/*
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* TLS PRF P_SHA256
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* Copyright (c) 2011, Jouni Malinen <j@w1.fi>
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*
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* This software may be distributed under the terms of the BSD license.
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* See README for more details.
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*/
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#include "includes.h"
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#include "common.h"
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#include "sha256.h"
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/**
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* tls_prf_sha256 - Pseudo-Random Function for TLS v1.2 (P_SHA256, RFC 5246)
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* @secret: Key for PRF
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* @secret_len: Length of the key in bytes
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* @label: A unique label for each purpose of the PRF
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* @seed: Seed value to bind into the key
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* @seed_len: Length of the seed
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* @out: Buffer for the generated pseudo-random key
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* @outlen: Number of bytes of key to generate
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* Returns: 0 on success, -1 on failure.
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*
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* This function is used to derive new, cryptographically separate keys from a
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* given key in TLS. This PRF is defined in RFC 2246, Chapter 5.
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*/
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void tls_prf_sha256(const u8 *secret, size_t secret_len, const char *label,
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const u8 *seed, size_t seed_len, u8 *out, size_t outlen)
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{
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size_t clen;
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u8 A[SHA256_MAC_LEN];
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u8 P[SHA256_MAC_LEN];
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size_t pos;
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const unsigned char *addr[3];
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size_t len[3];
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addr[0] = A;
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len[0] = SHA256_MAC_LEN;
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addr[1] = (unsigned char *) label;
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len[1] = os_strlen(label);
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addr[2] = seed;
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len[2] = seed_len;
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/*
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* RFC 5246, Chapter 5
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* A(0) = seed, A(i) = HMAC(secret, A(i-1))
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* P_hash = HMAC(secret, A(1) + seed) + HMAC(secret, A(2) + seed) + ..
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* PRF(secret, label, seed) = P_SHA256(secret, label + seed)
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*/
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hmac_sha256_vector(secret, secret_len, 2, &addr[1], &len[1], A);
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pos = 0;
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while (pos < outlen) {
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hmac_sha256_vector(secret, secret_len, 3, addr, len, P);
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hmac_sha256(secret, secret_len, A, SHA256_MAC_LEN, A);
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clen = outlen - pos;
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if (clen > SHA256_MAC_LEN)
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clen = SHA256_MAC_LEN;
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os_memcpy(out + pos, P, clen);
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pos += clen;
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}
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}
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