text: /* $OpenBSD: bcrypt_pbkdf.c,v 1.13 2015/01/12 03:20:04 tedu Exp $ */
  /*
   * Copyright (c) 2013 Ted Unangst <tedu@openbsd.org>
   *
   * Permission to use, copy, modify, and distribute this software for any
   * purpose with or without fee is hereby granted, provided that the above
   * copyright notice and this permission notice appear in all copies.
   *
   * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
   * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
   * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
   * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
   * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
   * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
   * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
   */
  
  #include <sys/types.h>
  
  #include <stdint.h>
  #include <stdlib.h>
  #include <blf.h>
  #include <sha2.h>
  #include <string.h>
  #include <util.h>
  
  #define	MINIMUM(a,b) (((a) < (b)) ? (a) : (b))
  
  /*
   * pkcs #5 pbkdf2 implementation using the "bcrypt" hash
   *
   * The bcrypt hash function is derived from the bcrypt password hashing
   * function with the following modifications:
   * 1. The input password and salt are preprocessed with SHA512.
   * 2. The output length is expanded to 256 bits.
   * 3. Subsequently the magic string to be encrypted is lengthened and modifed
   *    to "OxychromaticBlowfishSwatDynamite"
   * 4. The hash function is defined to perform 64 rounds of initial state
   *    expansion. (More rounds are performed by iterating the hash.)
   *
   * Note that this implementation pulls the SHA512 operations into the caller
   * as a performance optimization.
   *
   * One modification from official pbkdf2. Instead of outputting key material
   * linearly, we mix it. pbkdf2 has a known weakness where if one uses it to
   * generate (e.g.) 512 bits of key material for use as two 256 bit keys, an
   * attacker can merely run once through the outer loop, but the user
   * always runs it twice. Shuffling output bytes requires computing the
   * entirety of the key material to assemble any subkey. This is something a
   * wise caller could do; we just do it for you.
   */
  
  #define BCRYPT_WORDS 8
  #define BCRYPT_HASHSIZE (BCRYPT_WORDS * 4)
  
  static void
  bcrypt_hash(uint8_t *sha2pass, uint8_t *sha2salt, uint8_t *out)
  {
  	blf_ctx state;
  	uint8_t ciphertext[BCRYPT_HASHSIZE] =
  	    "OxychromaticBlowfishSwatDynamite";
  	uint32_t cdata[BCRYPT_WORDS];
  	int i;
  	uint16_t j;
  	size_t shalen = SHA512_DIGEST_LENGTH;
  
  	/* key expansion */
  	Blowfish_initstate(&state);
  	Blowfish_expandstate(&state, sha2salt, shalen, sha2pass, shalen);
  	for (i = 0; i < 64; i++) {
  		Blowfish_expand0state(&state, sha2salt, shalen);
  		Blowfish_expand0state(&state, sha2pass, shalen);
  	}
  
  	/* encryption */
  	j = 0;
  	for (i = 0; i < BCRYPT_WORDS; i++)
  		cdata[i] = Blowfish_stream2word(ciphertext, sizeof(ciphertext),
  		    &j);
  	for (i = 0; i < 64; i++)
  		blf_enc(&state, cdata, sizeof(cdata) / sizeof(uint64_t));
  
  	/* copy out */
  	for (i = 0; i < BCRYPT_WORDS; i++) {
  		out[4 * i + 3] = (cdata[i] >> 24) & 0xff;
  		out[4 * i + 2] = (cdata[i] >> 16) & 0xff;
  		out[4 * i + 1] = (cdata[i] >> 8) & 0xff;
  		out[4 * i + 0] = cdata[i] & 0xff;
  	}
  
  	/* zap */
  	explicit_bzero(ciphertext, sizeof(ciphertext));
  	explicit_bzero(cdata, sizeof(cdata));
  	explicit_bzero(&state, sizeof(state));
  }
  
  int
  bcrypt_pbkdf(const char *pass, size_t passlen, const uint8_t *salt, size_t saltlen,
      uint8_t *key, size_t keylen, unsigned int rounds)
  {
  	SHA2_CTX ctx;
  	uint8_t sha2pass[SHA512_DIGEST_LENGTH];
  	uint8_t sha2salt[SHA512_DIGEST_LENGTH];
  	uint8_t out[BCRYPT_HASHSIZE];
  	uint8_t tmpout[BCRYPT_HASHSIZE];
  	uint8_t countsalt[4];
  	size_t i, j, amt, stride;
  	uint32_t count;
  	size_t origkeylen = keylen;
  
  	/* nothing crazy */
  	if (rounds < 1)
  		return -1;
  	if (passlen == 0 || saltlen == 0 || keylen == 0 ||
  	    keylen > sizeof(out) * sizeof(out))
  		return -1;
  	stride = (keylen + sizeof(out) - 1) / sizeof(out);
  	amt = (keylen + stride - 1) / stride;
  
  	/* collapse password */
  	SHA512Init(&ctx);
  	SHA512Update(&ctx, pass, passlen);
  	SHA512Final(sha2pass, &ctx);
  
  
  	/* generate key, sizeof(out) at a time */
  	for (count = 1; keylen > 0; count++) {
  		countsalt[0] = (count >> 24) & 0xff;
  		countsalt[1] = (count >> 16) & 0xff;
  		countsalt[2] = (count >> 8) & 0xff;
  		countsalt[3] = count & 0xff;
  
  		/* first round, salt is salt */
  		SHA512Init(&ctx);
  		SHA512Update(&ctx, salt, saltlen);
  		SHA512Update(&ctx, countsalt, sizeof(countsalt));
  		SHA512Final(sha2salt, &ctx);
  		bcrypt_hash(sha2pass, sha2salt, tmpout);
  		memcpy(out, tmpout, sizeof(out));
  
  		for (i = 1; i < rounds; i++) {
  			/* subsequent rounds, salt is previous output */
  			SHA512Init(&ctx);
  			SHA512Update(&ctx, tmpout, sizeof(tmpout));
  			SHA512Final(sha2salt, &ctx);
  			bcrypt_hash(sha2pass, sha2salt, tmpout);
  			for (j = 0; j < sizeof(out); j++)
  				out[j] ^= tmpout[j];
  		}
  
  		/*
  		 * pbkdf2 deviation: output the key material non-linearly.
  		 */
  		amt = MINIMUM(amt, keylen);
  		for (i = 0; i < amt; i++) {
  			size_t dest = i * stride + (count - 1);
  			if (dest >= origkeylen)
  				break;
  			key[dest] = out[i];
  		}
  		keylen -= i;
  	}
  
  	/* zap */
  	explicit_bzero(&ctx, sizeof(ctx));
  	explicit_bzero(out, sizeof(out));
  
  	return 0;
  }
  