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Versions: 00 01 draft-ietf-kitten-krb-spake-preauth

Internet Engineering Task Force                              N. McCallum
Internet-Draft                                                  S. Sorce
Intended status: Standards Track                              R. Harwood
Expires: June 15, 2017                                     Red Hat, Inc.
                                                               G. Hudson
                                                                     MIT
                                                       December 12, 2016


                        SPAKE Pre-Authentication
               draft-mccallum-kitten-krb-spake-preauth-01

Abstract

   This document defines a new pre-authentication mechanism for the
   Kerberos protocol that uses a password authenticated key exchange.
   This document has three goals.  First, increase the security of
   Kerberos pre-authentication exchanges by making offline brute-force
   attacks infeasible.  Second, enable the use of secure second factor
   authentication without relying on FAST.  This is achived using the
   existing trust relationship established by the shared first factor.
   Third, make Kerberos pre-authentication more resilient against time
   synchronization errors by removing the need to transfer an encrypted
   timestamp from the client.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 15, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Properties of PAKE  . . . . . . . . . . . . . . . . . . .   3
     1.2.  Which PAKE? . . . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  PAKE and Two-Factor Authentication  . . . . . . . . . . .   4
     1.4.  SPAKE Overview  . . . . . . . . . . . . . . . . . . . . .   5
   2.  Document Conventions  . . . . . . . . . . . . . . . . . . . .   5
   3.  Prerequisites . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  PA-ETYPE-INFO2  . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Cookie Support  . . . . . . . . . . . . . . . . . . . . .   6
     3.3.  More Pre-Authentication Data Required . . . . . . . . . .   6
   4.  SPAKE Pre-Authentication Message Protocol . . . . . . . . . .   6
     4.1.  First Pass  . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Second Pass . . . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Third Pass  . . . . . . . . . . . . . . . . . . . . . . .   8
     4.4.  Subsequent Passes . . . . . . . . . . . . . . . . . . . .   9
     4.5.  Reply Key Strengthening . . . . . . . . . . . . . . . . .  10
     4.6.  Optimizations . . . . . . . . . . . . . . . . . . . . . .  10
   5.  SPAKE Parameters and Conversions  . . . . . . . . . . . . . .  10
   6.  Transcript Checksum . . . . . . . . . . . . . . . . . . . . .  11
   7.  Key Derivation  . . . . . . . . . . . . . . . . . . . . . . .  12
   8.  Second Factor Types . . . . . . . . . . . . . . . . . . . . .  12
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   10. Assigned Constants  . . . . . . . . . . . . . . . . . . . . .  16
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     11.1.  Kerberos Second Factor Types . . . . . . . . . . . . . .  16
       11.1.1.  Registration Template  . . . . . . . . . . . . . . .  16
       11.1.2.  Initial Registry Contents  . . . . . . . . . . . . .  17
     11.2.  Kerberos SPAKE Groups  . . . . . . . . . . . . . . . . .  17
       11.2.1.  Registration Template  . . . . . . . . . . . . . . .  17
       11.2.2.  Initial Registry Contents  . . . . . . . . . . . . .  18
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     12.2.  Non-normative References . . . . . . . . . . . . . . . .  20
   Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .  21
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22



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1.  Introduction

   The most widely deployed Kerberos pre-authentication method, PA-ENC-
   TIMESTAMP, encrypts a timestamp in the client principal's long-term
   secret.  When a client uses this method, a passive attacker can
   perform an offline brute-force attack against the transferred
   ciphertext.  When the client principal's long-term key is based on a
   password, especially a weak password, offline dictionary attacks can
   successfully recover the key.

1.1.  Properties of PAKE

   Password authenticated key exchange (PAKE) algorithms provide several
   properties which are useful to overcome this problem and make them
   ideal for use as a Kerberos pre-authentication mechanism.

   1.  Each side of the exchange contributes entropy.

   2.  Passive attackers cannot determine the shared key.

   3.  Active attackers cannot perform a man-in-the-middle attack.

   4.  Either side can store a password or password equivalent.

   These properties of PAKE allow us to establish high-entropy
   encryption keys resistant to offline brute force attack, even when
   the passwords used are weak (low-entropy).

1.2.  Which PAKE?

   Diffie-Hellman Encrypted Key Exchange (DH-EKE) is the earliest widely
   deployed PAKE.  It works by encrypting the public keys of a Diffie-
   Hellman key exchange with a shared secret.  However, it requires both
   that unauthenticated encryption be used and that the public keys be
   indistinguishable from random data.  This last requirement makes it
   impossible to use this form of PAKE with elliptic curve cryptography.
   For these reasons, DH-EKE is not a good fit.

