pre-authentication mechanism [RFC4120].

   If the client is working in 2-pass mode and so is not responding to
   an initial KDC challenge then the values of the iteration count, hash
   algorithms and encryption type 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.

   Finally, the client generates a random value to include in the nonce
   of the response.  This value will then be returned encrypted by the
   KDC.

3.4.  Verifying the pre-auth Data

   The KDC validates the pre-authentication data by generating the same
   keys as the client and using the generated Client Key to decrypt the
   value of encData from the PA-OTP-REQUEST.

   If the otp-value field is included in the PA-OTP-REQUEST then the KDC
   MUST use that value in the key generation.  Otherwise, the KDC will
   need to generate or obtain the value.

   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 acts upon the values of otp-time, otp-
   counter, otp-format, otp-algID and otp-keyID if they are present in
   the PA-OTP-REQUEST.  If the KDC receives a request containing these
   values but cannot act upon theme then they MAY be ignored.



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   The KDC generates the Client Key and Reply Key as described in
   Section 3.6 from the OTP value using the hash algorithm and iteration
   count if present in the PA-OTP-REQUEST.  However, the client
   authentication MUST fail if the KDC requires hashed OTP values and
   the hashAlg field was not present in the PA-OTP-REQUEST, if the hash
   algorithm identifier or the value of iterationCount included in the
   PA-OTP-REQUEST do not conform to local KDC policy or if the value of
   the iterationCount is less than that specified in the PA-OTP-
   CHALLENGE.

   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 decrypted data will be a PA-OTP-ENC-REQUEST and
   the client authentication MUST fail 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 standard encrypted
   timestamp pre-authentication [RFC4120]

   The authentication MUST fail if the encryption type used by the
   client in the encData does not conform to policy.

   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 no longer
   being synchronized with the token used then the KDC SHALL return a
   PA-OTP-CHALLENGE with the "nextOTP" flag set as described in
   Section 2.4.

   If during the authentication process, the KDC determines that the
   user's PIN has expired then it MAY include a PA-OTP-PIN-CHANGE in the
   response as described in Section 2.3

3.5.  Confirming the Reply Key Change

   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 including in the AS-REP, a PA-OTP-CONFIRM
   containing the client's nonce from PA-OTP-REQUEST encrypted under the
   generated Reply Key.

   The nonce SHALL be returned within a PA-OTP-ENC-CONFIRM encrypted
   within the encData of the PA-OTP-CONFIRM.  The key usage SHALL be
   KEY_USAGE_OTP_CONFIRM and the encryption type SHOULD be the same as
   used by the client in the encData of the PA-OTP-REQUEST.




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   The PA-OTP-CONFIRM SHALL be sent to the client within the enc-fast-
   rep of a PA-FX-FAST-REPLY encrypted under the current Armor Key.

   The client then uses its generated Reply Key to decrypt the PA-OTP-
   ENC-CONFIRM from the encData of the PA-OTP-CONFIRM.  The client MUST
   fail the authentication process if the nonce value in the PA-OTP-ENC-
   CONFIRM is not the same as the nonce value sent in the PA-OTP-
   REQUEST.

3.6.  Reply Key Generation

   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 but also to be used by the KDC to
      encrypt and by the client to decrypt the encData value in the PA-
      OTP-CONFIRM.

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

   o  If the OTP value is included in the otp-value of the PA-OTP-
      REQUEST then all three keys SHALL be the same as the Armor Key
      (defined in [ZhHa07]).

   o  If the OTP value is not included in the otp-value of the PA-OTP-
      REQUEST then the three 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 SHALL be generated using the KRB_FX_CF2 algorithm from
   [ZhHa07]

             Client Key = KRB_FX_CF2(K1, K2, O1, O2)
             Reply Key = KRB_FX_CF2(K1, K2, O3, O4)

   The first input keys, K1, shall be the Armor Key. The second input
   key, K2, shall be derived from the OTP value using string-to-key
   (defined in [RFC3961]).

   The octet string parameters, O1, O2, O3 and O4, shall be the ASCII
   string "Combine1", "Combine2", "Combine3" and "Combine4" as shown
   below:




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      {0x43, 0x6f, 0x6d, 0x62, 0x69, 0x6e, 0x65, 0x31}
      {0x43, 0x6f, 0x6d, 0x62, 0x69, 0x6e, 0x65, 0x32}
      {0x43, 0x6f, 0x6d, 0x62, 0x69, 0x6e, 0x65, 0x33}
      {0x43, 0x6f, 0x6d, 0x62, 0x69, 0x6e, 0x65, 0x34}

   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(sname|nonce|OTP)

      Where:
      *  "|" denotes concatenation
      *  hash is the hash algorithm selected by the client.
      *  sname is the UTF-8 encoding of the KDC's fully qualified domain
         name.  If the domain name is an Internationalized Domain Name
         then the value shall be the output of nameprep [RFC3491] as
         described in [RFC3490]
      *  nonce is the random nonce value generated by the client to be
         included in the PA-OTP-REQUEST.
      *  OTP is the OTP value.

