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1// SPDX-License-Identifier: GPL-2.02/*3 * Implementation of HKDF ("HMAC-based Extract-and-Expand Key Derivation4 * Function"), aka RFC 5869. See also the original paper (Krawczyk 2010):5 * "Cryptographic Extraction and Key Derivation: The HKDF Scheme".6 *7 * This is used to derive keys from the fscrypt master keys.8 *9 * Copyright 2019 Google LLC10 */11 12#include <crypto/hash.h>13#include <crypto/sha2.h>14 15#include "fscrypt_private.h"16 17/*18 * HKDF supports any unkeyed cryptographic hash algorithm, but fscrypt uses19 * SHA-512 because it is well-established, secure, and reasonably efficient.20 *21 * HKDF-SHA256 was also considered, as its 256-bit security strength would be22 * sufficient here. A 512-bit security strength is "nice to have", though.23 * Also, on 64-bit CPUs, SHA-512 is usually just as fast as SHA-256. In the24 * common case of deriving an AES-256-XTS key (512 bits), that can result in25 * HKDF-SHA512 being much faster than HKDF-SHA256, as the longer digest size of26 * SHA-512 causes HKDF-Expand to only need to do one iteration rather than two.27 */28#define HKDF_HMAC_ALG "hmac(sha512)"29#define HKDF_HASHLEN SHA512_DIGEST_SIZE30 31/*32 * HKDF consists of two steps:33 *34 * 1. HKDF-Extract: extract a pseudorandom key of length HKDF_HASHLEN bytes from35 * the input keying material and optional salt.36 * 2. HKDF-Expand: expand the pseudorandom key into output keying material of37 * any length, parameterized by an application-specific info string.38 *39 * HKDF-Extract can be skipped if the input is already a pseudorandom key of40 * length HKDF_HASHLEN bytes. However, cipher modes other than AES-256-XTS take41 * shorter keys, and we don't want to force users of those modes to provide42 * unnecessarily long master keys. Thus fscrypt still does HKDF-Extract. No43 * salt is used, since fscrypt master keys should already be pseudorandom and44 * there's no way to persist a random salt per master key from kernel mode.45 */46 47/* HKDF-Extract (RFC 5869 section 2.2), unsalted */48static int hkdf_extract(struct crypto_shash *hmac_tfm, const u8 *ikm,49 unsigned int ikmlen, u8 prk[HKDF_HASHLEN])50{51 static const u8 default_salt[HKDF_HASHLEN];52 int err;53 54 err = crypto_shash_setkey(hmac_tfm, default_salt, HKDF_HASHLEN);55 if (err)56 return err;57 58 return crypto_shash_tfm_digest(hmac_tfm, ikm, ikmlen, prk);59}60 61/*62 * Compute HKDF-Extract using the given master key as the input keying material,63 * and prepare an HMAC transform object keyed by the resulting pseudorandom key.64 *65 * Afterwards, the keyed HMAC transform object can be used for HKDF-Expand many66 * times without having to recompute HKDF-Extract each time.67 */68int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,69 unsigned int master_key_size)70{71 struct crypto_shash *hmac_tfm;72 u8 prk[HKDF_HASHLEN];73 int err;74 75 hmac_tfm = crypto_alloc_shash(HKDF_HMAC_ALG, 0, 0);76 if (IS_ERR(hmac_tfm)) {77 fscrypt_err(NULL, "Error allocating " HKDF_HMAC_ALG ": %ld",78 PTR_ERR(hmac_tfm));79 return PTR_ERR(hmac_tfm);80 }81 82 if (WARN_ON_ONCE(crypto_shash_digestsize(hmac_tfm) != sizeof(prk))) {83 err = -EINVAL;84 goto err_free_tfm;85 }86 87 err = hkdf_extract(hmac_tfm, master_key, master_key_size, prk);88 if (err)89 goto err_free_tfm;90 91 err = crypto_shash_setkey(hmac_tfm, prk, sizeof(prk));92 if (err)93 goto err_free_tfm;94 95 hkdf->hmac_tfm = hmac_tfm;96 goto out;97 98err_free_tfm:99 crypto_free_shash(hmac_tfm);100out:101 memzero_explicit(prk, sizeof(prk));102 return err;103}104 105/*106 * HKDF-Expand (RFC 5869 section 2.3). This expands the pseudorandom key, which107 * was already keyed into 'hkdf->hmac_tfm' by fscrypt_init_hkdf(), into 'okmlen'108 * bytes of output keying material parameterized by the application-specific109 * 'info' of length 'infolen' bytes, prefixed by "fscrypt\0" and the 'context'110 * byte. This is thread-safe and may be called by multiple threads in parallel.111 *112 * ('context' isn't part of the HKDF specification; it's just a prefix fscrypt113 * adds to its application-specific info strings to guarantee that it doesn't114 * accidentally repeat an info string when using HKDF for different purposes.)115 */116int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,117 const u8 *info, unsigned int infolen,118 u8 *okm, unsigned int okmlen)119{120 SHASH_DESC_ON_STACK(desc, hkdf->hmac_tfm);121 u8 prefix[9];122 unsigned int i;123 int err;124 const u8 *prev = NULL;125 u8 counter = 1;126 u8 tmp[HKDF_HASHLEN];127 128 if (WARN_ON_ONCE(okmlen > 255 * HKDF_HASHLEN))129 return -EINVAL;130 131 desc->tfm = hkdf->hmac_tfm;132 133 memcpy(prefix, "fscrypt\0", 8);134 prefix[8] = context;135 136 for (i = 0; i < okmlen; i += HKDF_HASHLEN) {137 138 err = crypto_shash_init(desc);139 if (err)140 goto out;141 142 if (prev) {143 err = crypto_shash_update(desc, prev, HKDF_HASHLEN);144 if (err)145 goto out;146 }147 148 err = crypto_shash_update(desc, prefix, sizeof(prefix));149 if (err)150 goto out;151 152 err = crypto_shash_update(desc, info, infolen);153 if (err)154 goto out;155 156 BUILD_BUG_ON(sizeof(counter) != 1);157 if (okmlen - i < HKDF_HASHLEN) {158 err = crypto_shash_finup(desc, &counter, 1, tmp);159 if (err)160 goto out;161 memcpy(&okm[i], tmp, okmlen - i);162 memzero_explicit(tmp, sizeof(tmp));163 } else {164 err = crypto_shash_finup(desc, &counter, 1, &okm[i]);165 if (err)166 goto out;167 }168 counter++;169 prev = &okm[i];170 }171 err = 0;172out:173 if (unlikely(err))174 memzero_explicit(okm, okmlen); /* so caller doesn't need to */175 shash_desc_zero(desc);176 return err;177}178 179void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf)180{181 crypto_free_shash(hkdf->hmac_tfm);182}183