OPSAWG                                                    J. Merkle, Ed.
Internet-Draft                                 Secunet Security Networks
Intended status: Informational                                M. Lochter
Expires: June 15, July 23, 2015                                               BSI
                                                       December 12, 2014
                                                        January 19, 2015

          HMAC-SHA-2 Authentication Protocols in USM for SNMP
                draft-ietf-opsawg-hmac-sha-2-usm-snmp-00
                draft-ietf-opsawg-hmac-sha-2-usm-snmp-01

Abstract

   This memo specifies new HMAC-SHA-2 authentication protocols for the
   User-based Security Model (USM) for SNMPv3 defined in RFC 3414.

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
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   This Internet-Draft will expire on June 15, July 23, 2015.

Copyright Notice

   Copyright (c) 2014 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  The Internet-Standard Management Framework  . . . . . . . . .   3
   3.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  The HMAC-SHA-2 Authentication Protocols . . . . . . . . . . .   3
     4.1.  Deviations from the HMAC-SHA-96     Authentication
           Protocol  . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Processing  . . . . . . . . . . . . . . . . . . . . . . .   5
       4.2.1.  Processing an Outgoing Message  . . . . . . . . . . .   5
       4.2.2.  Processing an Incoming Message  . . . . . . . . . . .   6
   5.  Key Localization and Key Change . . . . . . . . . . . . . . .   6
   6.  Structure of the MIB Module . . . . . . . . . . . . . . . . .   6
   7.  Relationship to Other MIB Modules . . . . . . . . . . . . . .   7
     7.1.  Relationship to SNMP-USER-BASED-SM-MIB  . . . . . . . . .   7
     7.2.  Relationship to SNMP-FRAMEWORK-MIB  . . . . . . . . . . .   7
     7.3.  MIB modules required for IMPORTS  . . . . . . . . . . . .   7
   8.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   7
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
     9.1.  Use of the HMAC-SHA-2 authentication protocols in USM . .   9
     9.2.  Cryptographic strength of the authentication protocols  .   9
     9.3.  Derivation of keys from passwords . . . . . . . . . . . .  10
     9.4.  Access to the SNMP-USM-HMAC-SHA2-MIB  . . . . . . . . . .  11
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     11.2.  Informative References . . . . . . . . . . . . . . . . .  12

1.  Introduction

   This memo defines a portion of the Management Information Base (MIB)
   for use with network management protocols.  In particular it defines
   additional authentication protocols for the User-based Security Model
   (USM) for version 3 of the Simple Network Management Protocol
   (SNMPv3) specified in RFC 3414 [RFC3414].

   In RFC 3414, two different authentication protocols, HMAC-MD5-96 and
   HMAC-SHA-96, are defined based on the hash functions MD5 and SHA-1,
   respectively.  This memo specifies new HMAC-SHA-2 authentication
   protocols for USM using an HMAC based on the SHA-2 family of hash
   functions [SHA] and truncated to 128 bits for SHA-224, to 192 bits
   for SHA-256, to 256 bits for SHA-384, and to 384 bits for SHA-512.
   These protocols are straightforward adaptations of the authentication
   protocols HMAC-MD5-96 and HMAC-SHA-96 to the SHA-2 based HMAC.

2.  The Internet-Standard Management Framework

   For a detailed overview of the documents that describe the current
   Internet-Standard Management Framework, please refer to section 7 of
   RFC 3410 [RFC3410].

   Managed objects are accessed via a virtual information store, termed
   the Management Information Base or MIB.  MIB objects are generally
   accessed through the Simple Network Management Protocol (SNMP).
   Objects in the MIB are defined using the mechanisms defined in the
   Structure of Management Information (SMI).  This memo specifies a MIB
   module that is compliant to the SMIv2, which is described in STD 58,
   RFC 2578 [RFC2578], STD 58, RFC 2579 [RFC2579] and STD 58, RFC 2580
   [RFC2580].

3.  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 BCP 14, RFC 2119
   [RFC2119].