   Password authenticated key exchange by juggling (JPAKE) permits the
   use of elliptic curve cryptography.  However, it too has drawbacks.
   First, the additional computation required for the algorithm makes it
   resource intensive for servers under load.  Second, it requires an
   additional network round-trip, increasing latency and load on the
   network.

   SPAKE is a variant of the technique used by DH-EKE which ensures that
   all public key encryption and decryption operations result in a
   member of the underlying group.  This property allows SPAKE to be



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   used with elliptic curve cryptography, permitting the use of markedly
   smaller key sizes with equivalent security to a finite-field Diffie-
   Hellman key exchange.  Additionally, SPAKE can complete the key
   exchange in just a single round-trip.  These properties make SPAKE an
   ideal PAKE to use for Kerberos pre-authentication.

1.3.  PAKE and Two-Factor Authentication

   Using PAKE in a pre-authentication mechanism also has another benefit
   when coupled with two-factor authentication (2FA). 2FA methods often
   require the secure transfer of plaintext material to the KDC for
   verification.  This includes one-time passwords, challenge/response
   signatures and biometric data.  Attempting to encrypt this data using
   the long-term secret results in packets that are vulnerable to
   offline brute-force attack if either authenticated encryption is used
   or if the plaintext is distinguishable from random data.  This is a
   problem that PAKE solves for first factor authentication.  So a
   similar technique can be used with PAKE to encrypt second-factor
   data.

   In the OTP pre-authentication [RFC6560] specification, this problem
   has been mitigated by using FAST, which uses a secondary trust
   relationship to create a secure encryption channel within which pre-
   authentication data can be sent.  However, the requirement for a
   secondary trust relationship has proven to be cumbersome to deploy
   and often introduces third parties into the trust chain (such as
   certification authorities).  These requirements lead to a scenario
   where FAST cannot be enabled by default without sufficient
   configuration.  SPAKE pre-authentication, instead, can create a
   secure encryption channel implicitly, using the key exchange to
   negotiate a high-entropy encryption key.  This key can then be used
   to securely encrypt 2FA plaintext data without the need for a
   secondary trust relationship.  Further, if the second factor
   verifiers are sent at the same time as the first factor verifier, and
   the KDC is careful to prevent timing attacks, then an online brute-
   force attack cannot be used to attack the factors separately.

   For these reasons, this draft departs from the advice given in
   Section 1 of RFC 6113 [RFC6113] which states that "Mechanism
   designers should design FAST factors, instead of new pre-
   authentication mechanisms outside of FAST."  However, this pre-
   authentication mechanism does not intend to replace FAST, and may be
   used with it to further conceal the metadata of the Kerberos
   messages.







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1.4.  SPAKE Overview

   The SPAKE algorithm can be broadly described in a series of four
   steps:

   1.  Calculation and exchange of the public key

   2.  Calculation of the shared secret (K)

   3.  Derivation of an encryption key (K')

   4.  Verification of the derived encryption key (K')

   Higher level protocols must define their own verification step.  In
   the case of this mechanism, verification happens implicitly by a
   successful decryption of the 2FA data.

   This mechanism also provides its own method of deriving encryption
   keys from the calculated shared secret K, for several reasons: to fit
   within the framework of [RFC3961], to ensure negotiation integrity
   using a transcript hash, to derive different keys for each use, and
   to bind the KDC-REQ-BODY to the pre-authentication exchange.

2.  Document Conventions

   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 refers to numerous terms and protocol messages defined
   in [RFC4120].

   The terms "encryption type", "required checksum mechanism", and
   "get_mic" are defined in [RFC3961].

   The terms "FAST", "PA-FX-COOKIE", "KDC_ERR_PREAUTH_EXPIRED",
   "KDC_ERR_MORE_PREAUTH_DATA_REQUIRED", "pre-authentication facility",
   and "authentication set" are defined in [RFC6113].

   The [SPAKE] paper defines SPAKE as a family of two key exchange
   algorithms differing only in derivation of the final key.  This
   mechanism uses a derivation similar to the second algorithm (SPAKE2)
   with differences in detail.  For simplicity, this document refers to
   the algorithm as "SPAKE".  The normative reference for this algorithm
   is [I-D.irtf-cfrg-spake2].

   The terms "ASN.1" and "DER" are defined in [CCITT.X680.2002] and
   [CCITT.X690.2002].



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3.  Prerequisites

3.1.  PA-ETYPE-INFO2

   This mechanism requires the initial KDC pre-authentication state to
   contain a singular reply key.  Therefore, a KDC which offers SPAKE
   pre-authentication as a stand-alone mechanism MUST supply a PA-ETYPE-
   INFO2 value containing a single ETYPE-INFO2-ENTRY, as described in
   [RFC6113] section 2.1.

