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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