4.  The HMAC-SHA-2 Authentication Protocols

   This section describes the HMAC-SHA-2 authentication protocols.  They
   use the SHA-2 hash functions, which are described in [SHA] and
   [RFC6234], in HMAC mode described in [RFC2104] and [RFC6234],
   truncating the output to 128 bits for SHA-224, 192 bits for SHA-256,
   256 bits for SHA-384, and 384 bits for SHA-512.  [RFC6234] also
   provides source code for all the SHA-2 algorithms and HMAC (without
   truncation).  It also includes test harness and standard test vectors
   for all the defined hash functions and HMAC examples.

   The following protocols are defined:

      usmHMAC128SHA224AuthProtocol: uses SHA-224 and truncates the
      output to 128 bits (16 octets);

      usmHMAC192SHA256AuthProtocol: uses SHA-256 and truncates the
      output to 192 bits (24 octets);

      usmHMAC256SHA384AuthProtocol: uses SHA-384 and truncates the
      output to 256 bits (32 octets);

      usmHMAC384SHA512AuthProtocol: uses SHA-512 and truncates the
      output to 384 bits (48 octets).

   Implementations conforming to this specification MUST support
   usmHMAC192SHA256AuthProtocol and SHOULD support
   usmHMAC384SHA512AuthProtocol.  The protocols
   usmHMAC128SHA224AuthProtocol and usmHMAC256SHA384AuthProtocol are
   OPTIONAL.

4.1.  Deviations from the HMAC-SHA-96 Authentication Protocol

   All the HMAC-SHA-2 authentication protocols are straightforward
   adaptations of the HMAC-MD5-96 and HMAC-SHA-96 authentication
   protocols.  Precisely, they differ from the HMAC-MD5-96 and HMAC-
   SHA-96 authentication protocols in the following aspects:

   o  The SHA-2 hash function is used to compute the message digest in
      the HMAC computation according to [RFC2104], as opposed to the MD5
      hash function [RFC1321] and SHA-1 hash function [SHA] used in
      HMAC-MD5-96 and HMAC-SHA-96, respectively.  Consequently, the
      length of the message digest prior to truncation is 224 bits for
      SHA-224 based protocol, 256 bits for SHA-256 based protocol, 384
      bits for SHA-384 based protocol, and 512 bits for SHA-512 based
      protocol.

   o  The resulting message digest (output of HMAC) is truncated to

      *  16 octets for usm128HMACSHA224AuthProtocol usmHMAC128SHA224AuthProtocol

      *  24 octets for usm192HMACSHA256AuthProtocol usmHMAC192SHA256AuthProtocol

      *  32 octets for usm256HMACSHA384AuthProtocol usmHMAC256SHA384AuthProtocol

      *  48 octets for usm384HMACSHA512AuthProtocol usmHMAC384SHA512AuthProtocol

      as opposed to the truncation to 12 octets in HMAC-MD5-96 and HMAC-
      SHA-96.

   o  The user's secret key to be used when calculating a digest MUST
      be:

      *  28 octets long and derived with SHA-224 for the SHA-224 based
         protocol usmHMAC128SHA224AuthProtocol

      *  32 octets long and derived with SHA-256 for the SHA-256 based
         protocol usmHMAC192SHA256AuthProtocol

      *  48 octets long and derived with SHA-384 for the SHA-384 based
         protocol usmHMAC256SHA384AuthProtocol

      *  64 octets long and derived with SHA-512 for the SHA-512 based
         protocol usmHMAC384SHA512AuthProtocol

      as opposed to the keys being 16 and 20 octets long in HMAC-MD5-96
      and HMAC-SHA-96, respectively.

4.2.  Processing

   This section describes the procedures for the HMAC-SHA-2
   authentication protocols.  The descriptions are based on the
   definition of services and data elements defined for HMAC-SHA-96 in
   RFC 3414 [RFC3414] with the deviations listed in Section 4.1.

4.2.1.  Processing an Outgoing Message

   Values of constants M (the length of the secret key) and N (the
   length of the MAC output) used below, are:

      usmHMAC128SHA224AuthProtocol: M=28, N=16;

      usmHMAC192SHA256AuthProtocol: M=32, N=24;

      usmHMAC256SHA384AuthProtocol: M=48, N=32;

      usmHMAC384SHA512AuthProtocol: M=64, N=48.

   correspondingly.

