RFC 4868

Q: What does the Introduction section cover in RFC 4868 — HMAC-SHA-256 for IPsec?

The Introduction section provides detailed technical specifications, security considerations, and implementation guidance as documented in RFC 4868 — HMAC-SHA-256 for IPsec. This content is part of the BLACKLABS RFC Reference Library.

Q: What does the Algorithm Parameters section cover in RFC 4868 — HMAC-SHA-256 for IPsec?

The Algorithm Parameters section provides detailed technical specifications, security considerations, and implementation guidance as documented in RFC 4868 — HMAC-SHA-256 for IPsec. This content is part of the BLACKLABS RFC Reference Library.

Q: What does the HMAC Construction section cover in RFC 4868 — HMAC-SHA-256 for IPsec?

The HMAC Construction section provides detailed technical specifications, security considerations, and implementation guidance as documented in RFC 4868 — HMAC-SHA-256 for IPsec. This content is part of the BLACKLABS RFC Reference Library.

Q: What does the Security Considerations section cover in RFC 4868 — HMAC-SHA-256 for IPsec?

The Security Considerations section provides detailed technical specifications, security considerations, and implementation guidance as documented in RFC 4868 — HMAC-SHA-256 for IPsec. This content is part of the BLACKLABS RFC Reference Library.

BLACKLABS

Using HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512 with IPsec

Standards Track

May 2007

Abstract: Specifies the use of HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512 as integrity algorithms within IPsec ESP and AH. These algorithms replace HMAC-MD5 and HMAC-SHA-1 in 3GPP NDS/IP (TS 33.210) for modern deployments.

Canonical source: https://www.rfc-editor.org/rfc/rfc4868 | IETF Datatracker

1. Introduction

▶

HMAC (Hash-based Message Authentication Code) combined with SHA-2 family hash functions provides strong integrity protection for IPsec packets. This RFC defines how to use HMAC-SHA-256-128, HMAC-SHA-384-192, and HMAC-SHA-512-256 as ESP/AH integrity algorithms.

In 3GPP networks, HMAC-SHA-256-128 is the recommended integrity algorithm for IPsec on NDS/IP interfaces (Ze, Zb, Zd) protecting Diameter, GTP-C, and other core signaling.

2. Algorithm Parameters

▶

Algorithm	HMAC Output	Truncation	ICV Length	Key Length
HMAC-SHA-256-128	256 bits	128 bits	16 bytes	32 bytes
HMAC-SHA-384-192	384 bits	192 bits	24 bytes	48 bytes
HMAC-SHA-512-256	512 bits	256 bits	32 bytes	64 bytes

3. HMAC Construction

▶

HMAC computes a nested hash: H(K ⊕ opad || H(K ⊕ ipad || M)). The output is truncated to the specified ICV length. Truncation provides adequate security while reducing packet overhead.

  Key (K)                    Message (M)
|                           |
v                           |
K ⊕ ipad                     |
|                           |
+---------> H( (K ⊕ ipad) || M )
|
v
inner_hash
|
K ⊕ opad             |
|                   |
+---------> H( (K ⊕ opad) || inner_hash )
|
v
HMAC output
|
Truncate to ICV length
|
v
ICV (appended to ESP/AH packet)

4. Security Considerations

▶

HMAC-SHA-256-128 provides 128-bit security against forgery — sufficient for current requirements
Key length should match the hash output size — shorter keys reduce security
Truncation to 128 bits is considered safe for HMAC-SHA-256 (birthday bound is 2^64 for forgery)
These algorithms replace HMAC-MD5-96 and HMAC-SHA-1-96 which have reduced security margins