1/*
2 * SHA256-based PRF (IEEE 802.11r)
3 * Copyright (c) 2003-2013, 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#include "crypto.h"
14
15
16/**
17 * sha256_prf - SHA256-based Pseudo-Random Function (IEEE 802.11r, 8.5.1.5.2)
18 * @key: Key for PRF
19 * @key_len: Length of the key in bytes
20 * @label: A unique label for each purpose of the PRF
21 * @data: Extra data to bind into the key
22 * @data_len: Length of the data
23 * @buf: Buffer for the generated pseudo-random key
24 * @buf_len: Number of bytes of key to generate
25 *
26 * This function is used to derive new, cryptographically separate keys from a
27 * given key.
28 */
29void sha256_prf(const u8 *key, size_t key_len, const char *label,
30		const u8 *data, size_t data_len, u8 *buf, size_t buf_len)
31{
32	sha256_prf_bits(key, key_len, label, data, data_len, buf, buf_len * 8);
33}
34
35
36/**
37 * sha256_prf_bits - IEEE Std 802.11-2012, 11.6.1.7.2 Key derivation function
38 * @key: Key for KDF
39 * @key_len: Length of the key in bytes
40 * @label: A unique label for each purpose of the PRF
41 * @data: Extra data to bind into the key
42 * @data_len: Length of the data
43 * @buf: Buffer for the generated pseudo-random key
44 * @buf_len: Number of bits of key to generate
45 *
46 * This function is used to derive new, cryptographically separate keys from a
47 * given key. If the requested buf_len is not divisible by eight, the least
48 * significant 1-7 bits of the last octet in the output are not part of the
49 * requested output.
50 */
51void sha256_prf_bits(const u8 *key, size_t key_len, const char *label,
52		     const u8 *data, size_t data_len, u8 *buf,
53		     size_t buf_len_bits)
54{
55	u16 counter = 1;
56	size_t pos, plen;
57	u8 hash[SHA256_MAC_LEN];
58	const u8 *addr[4];
59	size_t len[4];
60	u8 counter_le[2], length_le[2];
61	size_t buf_len = (buf_len_bits + 7) / 8;
62
63	addr[0] = counter_le;
64	len[0] = 2;
65	addr[1] = (u8 *) label;
66	len[1] = os_strlen(label);
67	addr[2] = data;
68	len[2] = data_len;
69	addr[3] = length_le;
70	len[3] = sizeof(length_le);
71
72	WPA_PUT_LE16(length_le, buf_len_bits);
73	pos = 0;
74	while (pos < buf_len) {
75		plen = buf_len - pos;
76		WPA_PUT_LE16(counter_le, counter);
77		if (plen >= SHA256_MAC_LEN) {
78			hmac_sha256_vector(key, key_len, 4, addr, len,
79					   &buf[pos]);
80			pos += SHA256_MAC_LEN;
81		} else {
82			hmac_sha256_vector(key, key_len, 4, addr, len, hash);
83			os_memcpy(&buf[pos], hash, plen);
84			pos += plen;
85			break;
86		}
87		counter++;
88	}
89
90	/*
91	 * Mask out unused bits in the last octet if it does not use all the
92	 * bits.
93	 */
94	if (buf_len_bits % 8) {
95		u8 mask = 0xff << (8 - buf_len_bits % 8);
96		buf[pos - 1] &= mask;
97	}
98}
99