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![Eric Biggers](/assets/img/avatar_default.png)
Currently <crypto/sha.h> contains declarations for both SHA-1 and SHA-2, and <crypto/sha3.h> contains declarations for SHA-3. This organization is inconsistent, but more importantly SHA-1 is no longer considered to be cryptographically secure. So to the extent possible, SHA-1 shouldn't be grouped together with any of the other SHA versions, and usage of it should be phased out. Therefore, split <crypto/sha.h> into two headers <crypto/sha1.h> and <crypto/sha2.h>, and make everyone explicitly specify whether they want the declarations for SHA-1, SHA-2, or both. This avoids making the SHA-1 declarations visible to files that don't want anything to do with SHA-1. It also prepares for potentially moving sha1.h into a new insecure/ or dangerous/ directory. Signed-off-by: Eric Biggers <ebiggers@google.com> Acked-by: Ard Biesheuvel <ardb@kernel.org> Acked-by: Jason A. Donenfeld <Jason@zx2c4.com> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
177 lines
5.1 KiB
C
177 lines
5.1 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.
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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 "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 reasonably secure and efficient; and since it produces
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* a 64-byte digest, deriving an AES-256-XTS key preserves all 64 bytes of
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* entropy from the master key and requires only one iteration of HKDF-Expand.
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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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/* HKDF-Extract (RFC 5869 section 2.2), unsalted */
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static int hkdf_extract(struct crypto_shash *hmac_tfm, const u8 *ikm,
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unsigned int ikmlen, u8 prk[HKDF_HASHLEN])
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{
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static const u8 default_salt[HKDF_HASHLEN];
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int err;
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err = crypto_shash_setkey(hmac_tfm, default_salt, HKDF_HASHLEN);
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if (err)
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return err;
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return crypto_shash_tfm_digest(hmac_tfm, ikm, ikmlen, prk);
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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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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(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, 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 prefix[9];
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unsigned int i;
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int err;
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const u8 *prev = NULL;
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u8 counter = 1;
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u8 tmp[HKDF_HASHLEN];
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if (WARN_ON(okmlen > 255 * HKDF_HASHLEN))
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return -EINVAL;
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desc->tfm = hkdf->hmac_tfm;
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memcpy(prefix, "fscrypt\0", 8);
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prefix[8] = context;
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for (i = 0; i < okmlen; i += HKDF_HASHLEN) {
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err = crypto_shash_init(desc);
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if (err)
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goto out;
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if (prev) {
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err = crypto_shash_update(desc, prev, HKDF_HASHLEN);
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if (err)
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goto out;
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}
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err = crypto_shash_update(desc, prefix, sizeof(prefix));
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if (err)
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goto out;
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err = crypto_shash_update(desc, info, infolen);
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if (err)
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goto out;
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BUILD_BUG_ON(sizeof(counter) != 1);
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if (okmlen - i < HKDF_HASHLEN) {
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err = crypto_shash_finup(desc, &counter, 1, tmp);
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if (err)
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goto out;
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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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err = crypto_shash_finup(desc, &counter, 1, &okm[i]);
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if (err)
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goto out;
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}
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counter++;
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prev = &okm[i];
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}
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err = 0;
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out:
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if (unlikely(err))
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memzero_explicit(okm, okmlen); /* so caller doesn't need to */
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shash_desc_zero(desc);
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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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