   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 [RFC2045] encoding
      of this final hash value.

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

   If the OTP value is binary and 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")

   If the OTP value is not binary and the hash value is not used, then
   K2 SHALL be derived by running the OTP value once through string-to-
   key.

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

   The salt and any additional parameters for string-to-key will be
   derived as described in section 3.1.3 of [RFC4120] using preauth data
   or default values defined for the particular enctype.  The symbol



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   "||" denotes string concatenation.


4.  OTP Kerberos Message Types

4.1.  PA-OTP-CHALLENGE

   The PA_OTP_CHALLENGE padata type is sent by the KDC to the client in
   the PA-DATA of a KRB-ERROR when pre-authentication using an OTP value
   is required.  The corresponding padata-value field contains the DER
   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     << TBA >>

             PA-OTP-CHALLENGE ::= SEQUENCE {
               flags              OTPFlags,
               nonce              UInt32,
               etype              Int32,
               supportedHashAlg   SEQUENCE OF AlgorithmIdentifier
                                                  OPTIONAL,
               iterationCount     INTEGER         OPTIONAL,
               otp-challenge      OCTET STRING (SIZE(8..MAX)) OPTIONAL,
               otp-length     [0] Int32           OPTIONAL,
               otp-service        UTF8String      OPTIONAL,
               otp-keyID      [1] OCTET STRING    OPTIONAL,
               otp-algID          AnyURI          OPTIONAL,
               ...
             }

             OTPFlags ::= KerberosFlags
             -- nextOTP (0)
             -- combine (1)

   flags
      If the "nextOTP" flag is set then the OTP SHALL be based on the
      next token "state" rather than the current one.  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.

      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.




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   nonce
      A KDC-supplied nonce value to be encrypted by the client in the
      PA-OTP-REQUEST.

   etype
      The encryption type to be used by the client to encrypt the nonce
      in the PA-OTP-REQUEST.

   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 SHA1 by both the client and KDC is REQUIRED.  The
      AlgorithmIdentifer 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 and
      only if supportedHashAlg is 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.

   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 it is
      present in, and SHOULD be eight octets or longer when 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 to
      accept a provided OTP value or not.

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







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   otp-service
      Use of this field is OPTIONAL, but MAY be used by the KDC to
      identify the appropriate OTP tokens to be used.  For example, this
      field could be used when a client has multiple OTP tokens from
      different servers.

   otp-keyID
      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.

   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.

4.2.  PA-OTP-REQUEST

   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.

             PA_OTP_REQUEST     << TBA >>

             PA-OTP-REQUEST ::= SEQUENCE {
               flags             OTPFlags,
               nonce             UInt32,
               encData           EncryptedData,
                                 -- PA-OTP-ENC-REQUEST or PA-ENC-TS-ENC
                                 -- Key usage of KEY_USAGE_OTP_REQUEST
               hashAlg           AlgorithmIdentifier OPTIONAL,
               iterationCount    INTEGER         OPTIONAL,
               otp-value         OCTET STRING    OPTIONAL,
               otp-challenge [0] OCTET STRING    OPTIONAL,
               otp-time          KerberosTime    OPTIONAL,
               otp-counter   [1] OCTET STRING    OPTIONAL,
               otp-format    [2] OTPFormat       OPTIONAL,
               otp-keyID     [3] OCTET STRING    OPTIONAL,
               otp-algID         AnyURI          OPTIONAL,
               ...
             }




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             KEY_USAGE_OTP_REQUEST  << TBA >>


             PA-OTP-ENC-REQUEST ::= SEQUENCE {
               nonce     UInt32,
               ...
             }


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

   flags
      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.

   nonce
      A random nonce value generated by the client to be returned
      encrypted by the KDC in the PA-OTP-CONFIRM.

   encData
      This field contains 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 and only if a hash of the OTP value
      was used as input to string-to-key (see Section 3.6) and 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.







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   iterationCount
      This field MUST be present if and only if a hash of the OTP value
      was used as input to string-to-key (see Section 3.6) and 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 that specified in the PA-OTP-
      CHALLENGE.

   otp-value
      The generated OTP value.  This value MUST NOT be present unless
      allowed by the OTP algorithm profile.

   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.

