crypto/rsa/padding.c - boringssl - Git at Google

 /* Written by Dr Stephen N Henson (steve@openssl.org) for the OpenSSL
  * project 2005.
  */
 /* ====================================================================
  * Copyright (c) 2005 The OpenSSL Project.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  *
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in
  *    the documentation and/or other materials provided with the
  *    distribution.
  *
  * 3. All advertising materials mentioning features or use of this
  *    software must display the following acknowledgment:
  *    "This product includes software developed by the OpenSSL Project
  *    for use in the OpenSSL Toolkit. (http://www.OpenSSL.org/)"
  *
  * 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to
  *    endorse or promote products derived from this software without
  *    prior written permission. For written permission, please contact
  *    licensing@OpenSSL.org.
  *
  * 5. Products derived from this software may not be called "OpenSSL"
  *    nor may "OpenSSL" appear in their names without prior written
  *    permission of the OpenSSL Project.
  *
  * 6. Redistributions of any form whatsoever must retain the following
  *    acknowledgment:
  *    "This product includes software developed by the OpenSSL Project
  *    for use in the OpenSSL Toolkit (http://www.OpenSSL.org/)"
  *
  * THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
  * EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE OpenSSL PROJECT OR
  * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
  * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
  * OF THE POSSIBILITY OF SUCH DAMAGE.
  * ====================================================================
  *
  * This product includes cryptographic software written by Eric Young
  * (eay@cryptsoft.com).  This product includes software written by Tim
  * Hudson (tjh@cryptsoft.com). */

 #include <openssl/rsa.h>

 #include <assert.h>
 #include <limits.h>
 #include <string.h>

 #include <openssl/bn.h>
 #include <openssl/digest.h>
 #include <openssl/err.h>
 #include <openssl/mem.h>
 #include <openssl/rand.h>
 #include <openssl/sha.h>

 #include "internal.h"
 #include "../internal.h"

 /* TODO(fork): don't the check functions have to be constant time? */

 int RSA_padding_add_PKCS1_type_1(uint8_t *to, unsigned to_len,
                                  const uint8_t *from, unsigned from_len) {
   unsigned j;

   if (to_len < RSA_PKCS1_PADDING_SIZE) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_KEY_SIZE_TOO_SMALL);
     return 0;
   }

   if (from_len > to_len - RSA_PKCS1_PADDING_SIZE) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
     return 0;
   }

   uint8_t *p = to;

   *(p++) = 0;
   *(p++) = 1; /* Private Key BT (Block Type) */

   /* pad out with 0xff data */
   j = to_len - 3 - from_len;
   OPENSSL_memset(p, 0xff, j);
   p += j;
   *(p++) = 0;
   OPENSSL_memcpy(p, from, from_len);
   return 1;
 }

 int RSA_padding_check_PKCS1_type_1(uint8_t *to, unsigned to_len,
                                    const uint8_t *from, unsigned from_len) {
   unsigned i, j;
   const uint8_t *p;

   if (from_len < 2) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_SMALL);
     return -1;
   }

   p = from;
   if ((*(p++) != 0) || (*(p++) != 1)) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_BLOCK_TYPE_IS_NOT_01);
     return -1;
   }

   /* scan over padding data */
   j = from_len - 2; /* one for leading 00, one for type. */
   for (i = 0; i < j; i++) {
     /* should decrypt to 0xff */
     if (*p != 0xff) {
       if (*p == 0) {
         p++;
         break;
       } else {
         OPENSSL_PUT_ERROR(RSA, RSA_R_BAD_FIXED_HEADER_DECRYPT);
         return -1;
       }
     }
     p++;
   }

   if (i == j) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_NULL_BEFORE_BLOCK_MISSING);
     return -1;
   }

   if (i < 8) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_BAD_PAD_BYTE_COUNT);
     return -1;
   }
   i++; /* Skip over the '\0' */
   j -= i;
   if (j > to_len) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_LARGE);
     return -1;
   }
   OPENSSL_memcpy(to, p, j);

   return j;
 }

 int RSA_padding_add_PKCS1_type_2(uint8_t *to, unsigned to_len,
                                  const uint8_t *from, unsigned from_len) {
   unsigned i, j;

   if (to_len < RSA_PKCS1_PADDING_SIZE) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_KEY_SIZE_TOO_SMALL);
     return 0;
   }

   if (from_len > to_len - RSA_PKCS1_PADDING_SIZE) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
     return 0;
   }

   uint8_t *p = to;