   This section describes the procedure followed by an SNMP engine
   whenever it must authenticate an outgoing message using one of the
   authentication protocols defined above.

   1.  The msgAuthenticationParameters field is set to serialization,
       according to the rules in [RFC3417], of an OCTET STRING
       containing N zero octets.

   2.  From the secret authKey of M octets, calculate the HMAC-SHA-2
       digest over it according to HMAC [RFC6234].  Take the first N
       octets of the final digest - this is the Message Authentication
       Code (MAC).

   3.  Replace the msgAuthenticationParameters field with the MAC
       obtained in the previous step.

   4.  The authenticatedWholeMsg is then returned to the caller together
       with statusInformation indicating success.

4.2.2.  Processing an Incoming Message

   Values of the constants M and N are the same as in Section 4.2.1, and
   are selected based on which authentication protocol is configured for
   the given USM usmUserTable entry.

   This section describes the procedure followed by an SNMP engine
   whenever it must authenticate an incoming message using one of the
   HMAC-SHA-2 authentication protocols.

   1.  If the digest received in the msgAuthenticationParameters field
       is not N octets long, then an failure and an errorIndication
       (authenticationError) is returned to the calling module.

   2.  The MAC received in the msgAuthenticationParameters field is
       saved.

   3.  The digest in the msgAuthenticationParameters field is replaced
       by the N zero octets.

   4.  Using the secret authKey, the HMAC is calculated over the
       wholeMsg.

   5.  N first octets of the above HMAC are taken as the computed MAC
       value.

   6.  The msgAuthenticationParameters field is replaced with the MAC
       value that was saved in step 2.

   7.  The newly calculated MAC is compared with the MAC saved in step
       2.  If they do not match, then a failure and an errorIndication
       (authenticationFailure) are returned to the calling module.

   8.  The authenticatedWholeMsg and statusInformation indicating
       success are then returned to the caller.

5.  Key Localization and Key Change

   For any of the protocols defined in Section 4, key localization and
   key change SHALL be performed according to RFC 3414 [RFC3414] using
   the SHA-2 hash function applied in the respective protocol.

6.  Structure of the MIB Module

   The MIB module specified in this memo does not define any managed
   objects, subtrees, notifications or tables, but only object
   identities (for authentication protocols) under a subtree of an
   existing MIB.

7.  Relationship to Other MIB Modules

7.1.  Relationship to SNMP-USER-BASED-SM-MIB

   RFC 3414 [RFC3414] specifies the MIB for the User-based Security
   Model (USM) for SNMPv3 (SNMP-USER-BASED-SM-MIB), which defines
   authentication protocols for USM based on the hash functions MD5 and
   SHA-1, respectively.  The following MIB module defines new HMAC-SHA2
   authentication protocols for USM based on the SHA-2 hash functions
   [SHA].  The use of the HMAC-SHA2 authentication protocols requires
   the usage of the objects defined in the SNMP-USER-BASED-SM-MIB.

7.2.  Relationship to SNMP-FRAMEWORK-MIB

   RFC 3411 [RFC3411] specifies the The SNMP Management Architecture MIB
   (SNMP-FRAMEWORK-MIB), which defines a subtree snmpAuthProtocols for
   SNMP authentication protocols.  The following MIB module defines new
   authentication protocols in the snmpAuthProtocols subtree.
   Therefore, the use of the HMAC-SHA2 authentication protocols requires
   the usage of the objects defined in the SNMP-FRAMEWORK-MIB.

7.3.  MIB modules required for IMPORTS

   The following MIB module IMPORTS objects from SNMPv2-SMI [RFC2578]
   and SNMP-FRAMEWORK-MIB [RFC3411].

