fs/crypto/hkdf.c

   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 }