1 /*
2 * TLS PRF P_SHA256
3 * Copyright (c) 2011, Jouni Malinen <j@w1.fi>
4 *
5 * This software may be distributed under the terms of the BSD license.
6 * See README for more details.
7 */
8
9 #include "includes.h"
10
11 #include "common.h"
12 #include "sha256.h"
13
14
15 /**
16 * tls_prf_sha256 - Pseudo-Random Function for TLS v1.2 (P_SHA256, RFC 5246)
17 * @secret: Key for PRF
18 * @secret_len: Length of the key in bytes
19 * @label: A unique label for each purpose of the PRF
20 * @seed: Seed value to bind into the key
21 * @seed_len: Length of the seed
22 * @out: Buffer for the generated pseudo-random key
23 * @outlen: Number of bytes of key to generate
24 * Returns: 0 on success, -1 on failure.
25 *
26 * This function is used to derive new, cryptographically separate keys from a
27 * given key in TLS. This PRF is defined in RFC 2246, Chapter 5.
28 */
tls_prf_sha256(const u8 * secret,size_t secret_len,const char * label,const u8 * seed,size_t seed_len,u8 * out,size_t outlen)29 void tls_prf_sha256(const u8 *secret, size_t secret_len, const char *label,
30 const u8 *seed, size_t seed_len, u8 *out, size_t outlen)
31 {
32 size_t clen;
33 u8 A[SHA256_MAC_LEN];
34 u8 P[SHA256_MAC_LEN];
35 size_t pos;
36 const unsigned char *addr[3];
37 size_t len[3];
38
39 addr[0] = A;
40 len[0] = SHA256_MAC_LEN;
41 addr[1] = (unsigned char *) label;
42 len[1] = os_strlen(label);
43 addr[2] = seed;
44 len[2] = seed_len;
45
46 /*
47 * RFC 5246, Chapter 5
48 * A(0) = seed, A(i) = HMAC(secret, A(i-1))
49 * P_hash = HMAC(secret, A(1) + seed) + HMAC(secret, A(2) + seed) + ..
50 * PRF(secret, label, seed) = P_SHA256(secret, label + seed)
51 */
52
53 hmac_sha256_vector(secret, secret_len, 2, &addr[1], &len[1], A);
54
55 pos = 0;
56 while (pos < outlen) {
57 hmac_sha256_vector(secret, secret_len, 3, addr, len, P);
58 hmac_sha256(secret, secret_len, A, SHA256_MAC_LEN, A);
59
60 clen = outlen - pos;
61 if (clen > SHA256_MAC_LEN)
62 clen = SHA256_MAC_LEN;
63 os_memcpy(out + pos, P, clen);
64 pos += clen;
65 }
66 }
67