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