   *(p++) = 0;
   *(p++) = 2; /* Public Key BT (Block Type) */

   /* pad out with non-zero random data */
   j = to_len - 3 - from_len;

   if (!RAND_bytes(p, j)) {
     return 0;
   }

   for (i = 0; i < j; i++) {
     while (*p == 0) {
       if (!RAND_bytes(p, 1)) {
         return 0;
       }
     }
     p++;
   }

   *(p++) = 0;

   OPENSSL_memcpy(p, from, from_len);
   return 1;
 }

 int RSA_padding_check_PKCS1_type_2(uint8_t *to, unsigned to_len,
                                    const uint8_t *from, unsigned from_len) {
   if (from_len == 0) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_EMPTY_PUBLIC_KEY);
     return -1;
   }

   /* PKCS#1 v1.5 decryption. See "PKCS #1 v2.2: RSA Cryptography
    * Standard", section 7.2.2. */
   if (from_len < RSA_PKCS1_PADDING_SIZE) {
     /* |from| is zero-padded to the size of the RSA modulus, a public value, so
      * this can be rejected in non-constant time. */
     OPENSSL_PUT_ERROR(RSA, RSA_R_KEY_SIZE_TOO_SMALL);
     return -1;
   }

   unsigned first_byte_is_zero = constant_time_eq(from[0], 0);
   unsigned second_byte_is_two = constant_time_eq(from[1], 2);

   unsigned i, zero_index = 0, looking_for_index = ~0u;
   for (i = 2; i < from_len; i++) {
     unsigned equals0 = constant_time_is_zero(from[i]);
     zero_index = constant_time_select(looking_for_index & equals0, (unsigned)i,
                                       zero_index);
     looking_for_index = constant_time_select(equals0, 0, looking_for_index);
   }

   /* The input must begin with 00 02. */
   unsigned valid_index = first_byte_is_zero;
   valid_index &= second_byte_is_two;

   /* We must have found the end of PS. */
   valid_index &= ~looking_for_index;

   /* PS must be at least 8 bytes long, and it starts two bytes into |from|. */
   valid_index &= constant_time_ge(zero_index, 2 + 8);

   /* Skip the zero byte. */
   zero_index++;

   /* NOTE: Although this logic attempts to be constant time, the API contracts
    * of this function and |RSA_decrypt| with |RSA_PKCS1_PADDING| make it
    * impossible to completely avoid Bleichenbacher's attack. Consumers should
    * use |RSA_unpad_key_pkcs1|. */
   if (!valid_index) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_PKCS_DECODING_ERROR);
     return -1;
   }

   const unsigned msg_len = from_len - zero_index;
   if (msg_len > to_len) {
     /* This shouldn't happen because this function is always called with
      * |to_len| as the key size and |from_len| is bounded by the key size. */
     OPENSSL_PUT_ERROR(RSA, RSA_R_PKCS_DECODING_ERROR);
     return -1;
   }

   if (msg_len > INT_MAX) {
     OPENSSL_PUT_ERROR(RSA, ERR_R_OVERFLOW);
     return -1;
   }

   OPENSSL_memcpy(to, &from[zero_index], msg_len);
   return (int)msg_len;
 }

 int RSA_padding_add_none(uint8_t *to, unsigned to_len, const uint8_t *from,
                          unsigned from_len) {
   if (from_len > to_len) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
     return 0;
   }

   if (from_len < to_len) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_SMALL_FOR_KEY_SIZE);
     return 0;
   }

   OPENSSL_memcpy(to, from, from_len);
   return 1;
 }

 static int PKCS1_MGF1(uint8_t *out, size_t len, const uint8_t *seed,
                       size_t seed_len, const EVP_MD *md) {
   int ret = 0;
   EVP_MD_CTX ctx;
   EVP_MD_CTX_init(&ctx);

   size_t md_len = EVP_MD_size(md);

   for (uint32_t i = 0; len > 0; i++) {
     uint8_t counter[4];
     counter[0] = (uint8_t)(i >> 24);
     counter[1] = (uint8_t)(i >> 16);
     counter[2] = (uint8_t)(i >> 8);
     counter[3] = (uint8_t)i;
     if (!EVP_DigestInit_ex(&ctx, md, NULL) ||
         !EVP_DigestUpdate(&ctx, seed, seed_len) ||
         !EVP_DigestUpdate(&ctx, counter, sizeof(counter))) {
       goto err;
     }