3.2.  Cookie Support

   KDCs which implement SPAKE pre-authentication MUST have some secure
   mechanism for retaining state between AS-REQs.  For stateless KDC
   implementations, this method will most commonly be an encrypted PA-
   FX-COOKIE.  Clients which implement SPAKE pre-authentication MUST
   support PA-FX-COOKIE.

3.3.  More Pre-Authentication Data Required

   Both KDCs and clients which implement SPAKE pre-authentication MUST
   support the use of KDC_ERR_MORE_PREAUTH_DATA_REQUIRED.

4.  SPAKE Pre-Authentication Message Protocol

   This mechanism uses the reply key and provides the Client
   Authentication and Strengthening Reply Key pre-authentication
   facilities.  When the mechanism completes successfully, the client
   will have proved knowledge of the original reply key and possibly a
   second factor, and the reply key will be strengthened to a more
   uniform distribution based on the PAKE exchange.  This mechanism also
   ensures the integrity of the KDC-REQ-BODY contents.  This mechanism
   can be used in an authentication set; no pa-hint value is required or
   defined.

   This section will describe the flow of messages when performing SPAKE
   pre-authentication.  We will begin by explaining the most verbose
   version of the protocol which all implementations MUST support.  Then
   we will describe several optional optimizations to reduce round-
   trips.

   Mechanism messages are communicated using PA-DATA elements within the
   padata field of KDC-REQ messages or within the METHOD-DATA in the
   e-data field of KRB-ERROR messages.  All PA-DATA elements for this
   mechanism MUST use the following padata-type:

   PA-SPAKE  TBD




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   The padata-value for all PA-SPAKE PA-DATA values MUST be empty or
   contain a DER encoding for the ASN.1 type PA-SPAKE.

   PA-SPAKE ::= CHOICE {
       support     [0] SPAKESupport,
       challenge   [1] SPAKEChallenge,
       response    [2] SPAKEResponse,
       encdata     [3] EncryptedData,
       ...
   }

4.1.  First Pass

   The SPAKE pre-authentication exchange begins when the client sends an
   initial authentication service request (AS-REQ) without pre-
   authentication data.  Upon receipt of this AS-REQ, a KDC which
   requires pre-authentication and supports SPAKE SHOULD reply with a
   KDC_ERR_PREAUTH_REQUIRED error, with METHOD-DATA containing an empty
   PA-SPAKE PA-DATA element.  This message indicates to the client that
   the KDC supports SPAKE pre-authentication.

4.2.  Second Pass

   Once the client knows that the KDC supports SPAKE pre-authentication
   and the client desires to use it, the client will generate a new AS-
   REQ message containing a PA-SPAKE PA-DATA element using the support
   choice.  This message indicates to the KDC which groups the client
   prefers for the SPAKE operation.  The group numbers are defined in
   the IANA "Kerberos SPAKE Groups" registry created by this document.
   The groups sequence is ordered from the most preferred group to the
   least preferred group.

   SPAKESupport ::= SEQUENCE {
       groups      [0] SEQUENCE (SIZE(1..MAX)) OF Int32,
       ...
   }

   The client and KDC initialize a transcript checksum (Section 6) and
   update it with the DER-encoded PA-SPAKE message.

   Upon receipt of the support message, the KDC will select a group.
   The KDC SHOULD choose a group from the groups provided by the support
   message.  However, if the support message does not contain any group
   that is supported by the KDC, the KDC MAY select another group in
   hopes that the client might support it.

   Once the KDC has selected a group, the KDC will reply to the client
   with a KDC_ERR_MORE_PREAUTH_DATA_REQUIRED error containing a PA-SPAKE



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   PA-DATA element using the challenge choice.  The client and KDC
   update the transcript checksum with the DER-encoded PA-SPAKE message.

   SPAKEChallenge ::= SEQUENCE {
       group       [0] Int32,
       pubkey      [1] OCTET STRING,
       factors     [2] SEQUENCE (SIZE(1..MAX)) OF SPAKESecondFactor,
       ...
   }

   The group field indicates the KDC-selected group used for all SPAKE
   calculations as defined in the IANA "Kerberos SPAKE Groups" registry
   created by this document.

   The pubkey field indicates the KDC's public key generated using the M
   constant in the SPAKE algorithm, with inputs and conversions as
   specified in Section 5.

   The factors field contains an unordered list of second factors which
   can be used to complete the authentication.  Each second factor is
   represented by a SPAKESecondFactor.

   SPAKESecondFactor ::= SEQUENCE {
       type        [0] Int32,
       data        [1] OCTET STRING OPTIONAL
   }

   The type field is a unique integer which identifies the second factor
   type.  The factors field of SPAKEChallenge MUST NOT contain more than
   one SPAKESecondFactor with the same type value.