   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 of 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 of 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
      calculation.  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-keyID
      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



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      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 calculation.  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.

4.3.  PA-OTP-CONFIRM

   The padata-type PA_OTP_CONFIRM is returned by the KDC in the enc-
   fast-rep of a PA-FX-FAST-REPLY in the AS-REP of the KDC.  It is used
   to return the client's nonce encrypted under the new Reply Key in
   order to authenticate the KDC and confirm the Reply Key change.

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

             PA_OTP_CONFIRM     << TBA >>

             PA-OTP-CONFIRM ::= SEQUENCE {
               encData        EncryptedData,
                                   -- PA-OTP-ENC-CONFIRM
                                   -- Key usage of KEY_USAGE_OTP_CONFIRM
               ...
             }

             KEY_USAGE_OTP_CONFIRM  << TBA >>


             PA-OTP-ENC-CONFIRM ::= SEQUENCE {
               nonce     UInt32,
               ...
             }

   encData
      An EncryptedData containing a PA-OTP-ENC-CONFIRM containing the
      value of the nonce from the corresponding PA-OTP-REQUEST encrypted
      under the current Reply Key. The key usage SHALL be
      KEY_USAGE_OTP_CONFIRM and the encryption type SHOULD be the same
      as that used by the client in the PA-OTP-REQUEST.







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4.4.  PA-OTP-PIN-CHANGE

   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 or if the user's PIN has been changed.

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

             PA_OTP_PIN_CHANGE     << TBA >>

             PA-OTP-PIN-CHANGE ::= SEQUENCE {
               flags         PinFlags,
               pin           UTF8String OPTIONAL,
               minLength     INTEGER    OPTIONAL,
               maxLength [1] INTEGER    OPTIONAL,
               ...
             }

             PinFlags ::= KerberosFlags
               -- systemSetPin (0)

   If the "systemSetPin" flag is set then the user's PIN has been
   changed and the new PIN value is contained in the pin field.  The pin
   field MUST therefore be present.

   If the "systemSetPin" flag is not set then the user's PIN has not
   been changed by the server but it MUST instead be changed by the
   user.  Restrictions on the size of the PIN MAY be given by the
   minLength and maxLength fields.  If the pin field is present then it
   contains a PIN value that MAY be used by the user when changing the
   PIN.


5.  IANA Considerations

   A registry may be required for the otp-algID values as introduced in
   Section 4.1.  No other IANA actions are anticipated.


6.  Security Considerations

6.1.  Man-in-the-Middle

   In the system described in this document, the OTP pre-authentication
   protocol is tunnelled within the FAST Armor channel provided by the
   pre-authentication framework.  As described in [AsNiNy02], tunneled
   protocols are potentially vulnerable to man-in-the-middle attacks if



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   the outer tunnel is compromised and it is generally considered good
   practice in such cases to bind the inner encryption to the outer
   tunnel.

   Even though no such attacks are known at this point, the proposed
   system uses the outer Armor Key in the derivation of the inner Client
   and Reply keys and so achieve crypto-binding to the outer channel.

6.2.  Reflection

   The 4-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.

6.3.  Replay

   The 2-pass version of the protocol does not involve a server nonce
   and so the client instead encrypts a timestamp.  To reduce the chance
   of replay attacks, the KDC must check that the client time used in
   such a request is later than that used in previous requests.

6.4.  Brute Force Attack

   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.

   As described in Section 3.6, 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.

6.5.  FAST Facilities

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




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7.  Acknowledgments

   Many significant contributions were made to this document by RSA
   employees but special thanks go to Magnus Nystrom, John Linn, 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 and Nicolas Williams.


8.  References

8.1.  Normative References

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, March 2003.

   [RFC3491]  Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
              Profile for Internationalized Domain Names (IDN)",
              RFC 3491, March 2003.

   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
              Kerberos 5", RFC 3961, February 2005.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 2005.

   [ZhHa07]   Znu, L. and S. Hartman, "A generalized Framework for
              Kerberos Pre-Authentication",
              draft-ietf-krb-wg-preauth-framework-08 (work in progress),
              July 2008.

8.2.  Informative References

   [AsNiNy02]
              Asokan, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle
              in Tunneled Authentication Protocols", Cryptology ePrint
              Archive Report 2002/163, November 2002.



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   [HoReNeZo04]
              Horstein, K., Renard, K., Neuman, C., and G. Zorn,
              "Integrating Single-use Authentication Mechanisms with
              Kerberos", draft-ietf-krb-wg-kerberos-sam-03 (work in
              progress), July 2004.

   [RFC2289]  Haller, N., Metz, C., Nesser, P., and M. Straw, "A One-
              Time Password System", RFC 2289, February 1998.