     if (md_len <= len) {
       if (!EVP_DigestFinal_ex(&ctx, out, NULL)) {
         goto err;
       }
       out += md_len;
       len -= md_len;
     } else {
       uint8_t digest[EVP_MAX_MD_SIZE];
       if (!EVP_DigestFinal_ex(&ctx, digest, NULL)) {
         goto err;
       }
       OPENSSL_memcpy(out, digest, len);
       len = 0;
     }
   }

   ret = 1;

 err:
   EVP_MD_CTX_cleanup(&ctx);
   return ret;
 }

 int RSA_padding_add_PKCS1_OAEP_mgf1(uint8_t *to, unsigned to_len,
                                     const uint8_t *from, unsigned from_len,
                                     const uint8_t *param, unsigned param_len,
                                     const EVP_MD *md, const EVP_MD *mgf1md) {
   unsigned i, emlen, mdlen;
   uint8_t *db, *seed;
   uint8_t *dbmask = NULL, seedmask[EVP_MAX_MD_SIZE];
   int ret = 0;

   if (md == NULL) {
     md = EVP_sha1();
   }
   if (mgf1md == NULL) {
     mgf1md = md;
   }

   mdlen = EVP_MD_size(md);

   if (to_len < 2 * mdlen + 2) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_KEY_SIZE_TOO_SMALL);
     return 0;
   }

   emlen = to_len - 1;
   if (from_len > emlen - 2 * mdlen - 1) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
     return 0;
   }

   if (emlen < 2 * mdlen + 1) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_KEY_SIZE_TOO_SMALL);
     return 0;
   }

   to[0] = 0;
   seed = to + 1;
   db = to + mdlen + 1;

   if (!EVP_Digest(param, param_len, db, NULL, md, NULL)) {
     return 0;
   }
   OPENSSL_memset(db + mdlen, 0, emlen - from_len - 2 * mdlen - 1);
   db[emlen - from_len - mdlen - 1] = 0x01;
   OPENSSL_memcpy(db + emlen - from_len - mdlen, from, from_len);
   if (!RAND_bytes(seed, mdlen)) {
     return 0;
   }

   dbmask = OPENSSL_malloc(emlen - mdlen);
   if (dbmask == NULL) {
     OPENSSL_PUT_ERROR(RSA, ERR_R_MALLOC_FAILURE);
     return 0;
   }

   if (!PKCS1_MGF1(dbmask, emlen - mdlen, seed, mdlen, mgf1md)) {
     goto out;
   }
   for (i = 0; i < emlen - mdlen; i++) {
     db[i] ^= dbmask[i];
   }

   if (!PKCS1_MGF1(seedmask, mdlen, db, emlen - mdlen, mgf1md)) {
     goto out;
   }
   for (i = 0; i < mdlen; i++) {
     seed[i] ^= seedmask[i];
   }
   ret = 1;

 out:
   OPENSSL_free(dbmask);
   return ret;
 }

 int RSA_padding_check_PKCS1_OAEP_mgf1(uint8_t *to, unsigned to_len,
                                       const uint8_t *from, unsigned from_len,
                                       const uint8_t *param, unsigned param_len,
                                       const EVP_MD *md, const EVP_MD *mgf1md) {
   unsigned i, dblen, mlen = -1, mdlen, bad, looking_for_one_byte, one_index = 0;
   const uint8_t *maskeddb, *maskedseed;
   uint8_t *db = NULL, seed[EVP_MAX_MD_SIZE], phash[EVP_MAX_MD_SIZE];

   if (md == NULL) {
     md = EVP_sha1();
   }
   if (mgf1md == NULL) {
     mgf1md = md;
   }

   mdlen = EVP_MD_size(md);

   /* The encoded message is one byte smaller than the modulus to ensure that it
    * doesn't end up greater than the modulus. Thus there's an extra "+1" here
    * compared to https://tools.ietf.org/html/rfc2437#section-9.1.1.2. */
   if (from_len < 1 + 2*mdlen + 1) {
     /* 'from_len' is the length of the modulus, i.e. does not depend on the
      * particular ciphertext. */
     goto decoding_err;
   }

   dblen = from_len - mdlen - 1;
   db = OPENSSL_malloc(dblen);
   if (db == NULL) {
     OPENSSL_PUT_ERROR(RSA, ERR_R_MALLOC_FAILURE);
     goto err;
   }

   maskedseed = from + 1;
   maskeddb = from + 1 + mdlen;