   The data field contains optional challenge data.  The contents in
   this field will depend upon the second factor type chosen.

4.3.  Third Pass

   Upon receipt of the challenge message, the client will complete its
   part of of the SPAKE process, resulting in the shared secret K.

   Next, the client chooses one of the second factor types listed in the
   factors field of the challenge message and gathers whatever data is
   required for this second factor type; possibly using the challenge
   data for this second factor type.  Finally, the client sends an AS-
   REQ containing a PA-SPAKE PA-DATA element using the response choice.







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   SPAKEResponse ::= SEQUENCE {
       pubkey      [0] OCTET STRING,
       factor      [1] EncryptedData, -- SPAKESecondFactor
       ...
   }

   The client and KDC update the transcript checksum with the pubkey
   value, and use the resulting checksum for all encryption key
   derivations.

   The pubkey field indicates the client's public key generated using
   the N constant in the SPAKE algorithm, with inputs and conversions as
   specified in Section 5.

   The factor field indicates the client's chosen second factor data.
   The key for this field is K'[1] as specified in Section 7.  The key
   usage number for the encryption is KEY_USAGE_SPAKE_FACTOR.  The plain
   text inside the EncryptedData is an encoding of SPAKESecondFactor.
   Once decoded, the SPAKESecondFactor contains the type of the second
   factor and any optional data used.  The contents of the data field
   will depend on the second factor type chosen.  The client MUST NOT
   send a response containing a second factor type which was not listed
   in the factors field of the challenge message.

   When the KDC receives the response message from the client, it will
   use the pubkey to compute the SPAKE result, derive K'[1], and decrypt
   the factors field.  If decryption is successful, the first factor is
   successfully validated.  The KDC then validates the second factor.
   If either factor fails to validate, the KDC responds with an
   appropriate KRB-ERROR message.

   If validation of the second factor requires further round-trips, the
   KDC MUST reply to the client with KDC_ERR_MORE_PREAUTH_DATA_REQUIRED
   containing a PA-SPAKE PA-DATA element using the encdata choice.  The
   key for the EncryptedData value is K'[2] as specified in Section 7,
   and the key usage number is KEY_USAGE_SPAKE_FACTOR.  The plain text
   of this message contains a DER-encoded SPAKESecondFactor message.  As
   before, the type field of this message will contain the second factor
   type, and the data field will optionally contain second factor type
   specific data.

   KEY_USAGE_SPAKE_FACTOR                  TBD

4.4.  Subsequent Passes

   Any number of additional round trips may occur using the encdata
   choice.  The contents of the plaintexts are specific to the second
   factor type.  If a client receives a PA-SPAKE PA-DATA element using



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   the encdata choice from the KDC, it MUST reply with a subsequent AS-
   REQ with a PA-SPAKE PA-DATA using the encdata choice, or abort the AS
   exchange.

   The key for client-originated encdata messages in subsequent passes
   is K'[3] as specified in Section 7 for the first subsequent pass,
   K'[5] for the second, and so on.  The key for KDC-originated encdata
   messages is K'[4] for the first subsequent pass, K'[6] for the
   second, and so on.

4.5.  Reply Key Strengthening

   When the KDC has successfully validated both factors, the reply key
   is strengthened and the mechanism is complete.  To strengthen the
   reply key, the client and KDC replace it with K'[0] as specified in
   Section 7.  The KDC then replies with a KDC-REP message, or continues
   on to the next mechanism in the authentication set.  There is no
   final PA-SPAKE PA-DATA message from the KDC to the client.

   Reply key strengthening occurs only once at the end of the exchange.
   The client and KDC MUST use the initial reply key as the base key for
   all K'[n] derivations.

4.6.  Optimizations

   The full protocol has two possible optimizations.

   First, the KDC MAY reply to the initial AS-REQ (containing no pre-
   authentication data) with a PA-SPAKE PA-DATA element using the
   challenge choice, instead of an empty padata-value.  In this case,
   the KDC optimistically selects a group which the client may not
   support.  If the group chosen by the challenge message is supported
   by the client, the client MUST skip to the third pass by issuing an
   AS-REQ with a PA-SPAKE message using the response choice.  If the
   KDC's chosen group is not supported by the client, the client MUST
   initialize and update the transcript hash with the KDC's challenge
   message, and then continue to the second pass.  Clients MUST support
   this optimization.

   Second, clients MAY skip the first pass and send an AS-REQ with a PA-
   SPAKE PA-DATA element using the support choice.  KDCs MUST support
   this optimization.

5.  SPAKE Parameters and Conversions

   Group elements are converted to octet strings for the SPAKEChallenge
   and SPAKEResponse pubkey fields and for key derivation using the




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   serialization method defined in the IANA "Kerberos SPAKE Groups"
   registry created by this document.

