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// SPDX-License-Identifier: GPL-2.0
/*
* Implementation of HKDF ("HMAC-based Extract-and-Expand Key Derivation
* Function"), aka RFC 5869. See also the original paper (Krawczyk 2010):
* "Cryptographic Extraction and Key Derivation: The HKDF Scheme".
*
* This is used to derive keys from the fscrypt master keys (or from the
* "software secrets" which hardware derives from the fscrypt master keys, in
* the case that the fscrypt master keys are hardware-wrapped keys).
*
* Copyright 2019 Google LLC
*/
#include "fscrypt_private.h"
/*
* HKDF supports any unkeyed cryptographic hash algorithm, but fscrypt uses
* SHA-512 because it is well-established, secure, and reasonably efficient.
*
* HKDF-SHA256 was also considered, as its 256-bit security strength would be
* sufficient here. A 512-bit security strength is "nice to have", though.
* Also, on 64-bit CPUs, SHA-512 is usually just as fast as SHA-256. In the
* common case of deriving an AES-256-XTS key (512 bits), that can result in
* HKDF-SHA512 being much faster than HKDF-SHA256, as the longer digest size of
* SHA-512 causes HKDF-Expand to only need to do one iteration rather than two.
*/
#define HKDF_HASHLEN SHA512_DIGEST_SIZE
/*
* HKDF consists of two steps:
*
* 1. HKDF-Extract: extract a pseudorandom key of length HKDF_HASHLEN bytes from
* the input keying material and optional salt.
* 2. HKDF-Expand: expand the pseudorandom key into output keying material of
* any length, parameterized by an application-specific info string.
*
* HKDF-Extract can be skipped if the input is already a pseudorandom key of
* length HKDF_HASHLEN bytes. However, cipher modes other than AES-256-XTS take
* shorter keys, and we don't want to force users of those modes to provide
* unnecessarily long master keys. Thus fscrypt still does HKDF-Extract. No
* salt is used, since fscrypt master keys should already be pseudorandom and
* there's no way to persist a random salt per master key from kernel mode.
*/
/*
* Compute HKDF-Extract using 'master_key' as the input keying material, and
* prepare the resulting HMAC key in 'hkdf'. Afterwards, 'hkdf' can be used for
* HKDF-Expand many times without having to recompute HKDF-Extract each time.
*/
void fscrypt_init_hkdf(struct hmac_sha512_key *hkdf, const u8 *master_key,
unsigned int master_key_size)
{
static const u8 default_salt[HKDF_HASHLEN];
u8 prk[HKDF_HASHLEN];
hmac_sha512_usingrawkey(default_salt, sizeof(default_salt),
master_key, master_key_size, prk);
hmac_sha512_preparekey(hkdf, prk, sizeof(prk));
memzero_explicit(prk, sizeof(prk));
}
/*
* HKDF-Expand (RFC 5869 section 2.3). Expand the HMAC key 'hkdf' into 'okmlen'
* bytes of output keying material parameterized by the application-specific
* 'info' of length 'infolen' bytes, prefixed by "fscrypt\0" and the 'context'
* byte. This is thread-safe and may be called by multiple threads in parallel.
*
* ('context' isn't part of the HKDF specification; it's just a prefix fscrypt
* adds to its application-specific info strings to guarantee that it doesn't
* accidentally repeat an info string when using HKDF for different purposes.)
*/
void fscrypt_hkdf_expand(const struct hmac_sha512_key *hkdf, u8 context,
const u8 *info, unsigned int infolen,
u8 *okm, unsigned int okmlen)
{
struct hmac_sha512_ctx ctx;
u8 counter = 1;
u8 tmp[HKDF_HASHLEN];
WARN_ON_ONCE(okmlen > 255 * HKDF_HASHLEN);
for (unsigned int i = 0; i < okmlen; i += HKDF_HASHLEN) {
hmac_sha512_init(&ctx, hkdf);
if (i != 0)
hmac_sha512_update(&ctx, &okm[i - HKDF_HASHLEN],
HKDF_HASHLEN);
hmac_sha512_update(&ctx, "fscrypt\0", 8);
hmac_sha512_update(&ctx, &context, 1);
hmac_sha512_update(&ctx, info, infolen);
hmac_sha512_update(&ctx, &counter, 1);
if (okmlen - i < HKDF_HASHLEN) {
hmac_sha512_final(&ctx, tmp);
memcpy(&okm[i], tmp, okmlen - i);
memzero_explicit(tmp, sizeof(tmp));
} else {
hmac_sha512_final(&ctx, &okm[i]);
}
counter++;
}
}
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