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3241cd0c6c
Separate out the HKDF functions into a separate module to to make them available to other callers. And add a testsuite to the module with test vectors from RFC 5869 (and additional vectors for SHA384 and SHA512) to ensure the integrity of the algorithm. Signed-off-by: Hannes Reinecke <hare@kernel.org> Acked-by: Eric Biggers <ebiggers@kernel.org> Acked-by: Herbert Xu <herbert@gondor.apana.org.au> Signed-off-by: Keith Busch <kbusch@kernel.org>
128 lines
4.1 KiB
C
128 lines
4.1 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/*
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* This is used to derive keys from the fscrypt master keys.
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*
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* Copyright 2019 Google LLC
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*/
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#include <crypto/hash.h>
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#include <crypto/sha2.h>
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#include <crypto/hkdf.h>
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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_HMAC_ALG "hmac(sha512)"
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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 the given master key as the input keying material,
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* and prepare an HMAC transform object keyed by the resulting pseudorandom key.
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*
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* Afterwards, the keyed HMAC transform object can be used for HKDF-Expand many
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* times without having to recompute HKDF-Extract each time.
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*/
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int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
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unsigned int master_key_size)
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{
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struct crypto_shash *hmac_tfm;
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static const u8 default_salt[HKDF_HASHLEN];
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u8 prk[HKDF_HASHLEN];
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int err;
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hmac_tfm = crypto_alloc_shash(HKDF_HMAC_ALG, 0, 0);
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if (IS_ERR(hmac_tfm)) {
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fscrypt_err(NULL, "Error allocating " HKDF_HMAC_ALG ": %ld",
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PTR_ERR(hmac_tfm));
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return PTR_ERR(hmac_tfm);
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}
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if (WARN_ON_ONCE(crypto_shash_digestsize(hmac_tfm) != sizeof(prk))) {
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err = -EINVAL;
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goto err_free_tfm;
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}
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err = hkdf_extract(hmac_tfm, master_key, master_key_size,
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default_salt, HKDF_HASHLEN, prk);
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if (err)
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goto err_free_tfm;
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err = crypto_shash_setkey(hmac_tfm, prk, sizeof(prk));
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if (err)
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goto err_free_tfm;
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hkdf->hmac_tfm = hmac_tfm;
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goto out;
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err_free_tfm:
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crypto_free_shash(hmac_tfm);
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out:
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memzero_explicit(prk, sizeof(prk));
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return err;
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}
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/*
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* HKDF-Expand (RFC 5869 section 2.3). This expands the pseudorandom key, which
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* was already keyed into 'hkdf->hmac_tfm' by fscrypt_init_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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int fscrypt_hkdf_expand(const struct fscrypt_hkdf *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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SHASH_DESC_ON_STACK(desc, hkdf->hmac_tfm);
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u8 *full_info;
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int err;
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full_info = kzalloc(infolen + 9, GFP_KERNEL);
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if (!full_info)
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return -ENOMEM;
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desc->tfm = hkdf->hmac_tfm;
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memcpy(full_info, "fscrypt\0", 8);
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full_info[8] = context;
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memcpy(full_info + 9, info, infolen);
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err = hkdf_expand(hkdf->hmac_tfm, full_info, infolen + 9,
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okm, okmlen);
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kfree_sensitive(full_info);
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return err;
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}
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void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf)
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{
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crypto_free_shash(hkdf->hmac_tfm);
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}
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