8.  Definitions

SNMP-USM-HMAC-SHA2-MIB DEFINITIONS ::= BEGIN
    IMPORTS
        MODULE-IDENTITY, OBJECT-IDENTITY,
    snmpModules             FROM SNMPv2-SMI          -- [RFC2578]
    snmpAuthProtocols       FROM SNMP-FRAMEWORK-MIB; -- [RFC3411]

snmpUsmHmacSha2MIB MODULE-IDENTITY
    LAST-UPDATED "201408280000Z" "201501150000Z"               -- 28 August 2014, 25 January 2015, midnight
    ORGANIZATION "SNMPv3 Working Group"
    CONTACT-INFO    "WG email:  OPSAWG@ietf.org
                    Subscribe:  https://www.ietf.org/mailman/listinfo/opsawg
                    Editor:     Johannes Merkle
                                secunet Security Networks
                    postal:     Mergenthaler Allee 77
                                D-65760 Eschborn
                                Germany
                    phone:      +49 20154543091
                    email:      johannes.merkle@secunet.com
                    Co-Editor:  Manfred Lochter
                                Bundesamt fuer Sicherheit in der
                                Informationstechnik (BSI)
                    postal:     Postfach 200363
                                D-53133 Bonn
                                Germany
                    phone:      +49 228 9582 5643
                    email:      manfred.lochter@bsi.bund.de"

    DESCRIPTION     "Definitions of Object Identities needed
                    for the use of HMAC-SHA2 by SNMP's User-based
                    Security Model.

                    Copyright (C) The Internet Society (2004).

                    This version of this MIB module is part of RFC TBD;
                    see the RFC itself for full legal notices.
                    Supplementary information may be available on
                    http://www.ietf.org/copyrights/ianamib.html."
    -- RFC Ed.: replace TBD with actual RFC number & remove this line

    REVISION     "201403060000Z"     "201501150000Z"
DESCRIPTION  "Initial version, published as RFC TBD"
-- RFC Ed.: replace TBD with actual RFC number & remove this line

::= { snmpModules nn }        -- nn to be assigned by IANA
-- RFC Ed.: replace nn with actual number assigned by IANA & remove this line

usmHmac128Sha224Protocol

usmHMAC128SHA224Protocol OBJECT-IDENTITY
    STATUS          current
    DESCRIPTION "The HMAC-SHA-224-128 Authentication Protocol. Protocol usmHMAC128SHA224AuthProtocol.
                Uses HMAC-SHA-224 and truncates output to 128 bits."
REFERENCE       "- Krawczyk, H., Bellare, M., and R. Canetti, HMAC:
                Keyed-Hashing for Message Authentication, RFC 2104.
                - National Institute of Standards and Technology,
                Secure Hash Standard (SHS), FIPS PUB 180-4, 2012."
    ::= { snmpAuthProtocols aa }  -- aa to be assigned by IANA
    -- RFC Ed.: replace aa with actual number assigned by IANA & remove this line

usmHmac192Sha256Protocol

usmHMAC192SHA256Protocol OBJECT-IDENTITY
    STATUS      current
    DESCRIPTION "The HMAC-SHA-256-192 Authentication Protocol. Protocol usmHMAC192SHA256AuthProtocol.
                Uses HMAC-SHA-256 and truncates output to 192 bits."
    REFERENCE   "- Krawczyk, H., Bellare, M., and R. Canetti, HMAC:
                Keyed-Hashing for Message Authentication, RFC 2104.
                - National Institute of Standards and Technology,
                Secure Hash Standard (SHS), FIPS PUB 180-4, 2012."
    ::= { snmpAuthProtocols bb }  -- bb to be assigned by IANA
    -- RFC Ed.: replace cc with actual number assigned by IANA & remove this line

usmHmac256Sha384Protocol

usmHMAC256SHA384Protocol OBJECT-IDENTITY
    STATUS      current
    DESCRIPTION "The HMAC-SHA-384-256 Authentication Protocol. Protocol usmHMAC256SHA384AuthProtocol.
                Uses HMAC-SHA-384 and truncates output to 256 bits."
    REFERENCE   "- Krawczyk, H., Bellare, M., and R. Canetti, HMAC:
                Keyed-Hashing for Message Authentication, RFC 2104.
                - National Institute of Standards and Technology,
                Secure Hash Standard (SHS), FIPS PUB 180-4, 2012."
::= { snmpAuthProtocols cc }  -- cc to be assigned by IANA
-- RFC Ed.: replace dd with actual number assigned by IANA & remove this line

