// 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.
*
* Copyright 2019 Google LLC
*/
#include <crypto/hash.h>
#include <crypto/sha2.h>
#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_HMAC_ALG "hmac(sha512)"
#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.
*/
/* HKDF-Extract (RFC 5869 section 2.2), unsalted */
static int hkdf_extract(struct crypto_shash *hmac_tfm, const u8 *ikm,
unsigned int ikmlen, u8 prk[HKDF_HASHLEN])
{
static const u8 default_salt[HKDF_HASHLEN];
int err;
err = crypto_shash_setkey(hmac_tfm, default_salt, HKDF_HASHLEN);
if (err)
return err;
return crypto_shash_tfm_digest(hmac_tfm, ikm, ikmlen, prk);
}
/*
* Compute HKDF-Extract using the given master key as the input keying material,
* and prepare an HMAC transform object keyed by the resulting pseudorandom key.
*
* Afterwards, the keyed HMAC transform object can be used for HKDF-Expand many
* times without having to recompute HKDF-Extract each time.
*/
int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
unsigned int master_key_size)
{
struct crypto_shash *hmac_tfm;
u8 prk[HKDF_HASHLEN];
int err;
hmac_tfm = crypto_alloc_shash(HKDF_HMAC_ALG, 0, 0);
if (IS_ERR(hmac_tfm)) {
fscrypt_err(NULL, "Error allocating " HKDF_HMAC_ALG ": %ld",
PTR_ERR(hmac_tfm));
return PTR_ERR(hmac_tfm);
}
if (WARN_ON_ONCE(crypto_shash_digestsize(hmac_tfm) != sizeof(prk))) {
err = -EINVAL;
goto err_free_tfm;
}
err = hkdf_extract(hmac_tfm, master_key, master_key_size, prk);
if (err)
goto err_free_tfm;
err = crypto_shash_setkey(hmac_tfm, prk, sizeof(prk));
if (err)
goto err_free_tfm;
hkdf->hmac_tfm = hmac_tfm;
goto out;
err_free_tfm:
crypto_free_shash(hmac_tfm);
out:
memzero_explicit(prk, sizeof(prk));
return err;
}
/*
* HKDF-Expand (RFC 5869 section 2.3). This expands the pseudorandom key, which
* was already keyed into 'hkdf->hmac_tfm' by fscrypt_init_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.)
*/
int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
const u8 *info, unsigned int infolen,
u8 *okm, unsigned int okmlen)
{
SHASH_DESC_ON_STACK(desc, hkdf->hmac_tfm);
u8 prefix[9];
unsigned int i;
int err;
const u8 *prev = NULL;
u8 counter = 1;
u8 tmp[HKDF_HASHLEN];
if (WARN_ON_ONCE(okmlen > 255 * HKDF_HASHLEN))
return -EINVAL;
desc->tfm = hkdf->hmac_tfm;
memcpy(prefix, "fscrypt\0", 8);
prefix[8] = context;
for (i = 0; i < okmlen; i += HKDF_HASHLEN) {
err = crypto_shash_init(desc);
if (err)
goto out;
if (prev) {
err = crypto_shash_update(desc, prev, HKDF_HASHLEN);
if (err)
goto out;
}
err = crypto_shash_update(desc, prefix, sizeof(prefix));
if (err)
goto out;
err = crypto_shash_update(desc, info, infolen);
if (err)
goto out;
BUILD_BUG_ON(sizeof(counter) != 1);
if (okmlen - i < HKDF_HASHLEN) {
err = crypto_shash_finup(desc, &counter, 1, tmp);
if (err)
goto out;
memcpy(&okm[i], tmp, okmlen - i);
memzero_explicit(tmp, sizeof(tmp));
} else {
err = crypto_shash_finup(desc, &counter, 1, &okm[i]);
if (err)
goto out;
}
counter++;
prev = &okm[i];
}
err = 0;
out:
if (unlikely(err))
memzero_explicit(okm, okmlen); /* so caller doesn't need to */
shash_desc_zero(desc);
return err;
}
void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf)
{
crypto_free_shash(hkdf->hmac_tfm);
}
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