1 // SPDX-License-Identifier: GPL-2.0 2 /* 3 * Implementation of HKDF ("HMAC-based Extract-and-Expand Key Derivation 4 * 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 LLC 10 */ 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 uses 19 * SHA-512 because it is reasonably secure and efficient; and since it produces 20 * a 64-byte digest, deriving an AES-256-XTS key preserves all 64 bytes of 21 * entropy from the master key and requires only one iteration of HKDF-Expand. 22 */ 23 #define HKDF_HMAC_ALG "hmac(sha512)" 24 #define HKDF_HASHLEN SHA512_DIGEST_SIZE 25 26 /* 27 * HKDF consists of two steps: 28 * 29 * 1. HKDF-Extract: extract a pseudorandom key of length HKDF_HASHLEN bytes from 30 * the input keying material and optional salt. 31 * 2. HKDF-Expand: expand the pseudorandom key into output keying material of 32 * any length, parameterized by an application-specific info string. 33 * 34 * HKDF-Extract can be skipped if the input is already a pseudorandom key of 35 * length HKDF_HASHLEN bytes. However, cipher modes other than AES-256-XTS take 36 * shorter keys, and we don't want to force users of those modes to provide 37 * unnecessarily long master keys. Thus fscrypt still does HKDF-Extract. No 38 * salt is used, since fscrypt master keys should already be pseudorandom and 39 * there's no way to persist a random salt per master key from kernel mode. 40 */ 41 42 /* HKDF-Extract (RFC 5869 section 2.2), unsalted */ 43 static int hkdf_extract(struct crypto_shash *hmac_tfm, const u8 *ikm, 44 unsigned int ikmlen, u8 prk[HKDF_HASHLEN]) 45 { 46 static const u8 default_salt[HKDF_HASHLEN]; 47 int err; 48 49 err = crypto_shash_setkey(hmac_tfm, default_salt, HKDF_HASHLEN); 50 if (err) 51 return err; 52 53 return crypto_shash_tfm_digest(hmac_tfm, ikm, ikmlen, prk); 54 } 55 56 /* 57 * Compute HKDF-Extract using the given master key as the input keying material, 58 * and prepare an HMAC transform object keyed by the resulting pseudorandom key. 59 * 60 * Afterwards, the keyed HMAC transform object can be used for HKDF-Expand many 61 * times without having to recompute HKDF-Extract each time. 62 */ 63 int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key, 64 unsigned int master_key_size) 65 { 66 struct crypto_shash *hmac_tfm; 67 u8 prk[HKDF_HASHLEN]; 68 int err; 69 70 hmac_tfm = crypto_alloc_shash(HKDF_HMAC_ALG, 0, 0); 71 if (IS_ERR(hmac_tfm)) { 72 fscrypt_err(NULL, "Error allocating " HKDF_HMAC_ALG ": %ld", 73 PTR_ERR(hmac_tfm)); 74 return PTR_ERR(hmac_tfm); 75 } 76 77 if (WARN_ON(crypto_shash_digestsize(hmac_tfm) != sizeof(prk))) { 78 err = -EINVAL; 79 goto err_free_tfm; 80 } 81 82 err = hkdf_extract(hmac_tfm, master_key, master_key_size, prk); 83 if (err) 84 goto err_free_tfm; 85 86 err = crypto_shash_setkey(hmac_tfm, prk, sizeof(prk)); 87 if (err) 88 goto err_free_tfm; 89 90 hkdf->hmac_tfm = hmac_tfm; 91 goto out; 92 93 err_free_tfm: 94 crypto_free_shash(hmac_tfm); 95 out: 96 memzero_explicit(prk, sizeof(prk)); 97 return err; 98 } 99 100 /* 101 * HKDF-Expand (RFC 5869 section 2.3). This expands the pseudorandom key, which 102 * was already keyed into 'hkdf->hmac_tfm' by fscrypt_init_hkdf(), into 'okmlen' 103 * bytes of output keying material parameterized by the application-specific 104 * 'info' of length 'infolen' bytes, prefixed by "fscrypt\0" and the 'context' 105 * byte. This is thread-safe and may be called by multiple threads in parallel. 106 * 107 * ('context' isn't part of the HKDF specification; it's just a prefix fscrypt 108 * adds to its application-specific info strings to guarantee that it doesn't 109 * accidentally repeat an info string when using HKDF for different purposes.) 110 */ 111 int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context, 112 const u8 *info, unsigned int infolen, 113 u8 *okm, unsigned int okmlen) 114 { 115 SHASH_DESC_ON_STACK(desc, hkdf->hmac_tfm); 116 u8 prefix[9]; 117 unsigned int i; 118 int err; 119 const u8 *prev = NULL; 120 u8 counter = 1; 121 u8 tmp[HKDF_HASHLEN]; 122 123 if (WARN_ON(okmlen > 255 * HKDF_HASHLEN)) 124 return -EINVAL; 125 126 desc->tfm = hkdf->hmac_tfm; 127 128 memcpy(prefix, "fscrypt\0", 8); 129 prefix[8] = context; 130 131 for (i = 0; i < okmlen; i += HKDF_HASHLEN) { 132 133 err = crypto_shash_init(desc); 134 if (err) 135 goto out; 136 137 if (prev) { 138 err = crypto_shash_update(desc, prev, HKDF_HASHLEN); 139 if (err) 140 goto out; 141 } 142 143 err = crypto_shash_update(desc, prefix, sizeof(prefix)); 144 if (err) 145 goto out; 146 147 err = crypto_shash_update(desc, info, infolen); 148 if (err) 149 goto out; 150 151 BUILD_BUG_ON(sizeof(counter) != 1); 152 if (okmlen - i < HKDF_HASHLEN) { 153 err = crypto_shash_finup(desc, &counter, 1, tmp); 154 if (err) 155 goto out; 156 memcpy(&okm[i], tmp, okmlen - i); 157 memzero_explicit(tmp, sizeof(tmp)); 158 } else { 159 err = crypto_shash_finup(desc, &counter, 1, &okm[i]); 160 if (err) 161 goto out; 162 } 163 counter++; 164 prev = &okm[i]; 165 } 166 err = 0; 167 out: 168 if (unlikely(err)) 169 memzero_explicit(okm, okmlen); /* so caller doesn't need to */ 170 shash_desc_zero(desc); 171 return err; 172 } 173 174 void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf) 175 { 176 crypto_free_shash(hkdf->hmac_tfm); 177 } 178