HMAC-SHA-2 for SNMPv3: HMAC-SHA-2 Authentication Protocols in the User-based Security Model (USM) for SNMPv3

Internet Engineering Task Force (IETF)                    J. Merkle, Ed.
Request for Comments: 7630                     Secunet Security Networks
Category: Standards Track                                     M. Lochter
ISSN: 2070-1721                                                      BSI
                                                            October 2015


                  HMAC-SHA-2 Authentication Protocols
           in the User-based Security Model (USM) for SNMPv3

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 is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7630.

Copyright Notice

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

   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  . . . . . . . . . . . . . . . . . . . . . . . .   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 . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

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 the Simple Network Management Protocol version 3 (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 a Hashed Message Authentication Code (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, which
   use the SHA-2 hash functions (described in FIPS PUB 180-4 [SHA] and
   RFC 6234 [RFC6234]) in the HMAC mode (described in RFC 2104 [RFC2104]
   and RFC 6234), 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.
   RFC 6234 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.  Specifically, 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 RFC 2104 and RFC 6234, 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 the SHA-224-based protocol, 256 bits for the
      SHA-256-based protocol, 384 bits for the SHA-384-based protocol,
      and 512 bits for the SHA-512-based protocol.

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

      *  16 octets for usmHMAC128SHA224AuthProtocol

      *  24 octets for usmHMAC192SHA256AuthProtocol

      *  32 octets for usmHMAC256SHA384AuthProtocol

      *  48 octets for 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 with the deviations listed in Section 4.1.

   Values of constants M (the length of the secret key in octets) and N
   (the length of the Message Authentication Code (MAC) output in
   octets), and the hash function H used below are:

      usmHMAC128SHA224AuthProtocol: M=28, N=16, H=SHA-224;

      usmHMAC192SHA256AuthProtocol: M=32, N=24, H=SHA-256;

      usmHMAC256SHA384AuthProtocol: M=48, N=32, H=SHA-384;

      usmHMAC384SHA512AuthProtocol: M=64, N=48, H=SHA-512.

4.2.1.  Processing an Outgoing Message

   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.  Values of the constants M
   and N, and the hash function H are as defined in Section 4.2 and are
   selected based on which authentication protocol is configured for the
   given USM usmUser Table entry.

   1.  The msgAuthenticationParameters field is set to the serialization
       of an OCTET STRING containing N zero octets; it is serialized
       according to the rules in RFC 3417 [RFC3417].

   2.  Using the secret authKey of M octets, the HMAC is calculated over
       the wholeMsg according to RFC 6234 with hash function H.

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

   4.  The msgAuthenticationParameters field is replaced with the MAC
       obtained in the previous step.

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

4.2.2.  Processing an Incoming Message

   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.  Values of the constants M and
   N, and the hash function H are as defined in Section 4.2 and are
   selected based on which authentication protocol is configured for the
   given USM usmUser Table entry.

   1.  If the digest received in the msgAuthenticationParameters field
       is not N octets long, then a failure and an errorIndication
       (authenticationError) are 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 of M octets, the HMAC is calculated over
       the wholeMsg according to RFC 6234 with hash function H.

   5.  The 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 same SHA-2 hash function as in the HMAC-SHA-2 authentication
   protocol.

6.  Structure of the MIB Module

   The MIB module specified in this memo does not define any managed
   objects, subtrees, notifications, or tables; rather, it only defines
   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 specifies the MIB module for 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 SNMP-FRAMEWORK-MIB, which defines a
   subtree snmpAuthProtocols for SNMP authentication protocols.  The
   following MIB module defines new authentication protocols in the
   snmpAuthProtocols subtree.

7.3.  MIB Modules Required for IMPORTS

   The following MIB module IMPORTS definitions 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    "201508130000Z"       -- 13 August 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) 2015 IETF Trust and the persons identified
             as authors of the code.  All rights reserved.

             Redistribution and use in source and binary forms, with
             or without modification, is permitted pursuant to, and
             subject to the license terms contained in, the Simplified
             BSD License set forth in Section 4.c of the IETF Trust's
             Legal Provisions Relating to IETF Documents
             (http://trustee.ietf.org/license-info)."

       REVISION    "201508130000Z"       -- 13 August 2015, midnight
       DESCRIPTION "Initial version, published as RFC 7630"
   ::= { snmpModules 235 }

   usmHMAC128SHA224AuthProtocol OBJECT-IDENTITY
       STATUS      current
       DESCRIPTION "The Authentication 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 4 }

   usmHMAC192SHA256AuthProtocol OBJECT-IDENTITY
       STATUS      current
       DESCRIPTION "The Authentication 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 5 }

   usmHMAC256SHA384AuthProtocol OBJECT-IDENTITY
       STATUS      current
       DESCRIPTION "The Authentication 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 6 }

   usmHMAC384SHA512AuthProtocol OBJECT-IDENTITY
       STATUS      current
       DESCRIPTION "The Authentication 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 7 }

   END

9.  Security Considerations

9.1.  Use of the HMAC-SHA-2 Authentication Protocols in USM

   The security considerations of RFC 3414 [RFC3414] also apply to the
   HMAC-SHA-2 authentication protocols defined in this document.