   The SPAKE algorithm requires constants M and N for each group.  These
   constants are defined in the IANA "Kerberos SPAKE Groups" registry
   created by this document.

   The SPAKE algorithm requires a shared secret input w to be used as a
   scalar multiplier (see [I-D.irtf-cfrg-spake2] section 2).  This value
   MUST be produced from the initial reply key as follows:

   1.  Determine the length of the multiplier octet string defined in
       the IANA "Kerberos SPAKE Groups" registry created by this
       document.

   2.  Produce an octet string of the above length using PRF+(K,
       "SPAKEsecret"), where K is the initial reply key and PRF+ is
       defined in [RFC6113] section 5.1.

   3.  Convert the octet array to a multiplier scalar using the
       multiplier conversion method defined in the IANA "Kerberos SPAKE
       Groups" registry created by this document.

   The KDC chooses a secret scalar value x and the client chooses a
   secret scalar value y.  As required by the SPAKE algorithm, these
   values are chosen randomly and uniformly.  The KDC and client MUST
   NOT reuse x or y values for authentications involving different
   initial reply keys.

6.  Transcript Checksum

   The transcript checksum is an octet string of length equal to the
   output length of the required checksum type of the encryption type of
   the initial reply key.  It initially contains all zero values.

   When the transcript checksum is updated with an octet string input,
   the new value is the get_mic result computed over the concatenation
   of the old value and the input, for the required checksum type of the
   initial reply key's encryption type, using the initial reply key and
   the key usage number KEY_USAGE_SPAKE_TRANSCRIPT.

   In the normal message flow or with the second optimization described
   in Section 4.6, the transcript checksum is first updated with the
   client's support message, then the KDC's challenge message, and
   finally with the client's pubkey value.  It therefore incorporates
   the client's supported groups, the KDC's chosen group, the KDC's
   initial second-factor messages, and the client and KDC public values.




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   If the first optimization described in Section 4.6 is used
   successfully, the transcript checksum is updated only with the KDC's
   challenge message and the client's pubkey value.

   If first optimization is used unsuccessfully (i.e. the client does
   not accept the KDC's selected group), the transcript checksum is
   updated with the KDC's optimistic challenge message, then with the
   client's support message, then the KDC's second challenge message,
   and finally with the client's pubkey value.

   KEY_USAGE_SPAKE_TRANSCRIPT              TBD

7.  Key Derivation

   Implementations MUST NOT use the SPAKE result (denoted by K in
   Section 2 of SPAKE [I-D.irtf-cfrg-spake2]) directly for any
   cryptographic operation.  Instead, the SPAKE result is used to derive
   keys K'[n] as defined in this section.  This method differs slightly
   from the method used to generate K' in Section 3 of SPAKE
   [I-D.irtf-cfrg-spake2].

   A PRF+ input string is assembled by concatenating the following
   values:

   o  The fixed string "SPAKEKey".

   o  The SPAKE result K, converted to an octet string as specified in
      Section 5.

   o  The transcript checksum.

   o  The KDC-REQ-BODY encoding for the request being sent or responded
      to.  Within a FAST channel, the inner KDC-REQ-BODY encoding MUST
      be used.

   o  The value n as a big-endian four-byte unsigned binary number.

   The derived key K'[n] has the same encryption type as the initial
   reply key, and has the value random-to-key(PRF+(initial-reply-key,
   input-string)).  PRF+ is defined in [RFC6113] section 5.1.

8.  Second Factor Types

   This document defines one second factor type:

   SF-NONE  1





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   This second factor type indicates that no second factor is used.
   Whenever a SPAKESecondFactor is used with SF-NONE, the data field
   MUST be omitted.  The SF-NONE second factor always successfully
   validates.

9.  Security Considerations

   All of the security considerations from SPAKE [I-D.irtf-cfrg-spake2]
   apply here as well.

   This mechanism includes unauthenticated plaintext in the support and
   challenge messages.  Beginning with the third pass, the integrity of
   this plaintext is ensured by incorporating the transcript checksum
   into the derivation of the final reply key and second factor
   encryption keys.  Downgrade attacks on support and challenge messages
   will result in the client and KDC deriving different reply keys and
   EncryptedData keys.  The KDC-REQ-BODY contents are also incorporated
   into key derivation, ensuring their integrity.  The unauthenticated
   plaintext in the KDC-REP message is not protected by this mechanism.

   Unless FAST is used, the factors field of a challenge message is not
   integrity-protected until the response is verified.  Second factor
   types MUST account for this when specifying the semantics of the data
   field.  Second factor data in the challenge should not be included in
   user prompts, as it could be modified by an attacker to contain
   misleading or offensive information.

