^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 1) // SPDX-License-Identifier: GPL-2.0
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 2) /*
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 3) * Implementation of HKDF ("HMAC-based Extract-and-Expand Key Derivation
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 4) * Function"), aka RFC 5869. See also the original paper (Krawczyk 2010):
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 5) * "Cryptographic Extraction and Key Derivation: The HKDF Scheme".
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 6) *
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 7) * This is used to derive keys from the fscrypt master keys.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 8) *
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 9) * Copyright 2019 Google LLC
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 10) */
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 11)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 12) #include <crypto/hash.h>
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 13) #include <crypto/sha.h>
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 14)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 15) #include "fscrypt_private.h"
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 16)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 17) /*
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 18) * HKDF supports any unkeyed cryptographic hash algorithm, but fscrypt uses
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 19) * SHA-512 because it is well-established, secure, and reasonably efficient.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 20) *
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 21) * HKDF-SHA256 was also considered, as its 256-bit security strength would be
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 22) * sufficient here. A 512-bit security strength is "nice to have", though.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 23) * Also, on 64-bit CPUs, SHA-512 is usually just as fast as SHA-256. In the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 24) * common case of deriving an AES-256-XTS key (512 bits), that can result in
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 25) * HKDF-SHA512 being much faster than HKDF-SHA256, as the longer digest size of
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 26) * SHA-512 causes HKDF-Expand to only need to do one iteration rather than two.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 27) */
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 28) #define HKDF_HMAC_ALG "hmac(sha512)"
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 29) #define HKDF_HASHLEN SHA512_DIGEST_SIZE
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 30)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 31) /*
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 32) * HKDF consists of two steps:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 33) *
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 34) * 1. HKDF-Extract: extract a pseudorandom key of length HKDF_HASHLEN bytes from
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 35) * the input keying material and optional salt.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 36) * 2. HKDF-Expand: expand the pseudorandom key into output keying material of
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 37) * any length, parameterized by an application-specific info string.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 38) *
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 39) * HKDF-Extract can be skipped if the input is already a pseudorandom key of
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 40) * length HKDF_HASHLEN bytes. However, cipher modes other than AES-256-XTS take
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 41) * shorter keys, and we don't want to force users of those modes to provide
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 42) * unnecessarily long master keys. Thus fscrypt still does HKDF-Extract. No
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 43) * salt is used, since fscrypt master keys should already be pseudorandom and
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 44) * there's no way to persist a random salt per master key from kernel mode.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 45) */
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 46)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 47) /* HKDF-Extract (RFC 5869 section 2.2), unsalted */
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 48) static int hkdf_extract(struct crypto_shash *hmac_tfm, const u8 *ikm,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 49) unsigned int ikmlen, u8 prk[HKDF_HASHLEN])
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 50) {
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 51) static const u8 default_salt[HKDF_HASHLEN];
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 52) int err;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 53)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 54) err = crypto_shash_setkey(hmac_tfm, default_salt, HKDF_HASHLEN);
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 55) if (err)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 56) return err;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 57)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 58) return crypto_shash_tfm_digest(hmac_tfm, ikm, ikmlen, prk);
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 59) }
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 60)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 61) /*
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 62) * Compute HKDF-Extract using the given master key as the input keying material,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 63) * and prepare an HMAC transform object keyed by the resulting pseudorandom key.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 64) *
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 65) * Afterwards, the keyed HMAC transform object can be used for HKDF-Expand many
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 66) * times without having to recompute HKDF-Extract each time.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 67) */
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 68) int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 69) unsigned int master_key_size)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 70) {
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 71) struct crypto_shash *hmac_tfm;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 72) u8 prk[HKDF_HASHLEN];
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 73) int err;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 74)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 75) hmac_tfm = crypto_alloc_shash(HKDF_HMAC_ALG, 0, 0);
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 76) if (IS_ERR(hmac_tfm)) {
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 77) fscrypt_err(NULL, "Error allocating " HKDF_HMAC_ALG ": %ld",
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 78) PTR_ERR(hmac_tfm));
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 79) return PTR_ERR(hmac_tfm);
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 80) }
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 81)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 82) if (WARN_ON(crypto_shash_digestsize(hmac_tfm) != sizeof(prk))) {
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 83) err = -EINVAL;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 84) goto err_free_tfm;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 85) }
