1 /*
2  * HMAC-SHA256 KDF (RFC 5295) and HKDF-Expand(SHA256) (RFC 5869)
3  * Copyright (c) 2014-2017, 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  * hmac_sha256_kdf - HMAC-SHA256 based KDF (RFC 5295)
17  * @secret: Key for KDF
18  * @secret_len: Length of the key in bytes
19  * @label: A unique label for each purpose of the KDF or %NULL to select
20  *	RFC 5869 HKDF-Expand() with arbitrary seed (= info)
21  * @seed: Seed value to bind into the key
22  * @seed_len: Length of the seed
23  * @out: Buffer for the generated pseudo-random key
24  * @outlen: Number of bytes of key to generate
25  * Returns: 0 on success, -1 on failure.
26  *
27  * This function is used to derive new, cryptographically separate keys from a
28  * given key in ERP. This KDF is defined in RFC 5295, Chapter 3.1.2. When used
29  * with label = NULL and seed = info, this matches HKDF-Expand() defined in
30  * RFC 5869, Chapter 2.3.
31  */
32 int hmac_sha256_kdf(const u8 *secret, size_t secret_len,
33 		    const char *label, const u8 *seed, size_t seed_len,
34 		    u8 *out, size_t outlen)
35 {
36 	u8 T[SHA256_MAC_LEN];
37 	u8 iter = 1;
38 	const unsigned char *addr[4];
39 	size_t len[4];
40 	size_t pos, clen;
41 
42 	addr[0] = T;
43 	len[0] = SHA256_MAC_LEN;
44 	if (label) {
45 		addr[1] = (const unsigned char *) label;
46 		len[1] = os_strlen(label) + 1;
47 	} else {
48 		addr[1] = (const u8 *) "";
49 		len[1] = 0;
50 	}
51 	addr[2] = seed;
52 	len[2] = seed_len;
53 	addr[3] = &iter;
54 	len[3] = 1;
55 
56 	if (hmac_sha256_vector(secret, secret_len, 3, &addr[1], &len[1], T) < 0)
57 		return -1;
58 
59 	pos = 0;
60 	for (;;) {
61 		clen = outlen - pos;
62 		if (clen > SHA256_MAC_LEN)
63 			clen = SHA256_MAC_LEN;
64 		os_memcpy(out + pos, T, clen);
65 		pos += clen;
66 
67 		if (pos == outlen)
68 			break;
69 
70 		if (iter == 255) {
71 			os_memset(out, 0, outlen);
72 			forced_memzero(T, SHA256_MAC_LEN);
73 			return -1;
74 		}
75 		iter++;
76 
77 		if (hmac_sha256_vector(secret, secret_len, 4, addr, len, T) < 0)
78 		{
79 			os_memset(out, 0, outlen);
80 			forced_memzero(T, SHA256_MAC_LEN);
81 			return -1;
82 		}
83 	}
84 
85 	forced_memzero(T, SHA256_MAC_LEN);
86 	return 0;
87 }
88