9.2.  Cryptographic Strength of the Authentication Protocols

   At the time of publication of this document, 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 (if the key has maximum entropy), 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.  RFC
   2104 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 RFC 2104.

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  forming a second string 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

   The SNMP-USM-HMAC-SHA2-MIB module defines OBJECT IDENTIFIER values
   for use in other MIB modules.  It does not define any objects that
   can be accessed.  As such, the SNMP-USM-HMAC-SHA2-MIB does not, by
   itself, have any effect on the security of the Internet.

   The values defined in this module are expected to be used with the
   usmUserTable defined in the SNMP-USER-BASED-SM-MIB [RFC3414].  The
   considerations in Section 11.5 of RFC 3414 should be taken into
   account.

10.  IANA Considerations

   IANA has assigned an OID for the MIB as follows.

             +--------------------+-------------------------+
             |     Descriptor     | OBJECT IDENTIFIER value |
             +--------------------+-------------------------+
             | snmpUsmHmacSha2MIB |   { snmpModules 235 }   |
             +--------------------+-------------------------+

                            Table 2: OID of MIB

   Furthermore, IANA has assigned a value in the SnmpAuthProtocols
   registry for each of the following protocols.

           +------------------------------+-------+-----------+
           |         Description          | Value | Reference |
           +------------------------------+-------+-----------+
           | usmHMAC128SHA224AuthProtocol |   4   |  RFC 7630 |
           | usmHMAC192SHA256AuthProtocol |   5   |  RFC 7630 |
           | usmHMAC256SHA384AuthProtocol |   6   |  RFC 7630 |
           | usmHMAC384SHA512AuthProtocol |   7   |  RFC 7630 |
           +------------------------------+-------+-----------+

   Table 3: Code Points Assigned to HMAC-SHA-2 Authentication Protocols

11.  References

11.1.  Normative References

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <http://www.rfc-editor.org/info/rfc2104>.

   [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>.

   [RFC2578]  McCloghrie, K., Ed., Perkins, D., Ed., and J.
              Schoenwaelder, Ed., "Structure of Management Information
              Version 2 (SMIv2)", STD 58, RFC 2578,
              DOI 10.17487/RFC2578, April 1999,
              <http://www.rfc-editor.org/info/rfc2578>.

   [RFC2579]  McCloghrie, K., Ed., Perkins, D., Ed., and J.
              Schoenwaelder, Ed., "Textual Conventions for SMIv2",
              STD 58, RFC 2579, DOI 10.17487/RFC2579, April 1999,
              <http://www.rfc-editor.org/info/rfc2579>.

   [RFC2580]  McCloghrie, K., Ed., Perkins, D., Ed., and J.
              Schoenwaelder, Ed., "Conformance Statements for SMIv2",
              STD 58, RFC 2580, DOI 10.17487/RFC2580, April 1999,
              <http://www.rfc-editor.org/info/rfc2580>.

   [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,
              DOI 10.17487/RFC3414, December 2002,
              <http://www.rfc-editor.org/info/rfc3414>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <http://www.rfc-editor.org/info/rfc6234>.

   [SHA]      National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", FIPS PUB 180-4,
              DOI 10.6028/NIST.FIPS.180-4, March 2012,
              <http://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.180-4.pdf>.

11.2.  Informative References

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              DOI 10.17487/RFC1321, April 1992,
              <http://www.rfc-editor.org/info/rfc1321>.

   [RFC3410]  Case, J., Mundy, R., Partain, D., and B. Stewart,
              "Introduction and Applicability Statements for Internet-
              Standard Management Framework", RFC 3410,
              DOI 10.17487/RFC3410, December 2002,
              <http://www.rfc-editor.org/info/rfc3410>.

   [RFC3411]  Harrington, D., Presuhn, R., and B. Wijnen, "An
              Architecture for Describing Simple Network Management
              Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
              DOI 10.17487/RFC3411, December 2002,
              <http://www.rfc-editor.org/info/rfc3411>.

   [RFC3417]  Presuhn, R., Ed., "Transport Mappings for the Simple
              Network Management Protocol (SNMP)", STD 62, RFC 3417,
              DOI 10.17487/RFC3417, December 2002,
              <http://www.rfc-editor.org/info/rfc3417>.

   [BCK]      Bellare, M., Canetti, R., and H. Krawczyk, "Keyed Hash
              Functions for Message Authentication", Advances in
              Cryptology - CRYPTO 96, Lecture Notes in Computer Science
              1109, Springer-Verlag Berlin Heidelberg,
              DOI 10.1007/3-540-68697-5_1, 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