   [RFC2808]  Nystrom, M., "The SecurID(r) SASL Mechanism", RFC 2808,
              April 2000.

   [RFC3244]  Swift, M., Trostle, J., and J. Brezak, "Microsoft Windows
              2000 Kerberos Change Password and Set Password Protocols",
              RFC 3244, February 2002.

   [RFC4226]  M'Raihi, D., Bellare, M., Hoornaert, F., Naccache, D., and
              O. Ranen, "HOTP: An HMAC-Based One-Time Password
              Algorithm", RFC 4226, December 2005.


Appendix A.  ASN.1 Module

   OTPKerberos
   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN

   IMPORTS
       AnyURI
       FROM XSD {joint-iso-itu-t asn1(1) specification(0)
                 modules(0) xsd-module(1)};

       KerberosTime, KerberosFlags, EncryptionKey, UInt32,
       Int32, EncryptedData
       FROM KerberosV5Spec2 {iso(1) identified-organization(3)
                             dod(6) internet(1) security(5)
                             kerberosV5(2) modules(4) krb5spec2(2)}
                             -- as defined in RFC 4120.
       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 RFC 3280.

       PA-OTP-CHALLENGE ::= SEQUENCE {
         flags              OTPFlags,
         nonce              UInt32,



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         etype              Int32,
         supportedHashAlg   SEQUENCE OF AlgorithmIdentifier
                                            OPTIONAL,
         iterationCount     INTEGER         OPTIONAL,
         otp-challenge      OCTET STRING (SIZE(8..MAX)) OPTIONAL,
         otp-length     [0] Int32           OPTIONAL,
         otp-service        UTF8String      OPTIONAL,
         otp-keyID      [1] OCTET STRING    OPTIONAL,
         otp-algID          AnyURI          OPTIONAL,
         ...
       }

       OTPFlags ::= KerberosFlags
       -- nextOTP (0)
       -- combine (1)

       PA-OTP-REQUEST ::= SEQUENCE {
         flags             OTPFlags,
         nonce             UInt32,
         encData           EncryptedData,
                           -- PA-OTP-ENC-REQUEST or PA-ENC-TS-ENC
                           -- Key usage of KEY_USAGE_OTP_REQUEST
         hashAlg           AlgorithmIdentifier OPTIONAL,
         iterationCount    INTEGER         OPTIONAL,
         otp-value         OCTET STRING    OPTIONAL,
         otp-challenge [0] OCTET STRING (SIZE(8..MAX)) OPTIONAL,
         otp-time          KerberosTime    OPTIONAL,
         otp-counter   [1] OCTET STRING    OPTIONAL,
         otp-format    [2] OTPFormat       OPTIONAL,
         otp-keyID     [3] OCTET STRING    OPTIONAL,
         otp-algID         AnyURI          OPTIONAL,
         ...
       }

       PA-OTP-ENC-REQUEST ::= SEQUENCE {
         nonce     UInt32,
         ...
       }

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

       PA-OTP-CONFIRM ::= SEQUENCE {
         encData        EncryptedData,



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                        -- PA-OTP-ENC-CONFIRM
                        -- Key usage of KEY_USAGE_OTP_CONFIRM
         ...
       }

       PA-OTP-ENC-CONFIRM ::= SEQUENCE {
         nonce     UInt32,
         ...
       }

       PA-OTP-PIN-CHANGE ::= SEQUENCE {
         flags         PinFlags,
         pin           UTF8String OPTIONAL,
         minLength     INTEGER    OPTIONAL,
         maxLength [0] INTEGER    OPTIONAL,
         ...
       }

       PinFlags ::= KerberosFlags
       -- systemSetPin (0)

   END


Appendix B.  Examples of OTP Pre-Authentication Exchanges

   This section is non-normative.

B.1.  Four Pass Authentication

   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.  It also requires that 256-bit AES be used to
   encrypt the nonce.  The local policy on the client supports SHA-256
   and requires 100,000 iterations 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.




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   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:

        flags
           0

        nonce
           A randomly generated value.

        etype
           aes256-cts-hmac-sha1-96

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

        iterationCount
           1024

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

   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 obtains the current OTP value from the token and
        records the time as reported by the token.

   8.   The client generates Client Key and Reply Key as described in
        Section 3.6 using SHA-256 from the list of algorithms sent by
        the KDC and the iteration count of 100,000 as required by local
        policy.

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

        flags
           0

        nonce
           A randomly generated value.






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        encData
           An EncryptedData containing a PA-OTP-ENC-REQUEST encrypted
           under the Client Key with a key usage of
           KEY_USAGE_OTP_REQUEST and an encryption type of aes256-cts-
           hmac-sha1-96.  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.

