1 // SPDX-License-Identifier: GPL-2.0 2 /* 3 * This is used to derive keys from the fscrypt master keys. 4 * 5 * Copyright 2019 Google LLC 6 */ 7 8 #include <crypto/hash.h> 9 #include <crypto/sha2.h> 10 #include <crypto/hkdf.h> 11 12 #include "fscrypt_private.h" 13 14 /* 15 * HKDF supports any unkeyed cryptographic hash algorithm, but fscrypt uses 16 * SHA-512 because it is well-established, secure, and reasonably efficient. 17 * 18 * HKDF-SHA256 was also considered, as its 256-bit security strength would be 19 * sufficient here. A 512-bit security strength is "nice to have", though. 20 * Also, on 64-bit CPUs, SHA-512 is usually just as fast as SHA-256. In the 21 * common case of deriving an AES-256-XTS key (512 bits), that can result in 22 * HKDF-SHA512 being much faster than HKDF-SHA256, as the longer digest size of 23 * SHA-512 causes HKDF-Expand to only need to do one iteration rather than two. 24 */ 25 #define HKDF_HMAC_ALG "hmac(sha512)" 26 #define HKDF_HASHLEN SHA512_DIGEST_SIZE 27 28 /* 29 * HKDF consists of two steps: 30 * 31 * 1. HKDF-Extract: extract a pseudorandom key of length HKDF_HASHLEN bytes from 32 * the input keying material and optional salt. 33 * 2. HKDF-Expand: expand the pseudorandom key into output keying material of 34 * any length, parameterized by an application-specific info string. 35 * 36 * HKDF-Extract can be skipped if the input is already a pseudorandom key of 37 * length HKDF_HASHLEN bytes. However, cipher modes other than AES-256-XTS take 38 * shorter keys, and we don't want to force users of those modes to provide 39 * unnecessarily long master keys. Thus fscrypt still does HKDF-Extract. No 40 * salt is used, since fscrypt master keys should already be pseudorandom and 41 * there's no way to persist a random salt per master key from kernel mode. 42 */ 43 44 /* 45 * Compute HKDF-Extract using the given master key as the input keying material, 46 * and prepare an HMAC transform object keyed by the resulting pseudorandom key. 47 * 48 * Afterwards, the keyed HMAC transform object can be used for HKDF-Expand many 49 * times without having to recompute HKDF-Extract each time. 50 */ 51 int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key, 52 unsigned int master_key_size) 53 { 54 struct crypto_shash *hmac_tfm; 55 static const u8 default_salt[HKDF_HASHLEN]; 56 u8 prk[HKDF_HASHLEN]; 57 int err; 58 59 hmac_tfm = crypto_alloc_shash(HKDF_HMAC_ALG, 0, 0); 60 if (IS_ERR(hmac_tfm)) { 61 fscrypt_err(NULL, "Error allocating " HKDF_HMAC_ALG ": %ld", 62 PTR_ERR(hmac_tfm)); 63 return PTR_ERR(hmac_tfm); 64 } 65 66 if (WARN_ON_ONCE(crypto_shash_digestsize(hmac_tfm) != sizeof(prk))) { 67 err = -EINVAL; 68 goto err_free_tfm; 69 } 70 71 err = hkdf_extract(hmac_tfm, master_key, master_key_size, 72 default_salt, HKDF_HASHLEN, prk); 73 if (err) 74 goto err_free_tfm; 75 76 err = crypto_shash_setkey(hmac_tfm, prk, sizeof(prk)); 77 if (err) 78 goto err_free_tfm; 79 80 hkdf->hmac_tfm = hmac_tfm; 81 goto out; 82 83 err_free_tfm: 84 crypto_free_shash(hmac_tfm); 85 out: 86 memzero_explicit(prk, sizeof(prk)); 87 return err; 88 } 89 90 /* 91 * HKDF-Expand (RFC 5869 section 2.3). This expands the pseudorandom key, which 92 * was already keyed into 'hkdf->hmac_tfm' by fscrypt_init_hkdf(), into 'okmlen' 93 * bytes of output keying material parameterized by the application-specific 94 * 'info' of length 'infolen' bytes, prefixed by "fscrypt\0" and the 'context' 95 * byte. This is thread-safe and may be called by multiple threads in parallel. 96 * 97 * ('context' isn't part of the HKDF specification; it's just a prefix fscrypt 98 * adds to its application-specific info strings to guarantee that it doesn't 99 * accidentally repeat an info string when using HKDF for different purposes.) 100 */ 101 int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context, 102 const u8 *info, unsigned int infolen, 103 u8 *okm, unsigned int okmlen) 104 { 105 SHASH_DESC_ON_STACK(desc, hkdf->hmac_tfm); 106 u8 *full_info; 107 int err; 108 109 full_info = kzalloc(infolen + 9, GFP_KERNEL); 110 if (!full_info) 111 return -ENOMEM; 112 desc->tfm = hkdf->hmac_tfm; 113 114 memcpy(full_info, "fscrypt\0", 8); 115 full_info[8] = context; 116 memcpy(full_info + 9, info, infolen); 117 118 err = hkdf_expand(hkdf->hmac_tfm, full_info, infolen + 9, 119 okm, okmlen); 120 kfree_sensitive(full_info); 121 return err; 122 } 123 124 void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf) 125 { 126 crypto_free_shash(hkdf->hmac_tfm); 127 } 128