usmHmac384Sha512Protocol

usmHMAC384SHA12Protocol OBJECT-IDENTITY
    STATUS      current
    DESCRIPTION "The HMAC-SHA-512-384 Authentication Protocol. Protocol usmHMAC384SHA512AuthProtocol.
                Uses HMAC-SHA-512 and truncates output to 384 bits."
    REFERENCE   "- Krawczyk, H., Bellare, M., and R. Canetti, HMAC:
                Keyed-Hashing for Message Authentication, RFC 2104.
                - National Institute of Standards and Technology,
                Secure Hash Standard (SHS), FIPS PUB 180-4, 2012."
::= { snmpAuthProtocols dd }  -- dd to be assigned by IANA
-- RFC Ed.: replace ff with actual number assigned by IANA & remove this line

END

9.  Security Considerations

9.1.  Use of the HMAC-SHA-2 authentication protocols in USM

   The security considerations of [RFC3414] also apply the use of all
   the HMAC-SHA-2 authentication protocols in USM.

9.2.  Cryptographic strength of the authentication protocols

   At the time of this writing, all of the HMAC-SHA-2 authentication
   protocols provide a very high level of security.  The security of
   each HMAC-SHA-2 authentication protocol depends on the parameters
   used in the corresponding HMAC computation, which are the length of
   the key, the size of the hash function's internal state, and the
   length of the truncated MAC.  For the HMAC-SHA-2 authentication
   protocols these values are as follows (values are given in bits).

   +------------------------------+---------+----------------+---------+
   |           Protocol           |   Key   |    Size of     |   MAC   |
   |                              |  length | internal state |  length |
   +------------------------------+---------+----------------+---------+
   | usmHMAC128SHA224AuthProtocol |   224   |      256       |   128   |
   | usmHMAC192SHA256AuthProtocol |   256   |      256       |   192   |
   | usmHMAC256SHA384AuthProtocol |   384   |      512       |   256   |
   | usmHMAC384SHA512AuthProtocol |   512   |      512       |   384   |
   +------------------------------+---------+----------------+---------+

    Table 1: HMAC parameters of the HMAC-SHA-2 authentication protocols

   The security of the HMAC scales with both the key length and the size
   of the internal state: longer keys render key guessing attacks more
   difficult, and a larger internal state decreases the success
   probability of MAC forgeries based on internal collisions of the hash
   function.

   The role of the truncated output length is more complicated:
   according to [BCK], there is a trade-off in that "by outputting less
   bits the attacker has less bits to predict in a MAC forgery but, on
   the other hand, the attacker also learns less about the output of the
   compression function from seeing the authentication tags computed by
   legitimate parties"; thus, truncation weakens the HMAC against
   forgery by guessing, but at the same time strengthens it against
   chosen message attacks aiming at MAC forgery based on internal
   collisions or at key guessing.  [RFC2104] and [BCK] allow truncation
   to any length that is not less than half the size of the internal
   state.

   Further discussion of the security of the HMAC construction is given
   in [RFC2104].

9.3.  Derivation of keys from passwords

   If secret keys to be used for HMAC-SHA-2 authentication protocols are
   derived from passwords, the derivation SHOULD be performed using the
   password-to-key algorithm from Appendix A.1 of RFC 3414 with MD5
   being replaced by the SHA-2 hash function H used in the HMAC-SHA-2
   authentication protocol.  Specifically, the password is converted
   into the required secret key by the following steps:

   o  forming a string of length 1,048,576 octets by repeating the value
      of the password as often as necessary, truncating accordingly, and
      using the resulting string as the input to the hash function H.
      The resulting digest, termed "digest1", is used in the next step.

   o  a second string is formed by concatenating digest1, the SNMP
      engine's snmpEngineID value, and digest1.  This string is used as
      input to the hash function H.