   if (!PKCS1_MGF1(seed, mdlen, maskeddb, dblen, mgf1md)) {
     goto err;
   }
   for (i = 0; i < mdlen; i++) {
     seed[i] ^= maskedseed[i];
   }

   if (!PKCS1_MGF1(db, dblen, seed, mdlen, mgf1md)) {
     goto err;
   }
   for (i = 0; i < dblen; i++) {
     db[i] ^= maskeddb[i];
   }

   if (!EVP_Digest(param, param_len, phash, NULL, md, NULL)) {
     goto err;
   }

   bad = ~constant_time_is_zero(CRYPTO_memcmp(db, phash, mdlen));
   bad |= ~constant_time_is_zero(from[0]);

   looking_for_one_byte = ~0u;
   for (i = mdlen; i < dblen; i++) {
     unsigned equals1 = constant_time_eq(db[i], 1);
     unsigned equals0 = constant_time_eq(db[i], 0);
     one_index = constant_time_select(looking_for_one_byte & equals1, i,
                                      one_index);
     looking_for_one_byte =
         constant_time_select(equals1, 0, looking_for_one_byte);
     bad |= looking_for_one_byte & ~equals0;
   }

   bad |= looking_for_one_byte;

   if (bad) {
     goto decoding_err;
   }

   one_index++;
   mlen = dblen - one_index;
   if (to_len < mlen) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_LARGE);
     mlen = -1;
   } else {
     OPENSSL_memcpy(to, db + one_index, mlen);
   }

   OPENSSL_free(db);
   return mlen;

 decoding_err:
   /* to avoid chosen ciphertext attacks, the error message should not reveal
    * which kind of decoding error happened */
   OPENSSL_PUT_ERROR(RSA, RSA_R_OAEP_DECODING_ERROR);
  err:
   OPENSSL_free(db);
   return -1;
 }

 static const unsigned char zeroes[] = {0,0,0,0,0,0,0,0};

 int RSA_verify_PKCS1_PSS_mgf1(RSA *rsa, const uint8_t *mHash,
                               const EVP_MD *Hash, const EVP_MD *mgf1Hash,
                               const uint8_t *EM, int sLen) {
   int i;
   int ret = 0;
   int maskedDBLen, MSBits, emLen;
   size_t hLen;
   const uint8_t *H;
   uint8_t *DB = NULL;
   EVP_MD_CTX ctx;
   uint8_t H_[EVP_MAX_MD_SIZE];
   EVP_MD_CTX_init(&ctx);

   if (mgf1Hash == NULL) {
     mgf1Hash = Hash;
   }

   hLen = EVP_MD_size(Hash);

   /* Negative sLen has special meanings:
    *	-1	sLen == hLen
    *	-2	salt length is autorecovered from signature
    *	-N	reserved */
   if (sLen == -1) {
     sLen = hLen;
   } else if (sLen == -2) {
     sLen = -2;
   } else if (sLen < -2) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_SLEN_CHECK_FAILED);
     goto err;
   }

   MSBits = (BN_num_bits(rsa->n) - 1) & 0x7;
   emLen = RSA_size(rsa);
   if (EM[0] & (0xFF << MSBits)) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_FIRST_OCTET_INVALID);
     goto err;
   }
   if (MSBits == 0) {
     EM++;
     emLen--;
   }
   if (emLen < ((int)hLen + sLen + 2)) {
     /* sLen can be small negative */
     OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_LARGE);
     goto err;
   }
   if (EM[emLen - 1] != 0xbc) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_LAST_OCTET_INVALID);
     goto err;
   }
   maskedDBLen = emLen - hLen - 1;
   H = EM + maskedDBLen;
   DB = OPENSSL_malloc(maskedDBLen);
   if (!DB) {
     OPENSSL_PUT_ERROR(RSA, ERR_R_MALLOC_FAILURE);
     goto err;
   }
   if (!PKCS1_MGF1(DB, maskedDBLen, H, hLen, mgf1Hash)) {
     goto err;
   }
   for (i = 0; i < maskedDBLen; i++) {
     DB[i] ^= EM[i];
   }
   if (MSBits) {
     DB[0] &= 0xFF >> (8 - MSBits);
   }
   for (i = 0; DB[i] == 0 && i < (maskedDBLen - 1); i++) {
     ;
   }
   if (DB[i++] != 0x1) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_SLEN_RECOVERY_FAILED);
     goto err;
   }
   if (sLen >= 0 && (maskedDBLen - i) != sLen) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_SLEN_CHECK_FAILED);
     goto err;
   }
   if (!EVP_DigestInit_ex(&ctx, Hash, NULL) ||
       !EVP_DigestUpdate(&ctx, zeroes, sizeof zeroes) ||
       !EVP_DigestUpdate(&ctx, mHash, hLen)) {
     goto err;
   }
   if (maskedDBLen - i) {
     if (!EVP_DigestUpdate(&ctx, DB + i, maskedDBLen - i)) {
       goto err;
     }
   }
   if (!EVP_DigestFinal_ex(&ctx, H_, NULL)) {
     goto err;
   }
   if (OPENSSL_memcmp(H_, H, hLen)) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_BAD_SIGNATURE);
     ret = 0;
   } else {
     ret = 1;
   }

