HMAC-SHA-2 Authentication Protocols in USM for SNMPv3

Within the Architecture for describing Simple Network Management Protocol (SNMP) Management Frameworks , the User-based Security Model (USM) 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 and truncated to 128 bits for SHA-224, to 192 bits for SHA-256, to 256 bits for SHA&nbhy;384, and to 384 bits for SHA&nbhy;512. These protocols are straightforward adaptations of the authentication protocols HMAC-MD5-96 and HMAC-SHA-96 to the SHA&nbhy;2&nbhy;based HMAC. This document obsoletes RFC 7630, in which the MIB MODULE-IDENTITY value was incorrectly specified.

For a detailed overview of the documents that describe the current Internet-Standard Management Framework, please refer to section 7 of . 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 , , and .

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 .

This section describes the HMAC-SHA-2 authentication protocols, which use the SHA-2 hash functions (described in FIPS PUB 180-4 and ) in the HMAC mode (described in 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.

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: 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 and SHA-1 hash function 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&nbhy;256-based protocol, 384 bits for the SHA-384-based protocol, and 512 bits for the SHA-512-based protocol. 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. 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.

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 . 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.

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 and are selected based on which authentication protocol is configured for the given USM usmUser Table entry. The msgAuthenticationParameters field is set to the serialization of an OCTET STRING containing N zero octets; it is serialized according to the rules in . Using the secret authKey of M octets, the HMAC is calculated over the wholeMsg according to RFC 6234 with hash function H. The N first octets of the above HMAC are taken as the computed MAC value. The msgAuthenticationParameters field is replaced with the MAC obtained in the previous step. The authenticatedWholeMsg is then returned to the caller together with the statusInformation indicating success.

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 and are selected based on which authentication protocol is configured for the given USM usmUser Table entry. 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. The MAC received in the msgAuthenticationParameters field is saved. The digest in the msgAuthenticationParameters field is replaced by the N zero octets. Using the secret authKey of M octets, the HMAC is calculated over the wholeMsg according to RFC 6234 with hash function H. The N first octets of the above HMAC are taken as the computed MAC value. The msgAuthenticationParameters field is replaced with the MAC value that was saved in step 2. 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. The authenticatedWholeMsg and statusInformation indicating success are then returned to the caller.

For any of the protocols defined in , key localization and key change SHALL be performed according to using the same SHA-2 hash function as in the HMAC-SHA-2 authentication protocol.

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.

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 . The use of the HMAC-SHA2 authentication protocols requires the usage of the objects defined in the SNMP-USER-BASED-SM-MIB.

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.

The following MIB module IMPORTS definitions from SNMPv2-SMI and SNMP-FRAMEWORK-MIB .

The security considerations of also apply to the HMAC-SHA-2 authentication protocols defined in this document.

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 length Size of internal state MAC length usmHMAC128SHA224AuthProtocol 224 256 128 usmHMAC192SHA256AuthProtocol 256 256 192 usmHMAC256SHA384AuthProtocol 384 512 256 usmHMAC384SHA512AuthProtocol 512 512 384 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 , 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 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.

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: 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. 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.

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 . The considerations in Section 11.5 of RFC 3414 should be taken into account.

IANA has assigned an OID for the MIB as follows. Descriptor OBJECT IDENTIFIER value snmpUsmHmacSha2MIB { mib-2 235 } Furthermore, IANA has assigned a value in the SnmpAuthProtocols registry for each of the following protocols. Description Value Reference usmHMAC128SHA224AuthProtocol 4 RFC XXX usmHMAC192SHA256AuthProtocol 5 RFC XXX usmHMAC256SHA384AuthProtocol 6 RFC XXX usmHMAC384SHA512AuthProtocol 7 RFC XXX -- RFC Ed.: replace XXX with actual RFC number and remove this line