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(raw_key: default_salt, raw_key_len: sizeof(default_salt),
57 data: master_key, data_len: master_key_size, out: prk);
58 hmac_sha512_preparekey(key: hkdf, raw_key: prk, raw_key_len: sizeof(prk));
59 memzero_explicit(s: prk, count: 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: &ctx, key: hkdf);
84 if (i != 0)
85 hmac_sha512_update(ctx: &ctx, data: &okm[i - HKDF_HASHLEN],
86 HKDF_HASHLEN);
87 hmac_sha512_update(ctx: &ctx, data: "fscrypt\0", data_len: 8);
88 hmac_sha512_update(ctx: &ctx, data: &context, data_len: 1);
89 hmac_sha512_update(ctx: &ctx, data: info, data_len: infolen);
90 hmac_sha512_update(ctx: &ctx, data: &counter, data_len: 1);
91 if (okmlen - i < HKDF_HASHLEN) {
92 hmac_sha512_final(ctx: &ctx, out: tmp);
93 memcpy(&okm[i], tmp, okmlen - i);
94 memzero_explicit(s: tmp, count: sizeof(tmp));
95 } else {
96 hmac_sha512_final(ctx: &ctx, out: &okm[i]);
97 }
98 counter++;
99 }
100}
101

source code of linux/fs/crypto/hkdf.c