 err:
   OPENSSL_free(DB);
   EVP_MD_CTX_cleanup(&ctx);

   return ret;
 }

 int RSA_padding_add_PKCS1_PSS_mgf1(RSA *rsa, unsigned char *EM,
                                    const unsigned char *mHash,
                                    const EVP_MD *Hash, const EVP_MD *mgf1Hash,
                                    int sLenRequested) {
   int ret = 0;
   size_t maskedDBLen, MSBits, emLen;
   size_t hLen;
   unsigned char *H, *salt = NULL, *p;
   EVP_MD_CTX ctx;

   if (mgf1Hash == NULL) {
     mgf1Hash = Hash;
   }

   hLen = EVP_MD_size(Hash);

   if (BN_is_zero(rsa->n)) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_EMPTY_PUBLIC_KEY);
     goto err;
   }

   MSBits = (BN_num_bits(rsa->n) - 1) & 0x7;
   emLen = RSA_size(rsa);
   if (MSBits == 0) {
     assert(emLen >= 1);
     *EM++ = 0;
     emLen--;
   }

   if (emLen < hLen + 2) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
     goto err;
   }

   /* Negative sLenRequested has special meanings:
    *   -1  sLen == hLen
    *   -2  salt length is maximized
    *   -N  reserved */
   size_t sLen;
   if (sLenRequested == -1) {
     sLen = hLen;
   } else if (sLenRequested == -2) {
     sLen = emLen - hLen - 2;
   } else if (sLenRequested < 0) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_SLEN_CHECK_FAILED);
     goto err;
   } else {
     sLen = (size_t)sLenRequested;
   }

   if (emLen - hLen - 2 < sLen) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
     goto err;
   }

   if (sLen > 0) {
     salt = OPENSSL_malloc(sLen);
     if (!salt) {
       OPENSSL_PUT_ERROR(RSA, ERR_R_MALLOC_FAILURE);
       goto err;
     }
     if (!RAND_bytes(salt, sLen)) {
       goto err;
     }
   }
   maskedDBLen = emLen - hLen - 1;
   H = EM + maskedDBLen;
   EVP_MD_CTX_init(&ctx);
   if (!EVP_DigestInit_ex(&ctx, Hash, NULL) ||
       !EVP_DigestUpdate(&ctx, zeroes, sizeof zeroes) ||
       !EVP_DigestUpdate(&ctx, mHash, hLen)) {
     goto err;
   }
   if (sLen && !EVP_DigestUpdate(&ctx, salt, sLen)) {
     goto err;
   }
   if (!EVP_DigestFinal_ex(&ctx, H, NULL)) {
     goto err;
   }
   EVP_MD_CTX_cleanup(&ctx);

   /* Generate dbMask in place then perform XOR on it */
   if (!PKCS1_MGF1(EM, maskedDBLen, H, hLen, mgf1Hash)) {
     goto err;
   }

   p = EM;

   /* Initial PS XORs with all zeroes which is a NOP so just update
    * pointer. Note from a test above this value is guaranteed to
    * be non-negative. */
   p += emLen - sLen - hLen - 2;
   *p++ ^= 0x1;
   if (sLen > 0) {
     for (size_t i = 0; i < sLen; i++) {
       *p++ ^= salt[i];
     }
   }
   if (MSBits) {
     EM[0] &= 0xFF >> (8 - MSBits);
   }

