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/* LibTomCrypt, modular cryptographic library -- Tom St Denis
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* LibTomCrypt is a library that provides various cryptographic
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* algorithms in a highly modular and flexible manner.
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* The library is free for all purposes without any express
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* Tom St Denis, tomstdenis@gmail.com, http://libtomcrypt.org
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@file pkcs_1_pss_encode.c
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PKCS #1 PSS Signature Padding, Tom St Denis
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PKCS #1 v2.00 Signature Encoding
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@param msghash The hash to encode
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@param msghashlen The length of the hash (octets)
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@param saltlen The length of the salt desired (octets)
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@param prng An active PRNG context
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@param prng_idx The index of the PRNG desired
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@param hash_idx The index of the hash desired
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@param modulus_bitlen The bit length of the RSA modulus
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@param out [out] The destination of the encoding
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@param outlen [in/out] The max size and resulting size of the encoded data
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@return CRYPT_OK if successful
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int pkcs_1_pss_encode(const unsigned char *msghash, unsigned long msghashlen,
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unsigned long saltlen, prng_state *prng,
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int prng_idx, int hash_idx,
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unsigned long modulus_bitlen,
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unsigned char *out, unsigned long *outlen)
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unsigned char *DB, *mask, *salt, *hash;
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unsigned long x, y, hLen, modulus_len;
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LTC_ARGCHK(msghash != NULL);
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LTC_ARGCHK(out != NULL);
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LTC_ARGCHK(outlen != NULL);
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/* ensure hash and PRNG are valid */
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if ((err = hash_is_valid(hash_idx)) != CRYPT_OK) {
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if ((err = prng_is_valid(prng_idx)) != CRYPT_OK) {
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hLen = hash_descriptor[hash_idx].hashsize;
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modulus_len = (modulus_bitlen>>3) + (modulus_bitlen & 7 ? 1 : 0);
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if ((saltlen > modulus_len) || (modulus_len < hLen + saltlen + 2)) {
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return CRYPT_PK_INVALID_SIZE;
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/* allocate ram for DB/mask/salt/hash of size modulus_len */
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DB = XMALLOC(modulus_len);
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mask = XMALLOC(modulus_len);
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salt = XMALLOC(modulus_len);
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hash = XMALLOC(modulus_len);
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if (DB == NULL || mask == NULL || salt == NULL || hash == NULL) {
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/* generate random salt */
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if (prng_descriptor[prng_idx].read(salt, saltlen, prng) != saltlen) {
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err = CRYPT_ERROR_READPRNG;
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/* M = (eight) 0x00 || msghash || salt, hash = H(M) */
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if ((err = hash_descriptor[hash_idx].init(&md)) != CRYPT_OK) {
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if ((err = hash_descriptor[hash_idx].process(&md, DB, 8)) != CRYPT_OK) {
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if ((err = hash_descriptor[hash_idx].process(&md, msghash, msghashlen)) != CRYPT_OK) {
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if ((err = hash_descriptor[hash_idx].process(&md, salt, saltlen)) != CRYPT_OK) {
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if ((err = hash_descriptor[hash_idx].done(&md, hash)) != CRYPT_OK) {
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/* generate DB = PS || 0x01 || salt, PS == modulus_len - saltlen - hLen - 2 zero bytes */
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XMEMSET(DB + x, 0, modulus_len - saltlen - hLen - 2);
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x += modulus_len - saltlen - hLen - 2;
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XMEMCPY(DB + x, salt, saltlen);
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/* generate mask of length modulus_len - hLen - 1 from hash */
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if ((err = pkcs_1_mgf1(hash, hLen, hash_idx, mask, modulus_len - hLen - 1)) != CRYPT_OK) {
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for (y = 0; y < (modulus_len - hLen - 1); y++) {
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/* output is DB || hash || 0xBC */
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if (*outlen < modulus_len) {
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err = CRYPT_BUFFER_OVERFLOW;
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/* DB len = modulus_len - hLen - 1 */
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XMEMCPY(out + y, DB, modulus_len - hLen - 1);
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y += modulus_len - hLen - 1;
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XMEMCPY(out + y, hash, hLen);
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/* now clear the 8*modulus_len - modulus_bitlen most significant bits */
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out[0] &= 0xFF >> ((modulus_len<<3) - (modulus_bitlen-1));
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/* store output size */
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*outlen = modulus_len;
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#ifdef LTC_CLEAN_STACK
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zeromem(DB, modulus_len);
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zeromem(mask, modulus_len);
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zeromem(salt, modulus_len);
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zeromem(hash, modulus_len);
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/* $Source: /cvs/libtom/libtomcrypt/src/pk/pkcs1/pkcs_1_pss_encode.c,v $ */
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/* $Revision: 1.4 $ */
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/* $Date: 2005/05/05 14:35:59 $ */