   Subsequent factor data, including the data in the response, are
   encrypted in a derivative of the shared secret K.  Therefore, it is
   not possible to exploit the untrustworthiness of challenge to turn
   the client into an encryption or signing oracle, unless the attacker
   knows the client's long-term key.

   An implementation of this pre-authentication mechanism can have the
   property of indistinguishability, meaning that an attacker who
   guesses a long-term key and a second factor value cannot determine
   whether one of the factors was correct unless both are correct.
   Indistinguishability is only maintained if the second factor can be
   validated solely based on the data in the response; the use of
   additional round trips will reveal to the attacker whether the long-
   term key is correct.  Indistinguishability also requires that there
   are no side channels.  When processing a response message, whether or
   not the KDC successfully decrypts the factor field, it must reply
   with the same error fields, take the same amount of time, and make
   the same observable communications to other servers.






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   Given that each EncryptedData will begin a new encryption context, a
   failure to properly derive the encryption keys will result in a
   situation where the risk of compromise is non-negligible.

   Weak checksums present a risk to the transcript hash.  Any etype with
   a checksum based on one of the following algorithms MUST NOT be used:

   o  CRC32

   o  MD4

   o  MD5

   Both the size of the EncryptedData and the number of EncryptedData
   messages may reveal information about the second factor used in an
   authentication.  Care should be taken to keep second factor messages
   as small and as few as possible.

   A stateless KDC implementation generally must use a PA-FX-COOKIE
   value to remember its private scalar value x and the transcript
   checksum.  The KDC MUST maintain confidentiality and integrity of the
   cookie value, perhaps by encrypting it in a key known only to the
   realm's KDCs.  Cookie values may be replayed by attackers.  The KDC
   SHOULD limit the time window of replays using a timestamp, and SHOULD
   prevent cookie values from being applied to other pre-authentication
   mechanisms or other client principals.  Within the validity period of
   a cookie, an attacker can replay the final message of a pre-
   authentication exchange to any of the realm's KDCs and make it appear
   that the client has authenticated.

   Any side channels in the creation of the shared secret input w, or in
   the multiplications wM and wN, could allow an attacker to recover the
   client long-term key.  Implementations MUST take care to avoid side
   channels, particularly timing channels.  Generation of the secret
   scalar values x and y need not take constant time, but the amount of
   time taken MUST NOT provide information about the resulting value.

   If an x or y value is reused for pre-authentications involving two
   different client long-term keys, an attacker who observes both
   authentications and knows one of the long-term keys can conduct an
   offline dictionary attack to recover the other one.

   This pre-authentication mechanism is not designed to provide forward
   secrecy.  Nevertheless, some measure of forward secrecy may result
   depending on implementation choices.  A passive attacker who
   determines the client long-term key after the exchange generally will
   not be able to recover the ticket session key; however, an attacker
   who also determines the PA-FX-COOKIE encryption key (if the KDC uses



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   an encrypted cookie) will be able to recover the ticket session key.
   The KDC can mitigate this risk by periodically rotating the cookie
   encryption key.  If the KDC or client retains the x or y value for
   reuse with the same client long-term key, an attacker who recovers
   the x or y value and the long-term key will be able to recover the
   ticket session key.

   Although this pre-authentication mechanism is designed to prevent an
   offline dictionary attack by an active attacker posing as the KDC,
   such an attacker can attempt to downgrade the client to encrypted
   timestamp.  Client implementations SHOULD provide a configuration
   option to disable encrypted timestamp on a per-realm basis to
   mitigate this attack.

   Like any other pre-authentication mechanism using the client long-
   term key, this pre-authentication mechanism does not prevent online
   password guessing attacks.  The KDC is made aware of unsuccessful
   guesses, and can apply facilities such as password lockout to
   mitigate the risk of online attacks.

   Elliptic curve group operations are more computationally expensive
   than secret-key operations.  As a result, the use of this mechanism
   may affect the KDC's performance under normal load and its resistance
   to denial of service attacks.

   The selected group's resistance to offline brute-force attacks may
   not correspond to the brute-force resistance of the secret key
   encryption type.  For performance reasons, a KDC MAY select a group
   whose brute-force resistance is weaker than the secret key.  A
   passive attacker who solves the group discrete logarithm problem
   after the exchange will be able to conduct an offline attack against
   the client long-term key.  Although the use of password policies and
   costly, salted string-to-key functions may increase the cost of such
   an attack, the resulting cost will likely not be higher than the cost
   of solving the group discrete logarithm.

   This mechanism does not directly provide the KDC Authentication pre-
   authentication facility, because it does not send a key confirmation
   from the KDC to the client.  When used as a stand-alone mechanism,
   the traditional KDC authentication provided by the KDC-REP enc-part
   still applies.