   /* H is already in place so just set final 0xbc */

   EM[emLen - 1] = 0xbc;

   ret = 1;

 err:
   OPENSSL_free(salt);

   return ret;
 }
	/* Written by Dr Stephen N Henson (steve@openssl.org) for the OpenSSL
	* project 2005.
	*/
	/* ====================================================================
	* Copyright (c) 2005 The OpenSSL Project. All rights reserved.
	*
	* Redistribution and use in source and binary forms, with or without
	* modification, are permitted provided that the following conditions
	* are met:
	*
	* 1. Redistributions of source code must retain the above copyright
	* notice, this list of conditions and the following disclaimer.
	*
	* 2. Redistributions in binary form must reproduce the above copyright
	* notice, this list of conditions and the following disclaimer in
	* the documentation and/or other materials provided with the
	* distribution.
	*
	* 3. All advertising materials mentioning features or use of this
	* software must display the following acknowledgment:
	* "This product includes software developed by the OpenSSL Project
	* for use in the OpenSSL Toolkit. (http://www.OpenSSL.org/)"
	*
	* 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to
	* endorse or promote products derived from this software without
	* prior written permission. For written permission, please contact
	* licensing@OpenSSL.org.
	*
	* 5. Products derived from this software may not be called "OpenSSL"
	* nor may "OpenSSL" appear in their names without prior written
	* permission of the OpenSSL Project.
	*
	* 6. Redistributions of any form whatsoever must retain the following
	* acknowledgment:
	* "This product includes software developed by the OpenSSL Project
	* for use in the OpenSSL Toolkit (http://www.OpenSSL.org/)"
	*
	* THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
	* EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
	* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE OpenSSL PROJECT OR
	* ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
	* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
	* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
	* LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
	* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
	* STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
	* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
	* OF THE POSSIBILITY OF SUCH DAMAGE.
	* ====================================================================
	*
	* This product includes cryptographic software written by Eric Young
	* (eay@cryptsoft.com). This product includes software written by Tim
	* Hudson (tjh@cryptsoft.com). */

	#include <openssl/rsa.h>

	#include <assert.h>
	#include <limits.h>
	#include <string.h>

	#include <openssl/bn.h>
	#include <openssl/digest.h>
	#include <openssl/err.h>
	#include <openssl/mem.h>
	#include <openssl/rand.h>
	#include <openssl/sha.h>

	#include "internal.h"
	#include "../internal.h"

	/* TODO(fork): don't the check functions have to be constant time? */

	int RSA_padding_add_PKCS1_type_1(uint8_t *to, unsigned to_len,
	const uint8_t *from, unsigned from_len) {
	unsigned j;

	if (to_len < RSA_PKCS1_PADDING_SIZE) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_KEY_SIZE_TOO_SMALL);
	return 0;
	}

	if (from_len > to_len - RSA_PKCS1_PADDING_SIZE) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
	return 0;
	}

	uint8_t *p = to;

	*(p++) = 0;
	(p++) = 1; / Private Key BT (Block Type) */

	/* pad out with 0xff data */
	j = to_len - 3 - from_len;
	OPENSSL_memset(p, 0xff, j);
	p += j;
	*(p++) = 0;
	OPENSSL_memcpy(p, from, from_len);
	return 1;
	}

	int RSA_padding_check_PKCS1_type_1(uint8_t *to, unsigned to_len,
	const uint8_t *from, unsigned from_len) {
	unsigned i, j;
	const uint8_t *p;

	if (from_len < 2) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_SMALL);
	return -1;
	}

	p = from;
	if (((p++) != 0) \|\| ((p++) != 1)) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_BLOCK_TYPE_IS_NOT_01);
	return -1;
	}

	/* scan over padding data */
	j = from_len - 2; /* one for leading 00, one for type. */
	for (i = 0; i < j; i++) {
	/* should decrypt to 0xff */
	if (*p != 0xff) {
	if (*p == 0) {
	p++;
	break;
	} else {
	OPENSSL_PUT_ERROR(RSA, RSA_R_BAD_FIXED_HEADER_DECRYPT);
	return -1;
	}
	}
	p++;
	}

	if (i == j) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_NULL_BEFORE_BLOCK_MISSING);
	return -1;
	}

	if (i < 8) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_BAD_PAD_BYTE_COUNT);
	return -1;
	}
	i++; /* Skip over the '\0' */
	j -= i;
	if (j > to_len) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_LARGE);
	return -1;
	}
	OPENSSL_memcpy(to, p, j);

	return j;
	}

	int RSA_padding_add_PKCS1_type_2(uint8_t *to, unsigned to_len,
	const uint8_t *from, unsigned from_len) {
	unsigned i, j;

	if (to_len < RSA_PKCS1_PADDING_SIZE) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_KEY_SIZE_TOO_SMALL);
	return 0;
	}

	if (from_len > to_len - RSA_PKCS1_PADDING_SIZE) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
	return 0;
	}

	uint8_t *p = to;

