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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.
- *
- * Copyright 2019 Google LLC
- */
- #include <crypto/hash.h>
- #include <crypto/sha.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(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(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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