   The conversion of the scalar multiplier for the SPAKE w parameter may
   produce a multiplier that is larger than the order of the group.
   Some group implementations may be unable to handle such a multiplier.
   Others may silently accept such a multiplier, but proceed to perform
   multiplication that is not constant time.  This is a minor risk in
   all known groups, but is a major risk for P-521 due to the extra



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   seven high bits in the input octet string.  A common solution to this
   problem is achieved by reducing the multiplier modulo the group
   order, taking care to ensure constant time operation.

10.  Assigned Constants

   The following key usage values are assigned for this mechanism:

   KEY_USAGE_SPAKE_TRANSCRIPT              TBD
   KEY_USAGE_SPAKE_FACTOR                  TBD

11.  IANA Considerations

   This document establishes two registries with the following
   procedure, in accordance with [RFC5226]:

   Registry entries are to be evaluated using the Specification Required
   method.  All specifications must be be published prior to entry
   inclusion in the registry.  There will be a three-week review period
   by Designated Experts on the krb5-spake-review@ietf.org mailing list.
   Prior to the end of the review, the Designated Experts must approve
   or deny the request.  This decision is to be conveyed to both the
   IANA and the list, and should include reasonably detailed explanation
   in the case of a denial as well as whether the request can be
   resubmitted.

11.1.  Kerberos Second Factor Types

   This section species the IANA "Kerberos Second Factor Types"
   registry.  This registry records the number, name, implementation
   requirements and reference for each second factor protocol.

11.1.1.  Registration Template

   ID Number:  This is a value that uniquely identifies this entry.  It
      is a signed integer in range -2147483648 to 2147483647, inclusive.
      Positive values must be assigned only for algorithms specified in
      accordance with these rules for use with Kerberos and related
      protocols.  Negative values should be used for private and
      experimental algorithms only.  Zero is reserved and must not be
      assigned.

   Name:  Brief, unique, human-readable name for this algorithm.

   Implementation Requirements:  The second factor implementation
      requirements, which must be one of the words Required,
      Recommended, Optional, Deprecated, or Prohibited.




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   Reference:  URI or otherwise unique identifier for where the details
      of this algorithm can be found.  It should be as specific as
      reasonably possible.

11.1.2.  Initial Registry Contents

   o ID Number: 1
   o Name: NONE
   o Implementation Requirements: Required
   o Reference: this draft.

11.2.  Kerberos SPAKE Groups

   This section specifies the IANA "Kerberos SPAKE Groups" registry.
   This registry records the number, name, implementation requirements,
   specification, serialization, multiplier length, multiplier
   conversion, SPAKE M constant and SPAKE N constant.

11.2.1.  Registration Template

   ID Number:  This is a value that uniquely identifies this entry.  It
      is a signed integer in range -2147483648 to 2147483647, inclusive.
      Positive values must be assigned only for algorithms specified in
      accordance with these rules for use with Kerberos and related
      protocols.  Negative values should be used for private and
      experimental use only.  Zero is reserved and must not be assigned.
      Values should be assigned in increasing order.

   Name:  Brief, unique, human readable name for this entry.

   Implementation Requirements:  The group implementation requirements,
      which must be one of the words Required, Recommended, Optional,
      Deprecated, or Prohibited.

   Specification:  Reference to the definition of the group parameters
      and operations.

   Serialization:  Reference to the definition of the method used to
      serialize group elements.

   Multiplier Length:  The length of the input octet string to
      multiplication operations.

   Multiplier Conversion:  Reference to the definition of the method
      used to convert an octet string to a multiplier scalar.

   SPAKE M Constant:  The serialized value of the SPAKE M constant in
      hexadecimal notation.



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   SPAKE N Constant:  The serialized value of the SPAKE N constant in
      hexadecimal notation.

11.2.2.  Initial Registry Contents

   o ID Number: 1
   o Name: P-256
   o Implementation Requirements: Required
   o Specification: [SEC2] section 2.4.2
   o Serialization: [SEC1] section 2.3.3 (compressed).
   o Multiplier Length: 32
   o Multiplier Conversion: [SEC1] section 2.3.8.
   o SPAKE M Constant:
     02886e2f97ace46e55ba9dd7242579f2993b64e16ef3dcab95afd497333d8fa12f
   o SPAKE N Constant:
     03d8bbd6c639c62937b04d997f38c3770719c629d7014d49a24b4f98baa1292b49

