hkdf.c

  1// SPDX-License-Identifier: GPL-2.0
  2/*
  3 * Implementation of HKDF ("HMAC-based Extract-and-Expand Key Derivation
  4 * Function"), aka RFC 5869.  See also the original paper (Krawczyk 2010):
  5 * "Cryptographic Extraction and Key Derivation: The HKDF Scheme".
  6 *
  7 * This is used to derive keys from the fscrypt master keys (or from the
  8 * "software secrets" which hardware derives from the fscrypt master keys, in
  9 * the case that the fscrypt master keys are hardware-wrapped keys).
 10 *
 11 * Copyright 2019 Google LLC
 12 */
 13
 14#include "fscrypt_private.h"
 15
 16/*
 17 * HKDF supports any unkeyed cryptographic hash algorithm, but fscrypt uses
 18 * SHA-512 because it is well-established, secure, and reasonably efficient.
 19 *
 20 * HKDF-SHA256 was also considered, as its 256-bit security strength would be
 21 * sufficient here.  A 512-bit security strength is "nice to have", though.
 22 * Also, on 64-bit CPUs, SHA-512 is usually just as fast as SHA-256.  In the
 23 * common case of deriving an AES-256-XTS key (512 bits), that can result in
 24 * HKDF-SHA512 being much faster than HKDF-SHA256, as the longer digest size of
 25 * SHA-512 causes HKDF-Expand to only need to do one iteration rather than two.
 26 */
 27#define HKDF_HASHLEN		SHA512_DIGEST_SIZE
 28
 29/*
 30 * HKDF consists of two steps:
 31 *
 32 * 1. HKDF-Extract: extract a pseudorandom key of length HKDF_HASHLEN bytes from
 33 *    the input keying material and optional salt.
 34 * 2. HKDF-Expand: expand the pseudorandom key into output keying material of
 35 *    any length, parameterized by an application-specific info string.
 36 *
 37 * HKDF-Extract can be skipped if the input is already a pseudorandom key of
 38 * length HKDF_HASHLEN bytes.  However, cipher modes other than AES-256-XTS take
 39 * shorter keys, and we don't want to force users of those modes to provide
 40 * unnecessarily long master keys.  Thus fscrypt still does HKDF-Extract.  No
 41 * salt is used, since fscrypt master keys should already be pseudorandom and
 42 * there's no way to persist a random salt per master key from kernel mode.
 43 */
 44
 45/*
 46 * Compute HKDF-Extract using 'master_key' as the input keying material, and
 47 * prepare the resulting HMAC key in 'hkdf'.  Afterwards, 'hkdf' can be used for
 48 * HKDF-Expand many times without having to recompute HKDF-Extract each time.
 49 */
 50void fscrypt_init_hkdf(struct hmac_sha512_key *hkdf, const u8 *master_key,
 51		       unsigned int master_key_size)
 52{
 53	static const u8 default_salt[HKDF_HASHLEN];
 54	u8 prk[HKDF_HASHLEN];
 55
 56	hmac_sha512_usingrawkey(default_salt, sizeof(default_salt),
 57				master_key, master_key_size, prk);
 58	hmac_sha512_preparekey(hkdf, prk, sizeof(prk));
 59	memzero_explicit(prk, sizeof(prk));
 60}
 61
 62/*
 63 * HKDF-Expand (RFC 5869 section 2.3).  Expand the HMAC key 'hkdf' into 'okmlen'
 64 * bytes of output keying material parameterized by the application-specific
 65 * 'info' of length 'infolen' bytes, prefixed by "fscrypt\0" and the 'context'
 66 * byte.  This is thread-safe and may be called by multiple threads in parallel.
 67 *
 68 * ('context' isn't part of the HKDF specification; it's just a prefix fscrypt
 69 * adds to its application-specific info strings to guarantee that it doesn't
 70 * accidentally repeat an info string when using HKDF for different purposes.)
 71 */
 72void fscrypt_hkdf_expand(const struct hmac_sha512_key *hkdf, u8 context,
 73			 const u8 *info, unsigned int infolen,
 74			 u8 *okm, unsigned int okmlen)
 75{
 76	struct hmac_sha512_ctx ctx;
 77	u8 counter = 1;
 78	u8 tmp[HKDF_HASHLEN];
 79
 80	WARN_ON_ONCE(okmlen > 255 * HKDF_HASHLEN);
 81
 82	for (unsigned int i = 0; i < okmlen; i += HKDF_HASHLEN) {
 83		hmac_sha512_init(&ctx, hkdf);
 84		if (i != 0)
 85			hmac_sha512_update(&ctx, &okm[i - HKDF_HASHLEN],
 86					   HKDF_HASHLEN);
 87		hmac_sha512_update(&ctx, "fscrypt\0", 8);
 88		hmac_sha512_update(&ctx, &context, 1);
 89		hmac_sha512_update(&ctx, info, infolen);
 90		hmac_sha512_update(&ctx, &counter, 1);
 91		if (okmlen - i < HKDF_HASHLEN) {
 92			hmac_sha512_final(&ctx, tmp);
 93			memcpy(&okm[i], tmp, okmlen - i);
 94			memzero_explicit(tmp, sizeof(tmp));
 95		} else {
 96			hmac_sha512_final(&ctx, &okm[i]);
 97		}
 98		counter++;
 99	}
100}