RFC7860
Internet Engineering Task Force (IETF) J. Merkle, Ed. Request for Comments: 7860 Secunet Security Networks Obsoletes: 7630 M. Lochter Category: Standards Track BSI ISSN: 2070-1721 April 2016
HMAC-SHA-2 Authentication Protocols in
User-Based Security Model (USM) for SNMPv3
Abstract
This document specifies several authentication protocols based on the SHA-2 hash functions for the User-based Security Model (USM) for SNMPv3 defined in RFC 3414. It obsoletes RFC 7630, in which the MIB MODULE-IDENTITY value was incorrectly specified.
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/rfc7860.
Copyright Notice
Copyright (c) 2016 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.
4.1. Deviations from the HMAC-SHA-96 Authentication Protocol . 4
9.1. Use of the HMAC-SHA-2 Authentication Protocols in USM . . 10 9.2. Cryptographic Strength of the Authentication Protocols . 10
Contents
Introduction
Within the Architecture for describing Simple Network Management Protocol (SNMP) Management Frameworks RFC3411, the User-based Security Model (USM) RFC3414 for SNMPv3 is defined as a Security Subsystem within an SNMP engine. 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.
This document obsoletes RFC 7630, in which the MIB MODULE-IDENTITY value was incorrectly specified.
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, RFC2578, STD 58, RFC2579 and STD 58, RFC2580.
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.
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 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. 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.
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 the 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.
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 specified 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.
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 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.
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.
Key Localization and Key Change
For any of the protocols defined in Section 4, key localization and key change SHALL be performed according to RFC3414 using the same SHA-2 hash function as in the HMAC-SHA-2 authentication protocol.
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.
Relationship to Other MIB Modules
Relationship to SNMP-USER-BASED-SM-MIB
RFC3414 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.
Relationship to SNMP-FRAMEWORK-MIB
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.
MIB Modules Required for IMPORTS
The following MIB module IMPORTS definitions from SNMPv2-SMI RFC2578 and SNMP-FRAMEWORK-MIB RFC3411.
Definitions
SNMP-USM-HMAC-SHA2-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-IDENTITY,
mib-2 FROM SNMPv2-SMI -- RFC 2578
snmpAuthProtocols FROM SNMP-FRAMEWORK-MIB; -- RFC 3411
snmpUsmHmacSha2MIB MODULE-IDENTITY
LAST-UPDATED "201604180000Z" -- 18 April 2016, midnight ORGANIZATION "SNMPv3 Working Group" CONTACT-INFO "WG email: [email protected] 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: [email protected]
Co-Editor: Manfred Lochter
Bundesamt fuer Sicherheit in der
Informationstechnik (BSI)
Postal: Postfach 200363
D-53133 Bonn
Germany
Phone: +49 228 9582 5643
Email: [email protected]"
DESCRIPTION
"Definitions of Object Identities needed for the use of
HMAC-SHA2 Authentication Protocols by SNMP's User-based Security
Model.
Copyright (c) 2016 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 "201604180000Z" -- 18 April 2016, midnight
DESCRIPTION
"Version correcting the MODULE-IDENTITY value,
published as RFC 7860"
REVISION "201510140000Z" -- 14 October 2015, midnight
DESCRIPTION
"Initial version, published as RFC 7630"
::= { mib-2 235 }
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
Security Considerations
Use of the HMAC-SHA-2 Authentication Protocols in USM
The security considerations of RFC3414 also apply to the HMAC-SHA-2 authentication protocols defined in this document.
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.
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.
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 | { mib-2 235 } |
+--------------------+-------------------------+
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 7860 |
| usmHMAC192SHA256AuthProtocol | 5 | RFC 7860 |
| usmHMAC256SHA384AuthProtocol | 6 | RFC 7860 |
| usmHMAC384SHA512AuthProtocol | 7 | RFC 7860 |
+------------------------------+-------+-----------+
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, August 2015,
<http://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.180-4.pdf>.
11.2. Informative References
[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.
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>.
Authors' Addresses
Johannes Merkle (editor) Secunet Security Networks Mergenthaler Allee 77 65760 Eschborn Germany
Phone: +49 201 5454 3091 Email: [email protected]
Manfred Lochter BSI Postfach 200363 53133 Bonn Germany
Phone: +49 228 9582 5643 Email: [email protected]