   o ID Number: 2
   o Name: P-384
   o Implementation Requirements: Optional
   o Specification: [SEC2] section 2.5.1
   o Serialization: [SEC1] section 2.3.3 (compressed).
   o Multiplier Length: 48
   o Multiplier Conversion: [SEC1] section 2.3.8.
   o SPAKE M Constant:
     030ff0895ae5ebf6187080a82d82b42e2765e3b2f8749c7e05eba3664
     34b363d3dc36f15314739074d2eb8613fceec2853
   o SPAKE N Constant:
     02c72cf2e390853a1c1c4ad816a62fd15824f56078918f43f922ca215
     18f9c543bb252c5490214cf9aa3f0baab4b665c10

   o ID Number: 3
   o Name: P-521
   o Implementation Requirements: Optional
   o Specification: [SEC2] section 2.6.1
   o Serialization: [SEC1] section 2.3.3 (compressed).
   o Multiplier Length: 66
   o Multiplier Conversion: [SEC1] section 2.3.8.
   o SPAKE M Constant:
     02003f06f38131b2ba2600791e82488e8d20ab889af753a41806c5db1
     8d37d85608cfae06b82e4a72cd744c719193562a653ea1f119eef9356907edc9b5
     6979962d7aa
   o SPAKE N Constant:
     0200c7924b9ec017f3094562894336a53c50167ba8c5963876880542b
     c669e494b2532d76c5b53dfb349fdf69154b9e0048c58a42e8ed04cef052a3bc34
     9d95575cd25





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12.  References

12.1.  Normative References

   [CCITT.X680.2002]
              International Telephone and Telegraph Consultative
              Committee, "Abstract Syntax Notation One (ASN.1):
              Specification of basic notation", CCITT Recommendation
              X.680, July 2002.

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

   [I-D.irtf-cfrg-spake2]
              Ladd, W., "SPAKE2, a PAKE", draft-irtf-cfrg-spake2-01
              (work in progress), February 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
              Kerberos 5", RFC 3961, DOI 10.17487/RFC3961, February
              2005, <http://www.rfc-editor.org/info/rfc3961>.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              DOI 10.17487/RFC4120, July 2005,
              <http://www.rfc-editor.org/info/rfc4120>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC6113]  Hartman, S. and L. Zhu, "A Generalized Framework for
              Kerberos Pre-Authentication", RFC 6113,
              DOI 10.17487/RFC6113, April 2011,
              <http://www.rfc-editor.org/info/rfc6113>.

   [SEC1]     Standards for Efficient Cryptography Group, "SEC 1:
              Elliptic Curve Cryptography", May 2009.




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   [SEC2]     Standards for Efficient Cryptography Group, "SEC 2:
              Recommended Elliptic Curve Domain Parameters", January
              2010.

12.2.  Non-normative References

   [RFC6560]  Richards, G., "One-Time Password (OTP) Pre-
              Authentication", RFC 6560, DOI 10.17487/RFC6560, April
              2012, <http://www.rfc-editor.org/info/rfc6560>.

   [SPAKE]    Abdalla, M. and D. Pointcheval, "Simple Password-Based
              Encrypted Key Exchange Protocols", February 2005.







































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Appendix A.  ASN.1 Module

   KerberosV5SPAKE {
           iso(1) identified-organization(3) dod(6) internet(1)
           security(5) kerberosV5(2) modules(4) spake(8)
   } DEFINITIONS EXPLICIT TAGS ::= BEGIN

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

   SPAKESupport ::= SEQUENCE {
       groups      [0] SEQUENCE (SIZE(1..MAX)) OF Int32,
       ...
   }

   SPAKEChallenge ::= SEQUENCE {
       group       [0] Int32,
       pubkey      [1] OCTET STRING,
       factors     [2] SEQUENCE (SIZE(1..MAX)) OF SPAKESecondFactor,
       ...
   }

   SPAKESecondFactor ::= SEQUENCE {
       type        [0] Int32,
       data        [1] OCTET STRING OPTIONAL
   }

   SPAKEResponse ::= SEQUENCE {
       pubkey      [0] OCTET STRING,
       factor      [1] EncryptedData, -- SPAKESecondFactor
       ...
   }

   PA-SPAKE ::= CHOICE {
       support     [0] SPAKESupport,
       challenge   [1] SPAKEChallenge,
       response    [2] SPAKEResponse,
       encdata     [3] EncryptedData,
       ...
   }

   END





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Appendix B.  Acknowledgements

     Nico Williams (Cryptonector)
     Tom Yu (MIT)

Authors' Addresses

   Nathaniel McCallum
   Red Hat, Inc.

   EMail: npmccallum@redhat.com


   Simo Sorce
   Red Hat, Inc.

   EMail: ssorce@redhat.com


   Robbie Harwood
   Red Hat, Inc.

   EMail: rharwood@redhat.com


   Greg Hudson
   MIT

   EMail: ghudson@mit.edu






















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