	*(p++) = 0;
	(p++) = 2; / Public Key BT (Block Type) */

	/* pad out with non-zero random data */
	j = to_len - 3 - from_len;

	if (!RAND_bytes(p, j)) {
	return 0;
	}

	for (i = 0; i < j; i++) {
	while (*p == 0) {
	if (!RAND_bytes(p, 1)) {
	return 0;
	}
	}
	p++;
	}

	*(p++) = 0;

	OPENSSL_memcpy(p, from, from_len);
	return 1;
	}

	int RSA_padding_check_PKCS1_type_2(uint8_t *to, unsigned to_len,
	const uint8_t *from, unsigned from_len) {
	if (from_len == 0) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_EMPTY_PUBLIC_KEY);
	return -1;
	}

	/* PKCS#1 v1.5 decryption. See "PKCS #1 v2.2: RSA Cryptography
	* Standard", section 7.2.2. */
	if (from_len < RSA_PKCS1_PADDING_SIZE) {
	/* \|from\| is zero-padded to the size of the RSA modulus, a public value, so
	* this can be rejected in non-constant time. */
	OPENSSL_PUT_ERROR(RSA, RSA_R_KEY_SIZE_TOO_SMALL);
	return -1;
	}

	unsigned first_byte_is_zero = constant_time_eq(from[0], 0);
	unsigned second_byte_is_two = constant_time_eq(from[1], 2);

	unsigned i, zero_index = 0, looking_for_index = ~0u;
	for (i = 2; i < from_len; i++) {
	unsigned equals0 = constant_time_is_zero(from[i]);
	zero_index = constant_time_select(looking_for_index & equals0, (unsigned)i,
	zero_index);
	looking_for_index = constant_time_select(equals0, 0, looking_for_index);
	}

	/* The input must begin with 00 02. */
	unsigned valid_index = first_byte_is_zero;
	valid_index &= second_byte_is_two;

	/* We must have found the end of PS. */
	valid_index &= ~looking_for_index;

	/* PS must be at least 8 bytes long, and it starts two bytes into \|from\|. */
	valid_index &= constant_time_ge(zero_index, 2 + 8);

	/* Skip the zero byte. */
	zero_index++;

	/* NOTE: Although this logic attempts to be constant time, the API contracts
	* of this function and \|RSA_decrypt\| with \|RSA_PKCS1_PADDING\| make it
	* impossible to completely avoid Bleichenbacher's attack. Consumers should
	* use \|RSA_unpad_key_pkcs1\|. */
	if (!valid_index) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_PKCS_DECODING_ERROR);
	return -1;
	}

	const unsigned msg_len = from_len - zero_index;
	if (msg_len > to_len) {
	/* This shouldn't happen because this function is always called with
	* \|to_len\| as the key size and \|from_len\| is bounded by the key size. */
	OPENSSL_PUT_ERROR(RSA, RSA_R_PKCS_DECODING_ERROR);
	return -1;
	}

	if (msg_len > INT_MAX) {
	OPENSSL_PUT_ERROR(RSA, ERR_R_OVERFLOW);
	return -1;
	}

	OPENSSL_memcpy(to, &from[zero_index], msg_len);
	return (int)msg_len;
	}

	int RSA_padding_add_none(uint8_t to, unsigned to_len, const uint8_t from,
	unsigned from_len) {
	if (from_len > to_len) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
	return 0;
	}

	if (from_len < to_len) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_SMALL_FOR_KEY_SIZE);
	return 0;
	}

	OPENSSL_memcpy(to, from, from_len);
	return 1;
	}

	static int PKCS1_MGF1(uint8_t out, size_t len, const uint8_t seed,
	size_t seed_len, const EVP_MD *md) {
	int ret = 0;
	EVP_MD_CTX ctx;
	EVP_MD_CTX_init(&ctx);

	size_t md_len = EVP_MD_size(md);

	for (uint32_t i = 0; len > 0; i++) {
	uint8_t counter[4];
	counter[0] = (uint8_t)(i >> 24);
	counter[1] = (uint8_t)(i >> 16);
	counter[2] = (uint8_t)(i >> 8);
	counter[3] = (uint8_t)i;
	if (!EVP_DigestInit_ex(&ctx, md, NULL) \|\|
	!EVP_DigestUpdate(&ctx, seed, seed_len) \|\|
	!EVP_DigestUpdate(&ctx, counter, sizeof(counter))) {
	goto err;
	}