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

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

   12.  The KDC validates the pre-authentication data in the PA-OTP-
        REQUEST:

        *  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 and hash algorithm of SHA-256 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.

   13.  The KDC constructs a TGT for the user.

   14.  The KDC constructs a PA-OTP-CONFIRM as follows:

        encData
           An EncryptedData containing a PA-OTP-ENC-CONFIRM encrypted
           under the Reply Key with a key usage of KEY_USAGE_OTP_CONFIRM
           and an encryption type of aes256-cts-hmac-sha1-96 (the
           encryption type used by the client in the PA-OTP-REQUEST).
           The PA-OTP-ENC-CONFIRM contains the nonce from the PA-OTP-



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           REQUEST.

   15.  The KDC encrypts the PA-OTP-CONFIRM within the enc-fast-rep of a
        PA-FX-FAST-REPLY.

   16.  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.

   17.  The client authenticates the KDC and verifies the Reply Key
        change.

        *  Uses the generated Reply Key to decrypt the PA-OTP-ENC-
           CONFIRM in the encData of the PA-OTP-CONFIRM.

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

B.2.  Two Pass Authentication

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

   In this example, the user has been issued with a hand-held token and
   so none of the OTP generation parameters (otp-length etc) are
   included in the PA-OTP-RESPONSE.  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 encryption type of aes128-cts-hmac-sha1-96 to use to encrypt
      the encData.
   o  The fact that 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.

   2.   The client generates Client Key and Reply Key using unhashed OTP
        values as described in Section 3.6.

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








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        flags
           0

        nonce
           A randomly generated value.

        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 aes128-cts-hmac-sha1-96.  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 pre-authentication data:

        *  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 constructs a PA-OTP-CONFIRM as follows:

        encData
           An EncryptedData containing a PA-OTP-ENC-CONFIRM encrypted
           under the Reply Key with a key usage of KEY_USAGE_OTP_CONFIRM
           and an encryption type of aes128-cts-hmac-sha1-96 (the
           encryption type used by the client in the PA-OTP-REQUEST).
           The PA-OTP-ENC-CONFIRM contains the nonce from the PA-OTP-
           REQUEST.

   9.   The KDC encrypts the PA-OTP-CONFIRM within the enc-fast-rep of a
        PA-FX-FAST-REPLY.

   10.  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.




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   11.  The client authenticates the KDC and verifies the key change.

        *  Uses the generated Reply Key to decrypt the PA-OTP-ENC-
           CONFIRM in the encData of the PA-OTP-CONFIRM.

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

B.3.  Pin Change

   This exchange follows from the point where the KDC receives the PA-
   OTP-REQUEST from the client in the examples in Appendix B.1 and
   Appendix B.2.  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 along with the PA-OTP-CONFIRM
   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 that requires the "initial" bit to be set in the
   service ticket.

   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.   KDC constructs a ticket for a PIN change service with the
        "initial" flag set.

   4.   KDC constructs a PA-OTP-CONFIRM as in the previous examples.

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

        flags
           0








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        minLength
           4

        maxLength
           8

   6.   KDC encrypts the PA-OTP-PIN-CHANGE and PA-OTP-CONFIRM within the
        enc-fast-rep of a PA-FX-FAST-REPLY.

   7.   KDC returns an AS-REP to the client containing the ticket to the
        PIN change service and padata containing the PA-FX-FAST-REPLY.

   8.   The client authenticates the KDC as in the previous examples.

   9.   The client uses the ticket in the AS-REP to call the PIN change
        service and change the user's PIN.

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

   11.  The KDC responds with an AS-REP containing a TGT and a PA-OTP-
        CONFRIM.


B.4.  Resynchronization

   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 re-synchronized 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 Appendix B.1 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 re-synchronizing.

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






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        flags
           nextOTP bit set

        nonce
           A randomly generated value.

        etype
           aes256-cts-hmac-sha1-96

        supportedHashAlg
           AlgorithmIdentifiers for SHA-256 and SHA-1

        iterationCount
           1024

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

   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.

   6.   The client generates Client Key Reply Key as described in
        Section 3.6 using SHA-256 from the list of algorithms sent by
        the KDC and the iteration count of 100,000 as required by local
        policy.

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

        flags
           nextOTP bit set

        nonce
           A 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 an encryption type of aes256-cts-
           hmac-sha1-96.  The PA-OTP-ENC-REQUEST contains the nonce from
           the PA-OTP-CHALLENGE.







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        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.  Authentication process now proceeds as with the standard
        sequence.


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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