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 86)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 87) err = hkdf_extract(hmac_tfm, master_key, master_key_size, prk);
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 88) if (err)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 89) goto err_free_tfm;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 90)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 91) err = crypto_shash_setkey(hmac_tfm, prk, sizeof(prk));
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 92) if (err)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 93) goto err_free_tfm;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 94)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 95) hkdf->hmac_tfm = hmac_tfm;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 96) goto out;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 97)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 98) err_free_tfm:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 99) crypto_free_shash(hmac_tfm);
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 100) out:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 101) memzero_explicit(prk, sizeof(prk));
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 102) return err;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 103) }
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 104)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 105) /*
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 106) * HKDF-Expand (RFC 5869 section 2.3). This expands the pseudorandom key, which
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 107) * was already keyed into 'hkdf->hmac_tfm' by fscrypt_init_hkdf(), into 'okmlen'
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 108) * bytes of output keying material parameterized by the application-specific
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 109) * 'info' of length 'infolen' bytes, prefixed by "fscrypt\0" and the 'context'
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 110) * byte. This is thread-safe and may be called by multiple threads in parallel.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 111) *
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 112) * ('context' isn't part of the HKDF specification; it's just a prefix fscrypt
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 113) * adds to its application-specific info strings to guarantee that it doesn't
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 114) * accidentally repeat an info string when using HKDF for different purposes.)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 115) */
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 116) int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 117) const u8 *info, unsigned int infolen,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 118) u8 *okm, unsigned int okmlen)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 119) {
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 120) SHASH_DESC_ON_STACK(desc, hkdf->hmac_tfm);
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 121) u8 prefix[9];
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 122) unsigned int i;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 123) int err;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 124) const u8 *prev = NULL;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 125) u8 counter = 1;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 126) u8 tmp[HKDF_HASHLEN];
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 127)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 128) if (WARN_ON(okmlen > 255 * HKDF_HASHLEN))
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 129) return -EINVAL;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 130)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 131) desc->tfm = hkdf->hmac_tfm;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 132)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 133) memcpy(prefix, "fscrypt\0", 8);
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 134) prefix[8] = context;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 135)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 136) for (i = 0; i < okmlen; i += HKDF_HASHLEN) {
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 137)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 138) err = crypto_shash_init(desc);
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 139) if (err)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 140) goto out;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 141)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 142) if (prev) {
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 143) err = crypto_shash_update(desc, prev, HKDF_HASHLEN);
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 144) if (err)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 145) goto out;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 146) }
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 147)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 148) err = crypto_shash_update(desc, prefix, sizeof(prefix));
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 149) if (err)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 150) goto out;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 151)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 152) err = crypto_shash_update(desc, info, infolen);
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 153) if (err)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 154) goto out;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 155)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 156) BUILD_BUG_ON(sizeof(counter) != 1);
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 157) if (okmlen - i < HKDF_HASHLEN) {
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 158) err = crypto_shash_finup(desc, &counter, 1, tmp);
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 159) if (err)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 160) goto out;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 161) memcpy(&okm[i], tmp, okmlen - i);
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 162) memzero_explicit(tmp, sizeof(tmp));
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 163) } else {
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 164) err = crypto_shash_finup(desc, &counter, 1, &okm[i]);
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 165) if (err)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 166) goto out;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 167) }
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 168) counter++;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 169) prev = &okm[i];
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 170) }
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 171) err = 0;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 172) out:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 173) if (unlikely(err))
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 174) memzero_explicit(okm, okmlen); /* so caller doesn't need to */
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 175) shash_desc_zero(desc);
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 176) return err;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 177) }
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 178)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 179) void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 180) {
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 181) crypto_free_shash(hkdf->hmac_tfm);
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 182) }
Orange Pi5 kernel
Deprecated Linux kernel 5.10.110 for OrangePi 5/5B/5+ boards
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