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/sha.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         SHASH_DESC_ON_STACK(desc, hmac_tfm);
  48         int err;
  49
  50         err = crypto_shash_setkey(hmac_tfm, default_salt, HKDF_HASHLEN);
  51         if (err)
  52                 return err;
  53
  54         desc->tfm = hmac_tfm;
  55         err = crypto_shash_digest(desc, ikm, ikmlen, prk);
  56         shash_desc_zero(desc);
  57         return err;
  58 }
  59
  60 /*
  61  * Compute HKDF-Extract using the given master key as the input keying material,
  62  * and prepare an HMAC transform object keyed by the resulting pseudorandom key.
  63  *
  64  * Afterwards, the keyed HMAC transform object can be used for HKDF-Expand many
  65  * times without having to recompute HKDF-Extract each time.
  66  */
  67 int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
  68                       unsigned int master_key_size)
  69 {
  70         struct crypto_shash *hmac_tfm;
  71         u8 prk[HKDF_HASHLEN];
  72         int err;
  73
  74         hmac_tfm = crypto_alloc_shash(HKDF_HMAC_ALG, 0, 0);
  75         if (IS_ERR(hmac_tfm)) {
  76                 fscrypt_err(NULL, "Error allocating " HKDF_HMAC_ALG ": %ld",
  77                             PTR_ERR(hmac_tfm));
  78                 return PTR_ERR(hmac_tfm);
  79         }
  80
  81         if (WARN_ON(crypto_shash_digestsize(hmac_tfm) != sizeof(prk))) {
  82                 err = -EINVAL;
  83                 goto err_free_tfm;
  84         }
  85
  86         err = hkdf_extract(hmac_tfm, master_key, master_key_size, prk);
  87         if (err)
  88                 goto err_free_tfm;
  89
  90         err = crypto_shash_setkey(hmac_tfm, prk, sizeof(prk));
  91         if (err)
  92                 goto err_free_tfm;
  93
  94         hkdf->hmac_tfm = hmac_tfm;
  95         goto out;
  96
  97 err_free_tfm:
  98         crypto_free_shash(hmac_tfm);
  99 out:
 100         memzero_explicit(prk, sizeof(prk));
 101         return err;
 102 }
 103
 104 /*
 105  * HKDF-Expand (RFC 5869 section 2.3).  This expands the pseudorandom key, which
 106  * was already keyed into 'hkdf->hmac_tfm' by fscrypt_init_hkdf(), into 'okmlen'
 107  * bytes of output keying material parameterized by the application-specific
 108  * 'info' of length 'infolen' bytes, prefixed by "fscrypt\0" and the 'context'
 109  * byte.  This is thread-safe and may be called by multiple threads in parallel.
 110  *
 111  * ('context' isn't part of the HKDF specification; it's just a prefix fscrypt
 112  * adds to its application-specific info strings to guarantee that it doesn't
 113  * accidentally repeat an info string when using HKDF for different purposes.)
 114  */
 115 int fscrypt_hkdf_expand(struct fscrypt_hkdf *hkdf, u8 context,
 116                         const u8 *info, unsigned int infolen,
 117                         u8 *okm, unsigned int okmlen)
 118 {
 119         SHASH_DESC_ON_STACK(desc, hkdf->hmac_tfm);
 120         u8 prefix[9];
 121         unsigned int i;
 122         int err;
 123         const u8 *prev = NULL;
 124         u8 counter = 1;
 125         u8 tmp[HKDF_HASHLEN];
 126
 127         if (WARN_ON(okmlen > 255 * HKDF_HASHLEN))
 128                 return -EINVAL;
 129
 130         desc->tfm = hkdf->hmac_tfm;
 131
 132         memcpy(prefix, "fscrypt\0", 8);
 133         prefix[8] = context;
 134
 135         for (i = 0; i < okmlen; i += HKDF_HASHLEN) {
 136
 137                 err = crypto_shash_init(desc);
 138                 if (err)
 139                         goto out;
 140
 141                 if (prev) {
 142                         err = crypto_shash_update(desc, prev, HKDF_HASHLEN);
 143                         if (err)
 144                                 goto out;
 145                 }
 146
 147                 err = crypto_shash_update(desc, prefix, sizeof(prefix));
 148                 if (err)
 149                         goto out;
 150
 151                 err = crypto_shash_update(desc, info, infolen);
 152                 if (err)
 153                         goto out;
 154
 155                 BUILD_BUG_ON(sizeof(counter) != 1);
 156                 if (okmlen - i < HKDF_HASHLEN) {
 157                         err = crypto_shash_finup(desc, &counter, 1, tmp);
 158                         if (err)
 159                                 goto out;
 160                         memcpy(&okm[i], tmp, okmlen - i);
 161                         memzero_explicit(tmp, sizeof(tmp));
 162                 } else {
 163                         err = crypto_shash_finup(desc, &counter, 1, &okm[i]);
 164                         if (err)
 165                                 goto out;
 166                 }
 167                 counter++;
 168                 prev = &okm[i];
 169         }
 170         err = 0;
 171 out:
 172         if (unlikely(err))
 173                 memzero_explicit(okm, okmlen); /* so caller doesn't need to */
 174         shash_desc_zero(desc);
 175         return err;
 176 }
 177
 178 void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf)
 179 {
 180         crypto_free_shash(hkdf->hmac_tfm);
 181 }