diff options
Diffstat (limited to 'fs/crypto/hkdf.c')
| -rw-r--r-- | fs/crypto/hkdf.c | 181 |
1 files changed, 181 insertions, 0 deletions
diff --git a/fs/crypto/hkdf.c b/fs/crypto/hkdf.c new file mode 100644 index 000000000000..f21873e1b467 --- /dev/null +++ b/fs/crypto/hkdf.c @@ -0,0 +1,181 @@ +// 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 reasonably secure and efficient; and since it produces + * a 64-byte digest, deriving an AES-256-XTS key preserves all 64 bytes of + * entropy from the master key and requires only one iteration of HKDF-Expand. + */ +#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]; + SHASH_DESC_ON_STACK(desc, hmac_tfm); + int err; + + err = crypto_shash_setkey(hmac_tfm, default_salt, HKDF_HASHLEN); + if (err) + return err; + + desc->tfm = hmac_tfm; + err = crypto_shash_digest(desc, ikm, ikmlen, prk); + shash_desc_zero(desc); + return err; +} + +/* + * 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(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); +} |