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 well-established, secure, and reasonably efficient.
  20  *
  21  * HKDF-SHA256 was also considered, as its 256-bit security strength would be
  22  * 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 the
  24  * common case of deriving an AES-256-XTS key (512 bits), that can result in
  25  * HKDF-SHA512 being much faster than HKDF-SHA256, as the longer digest size of
  26  * 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_SIZE
  30
  31 /*
  32  * HKDF consists of two steps:
  33  *
  34  * 1. HKDF-Extract: extract a pseudorandom key of length HKDF_HASHLEN bytes from
  35  *    the input keying material and optional salt.
  36  * 2. HKDF-Expand: expand the pseudorandom key into output keying material of
  37  *    any length, parameterized by an application-specific info string.
  38  *
  39  * HKDF-Extract can be skipped if the input is already a pseudorandom key of
  40  * length HKDF_HASHLEN bytes.  However, cipher modes other than AES-256-XTS take
  41  * shorter keys, and we don't want to force users of those modes to provide
  42  * unnecessarily long master keys.  Thus fscrypt still does HKDF-Extract.  No
  43  * salt is used, since fscrypt master keys should already be pseudorandom and
  44  * 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 */
  48 static 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 many
  66  * times without having to recompute HKDF-Extract each time.
  67  */
  68 int 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
  98 err_free_tfm:
  99         crypto_free_shash(hmac_tfm);
 100 out:
 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, which
 107  * was already keyed into 'hkdf->hmac_tfm' by fscrypt_init_hkdf(), into 'okmlen'
 108  * bytes of output keying material parameterized by the application-specific
 109  * '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 fscrypt
 113  * adds to its application-specific info strings to guarantee that it doesn't
 114  * accidentally repeat an info string when using HKDF for different purposes.)
 115  */
 116 int 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;
 172 out:
 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
 179 void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf)
 180 {
 181         crypto_free_shash(hkdf->hmac_tfm);
 182 }