9.4.  Access to the SNMP-USM-HMAC-SHA2-MIB

   None of the objects defined in SNMP-USM-HMAC-SHA2-MIB is writable,
   and the information they represent is not deemed to be particularly
   sensitive.  However, if they are deemed sensitive in a particular
   environment, access to them should be restricted through the use of
   appropriately configured Security and Access Control models.

10.  IANA Considerations

   IANA is requested to assign an OID for

             +--------------------+-------------------------+
             |     Descriptor     | OBJECT IDENTIFIER value |
             +--------------------+-------------------------+
             | snmpUsmHmacSha2MIB |    { snmpModules nn }   |
             +--------------------+-------------------------+

                            Table 2: OID of MIB

   with nn appearing in the MIB module definition in Section 8.

   Furthermore, IANA is requested to assign a value in the
   SnmpAuthProtocols registry for each of the following protocols.

           +------------------------------+-------+-----------+
           |         Description          | Value | Reference |
           +------------------------------+-------+-----------+
           | usmHMAC128SHA224AuthProtocol |   aa  |  RFC YYYY |
           | usmHMAC192SHA256AuthProtocol |   bb  |  RFC YYYY |
           | usmHMAC256SHA384AuthProtocol |   cc  |  RFC YYYY |
           | usmHMAC384SHA512AuthProtocol |   dd  |  RFC YYYY |
           +------------------------------+-------+-----------+

   Table 3: Code points assigned to HMAC-SHA-2 authentication protocols

   -- RFC Ed.: replace YYYY with actual RFC number and remove this line

   with aa, bb, cc, etc. appearing in the MIB module definition in
   Section 8.

11.  References

11.1.  Normative References

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104, February
              1997.

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

   [RFC2578]  McCloghrie, K., Ed., Perkins, D., Ed., and J.
              Schoenwaelder, Ed., "Structure of Management Information
              Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.

   [RFC2579]  McCloghrie, K., Ed., Perkins, D., Ed., and J.
              Schoenwaelder, Ed., "Textual Conventions for SMIv2", STD
              58, RFC 2579, April 1999.

   [RFC2580]  McCloghrie, K., Perkins, D., and J. Schoenwaelder,
              "Conformance Statements for SMIv2", STD 58, RFC 2580,
              April 1999.

   [RFC3414]  Blumenthal, U. and B. Wijnen, "User-based Security Model
              (USM) for version 3 of the Simple Network Management
              Protocol (SNMPv3)", STD 62, RFC 3414, December 2002.

   [SHA]      National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", FIPS PUB 180-4, March 2012.

11.2.  Informative References

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992.

   [RFC3410]  Case, J., Mundy, R., Partain, D., and B. Stewart,
              "Introduction and Applicability Statements for Internet-
              Standard Management Framework", RFC 3410, December 2002.

   [RFC3411]  Harrington, D., Presuhn, R., and B. Wijnen, "An
              Architecture for Describing Simple Network Management
              Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
              December 2002.

   [RFC3412]  Case, J., Harrington, D., Presuhn, R., and B. Wijnen,
              "Message Processing and Dispatching for the Simple Network
              Management Protocol (SNMP)", STD 62, RFC 3412, December
              2002.

   [RFC3417]  Presuhn, R., "Transport Mappings for the Simple Network
              Management Protocol (SNMP)", STD 62, RFC 3417, December
              2002.

   [RFC4231]  Nystrom, M., "Identifiers and Test Vectors for HMAC-SHA-
              224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512", RFC
              4231, December 2005.

   [RFC6234]  Eastlate 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.

   [BCK]      Bellare, M., Canetti, R., and H. Krawczyk, "Keyed Hash
              Functions for Message Authentication", Advances in
              Cryptology - CRYPTO 99, Lecture Notes in Computer Science
              1109, Springer Verlag, 1996.

Authors' Addresses

   Johannes Merkle (editor)
   Secunet Security Networks
   Mergenthaler Allee 77
   65760 Eschborn
   Germany

   Phone: +49 201 5454 3091
   EMail: johannes.merkle@secunet.com

   Manfred Lochter
   BSI
   Postfach 200363
   53133 Bonn
   Germany

   Phone: +49 228 9582 5643
   EMail: manfred.lochter@bsi.bund.de