kdf.h File Reference

SP800-108 counter-mode key derivation (HMAC-SHA256 PRF). More...

#include <stddef.h>
#include <stdint.h>

Go to the source code of this file.

Functions
bool	pc_kdf_ctr_hmac_sha256 (const uint8_t *ki, size_t ki_len, const uint8_t *fixed, size_t fixed_len, uint8_t *out, size_t out_len)
	SP800-108 KDF in counter mode with HMAC-SHA256 as the PRF (NIST SP800-108 §5.1; r = 32-bit counter placed before the fixed input).

Detailed Description

SP800-108 counter-mode key derivation (HMAC-SHA256 PRF).

The shared NIST SP800-108 §5.1 counter-mode KDF. SMB 3.x uses it to derive its signing and encryption keys (MS-SMB2 §3.1.4.2); the caller assembles the fixed input, keeping this independent of any protocol's label/context choices. Verified against the NIST CAVP KBKDF (KDFCTR) vectors.

Author

Douglas Quigg (dstroy0)

Date

2026

Definition in file kdf.h.

Function Documentation

pc_kdf_ctr_hmac_sha256()

bool pc_kdf_ctr_hmac_sha256	(	const uint8_t *	ki,
		size_t	ki_len,
		const uint8_t *	fixed,
		size_t	fixed_len,
		uint8_t *	out,
		size_t	out_len
	)

SP800-108 KDF in counter mode with HMAC-SHA256 as the PRF (NIST SP800-108 §5.1; r = 32-bit counter placed before the fixed input).

K(i) = HMAC-SHA256(Ki, [i]_32be || fixed); the blocks are concatenated for i = 1, 2, ... and the result truncated to out_len bytes. The caller assembles fixed as Label || 0x00 || Context || [L] (L = the output length in bits, 32-bit big-endian) and passes it whole.

Parameters

ki	the key-derivation key (e.g. the SMB 3.x session key).
ki_len	length of ki in bytes.
fixed	the fixed input (Label || 0x00 || Context || [L]).
fixed_len	length of fixed in bytes.
out	receives out_len derived bytes.
out_len	number of output bytes (>= 1); the caller must encode L = out_len * 8 into fixed.

Returns

true on success; false on a null pointer or out_len == 0.

Definition at line 20 of file kdf.cpp.

References pc_sha256(), pc_sha256_final(), pc_sha256_init(), pc_sha256_update(), and pc_wr32be().