	if (md_len <= len) {
	if (!EVP_DigestFinal_ex(&ctx, out, NULL)) {
	goto err;
	}
	out += md_len;
	len -= md_len;
	} else {
	uint8_t digest[EVP_MAX_MD_SIZE];
	if (!EVP_DigestFinal_ex(&ctx, digest, NULL)) {
	goto err;
	}
	OPENSSL_memcpy(out, digest, len);
	len = 0;
	}
	}

	ret = 1;

	err:
	EVP_MD_CTX_cleanup(&ctx);
	return ret;
	}

	int RSA_padding_add_PKCS1_OAEP_mgf1(uint8_t *to, unsigned to_len,
	const uint8_t *from, unsigned from_len,
	const uint8_t *param, unsigned param_len,
	const EVP_MD md, const EVP_MD mgf1md) {
	unsigned i, emlen, mdlen;
	uint8_t db, seed;
	uint8_t *dbmask = NULL, seedmask[EVP_MAX_MD_SIZE];
	int ret = 0;

	if (md == NULL) {
	md = EVP_sha1();
	}
	if (mgf1md == NULL) {
	mgf1md = md;
	}

	mdlen = EVP_MD_size(md);

	if (to_len < 2 * mdlen + 2) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_KEY_SIZE_TOO_SMALL);
	return 0;
	}

	emlen = to_len - 1;
	if (from_len > emlen - 2 * mdlen - 1) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
	return 0;
	}

	if (emlen < 2 * mdlen + 1) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_KEY_SIZE_TOO_SMALL);
	return 0;
	}

	to[0] = 0;
	seed = to + 1;
	db = to + mdlen + 1;

	if (!EVP_Digest(param, param_len, db, NULL, md, NULL)) {
	return 0;
	}
	OPENSSL_memset(db + mdlen, 0, emlen - from_len - 2 * mdlen - 1);
	db[emlen - from_len - mdlen - 1] = 0x01;
	OPENSSL_memcpy(db + emlen - from_len - mdlen, from, from_len);
	if (!RAND_bytes(seed, mdlen)) {
	return 0;
	}

	dbmask = OPENSSL_malloc(emlen - mdlen);
	if (dbmask == NULL) {
	OPENSSL_PUT_ERROR(RSA, ERR_R_MALLOC_FAILURE);
	return 0;
	}

	if (!PKCS1_MGF1(dbmask, emlen - mdlen, seed, mdlen, mgf1md)) {
	goto out;
	}
	for (i = 0; i < emlen - mdlen; i++) {
	db[i] ^= dbmask[i];
	}

	if (!PKCS1_MGF1(seedmask, mdlen, db, emlen - mdlen, mgf1md)) {
	goto out;
	}
	for (i = 0; i < mdlen; i++) {
	seed[i] ^= seedmask[i];
	}
	ret = 1;

	out:
	OPENSSL_free(dbmask);
	return ret;
	}

	int RSA_padding_check_PKCS1_OAEP_mgf1(uint8_t *to, unsigned to_len,
	const uint8_t *from, unsigned from_len,
	const uint8_t *param, unsigned param_len,
	const EVP_MD md, const EVP_MD mgf1md) {
	unsigned i, dblen, mlen = -1, mdlen, bad, looking_for_one_byte, one_index = 0;
	const uint8_t maskeddb, maskedseed;
	uint8_t *db = NULL, seed[EVP_MAX_MD_SIZE], phash[EVP_MAX_MD_SIZE];

	if (md == NULL) {
	md = EVP_sha1();
	}
	if (mgf1md == NULL) {
	mgf1md = md;
	}

	mdlen = EVP_MD_size(md);

	/* The encoded message is one byte smaller than the modulus to ensure that it
	* doesn't end up greater than the modulus. Thus there's an extra "+1" here
	* compared to https://tools.ietf.org/html/rfc2437#section-9.1.1.2. */
	if (from_len < 1 + 2*mdlen + 1) {
	/* 'from_len' is the length of the modulus, i.e. does not depend on the
	* particular ciphertext. */
	goto decoding_err;
	}

	dblen = from_len - mdlen - 1;
	db = OPENSSL_malloc(dblen);
	if (db == NULL) {
	OPENSSL_PUT_ERROR(RSA, ERR_R_MALLOC_FAILURE);
	goto err;
	}

	maskedseed = from + 1;
	maskeddb = from + 1 + mdlen;

	if (!PKCS1_MGF1(seed, mdlen, maskeddb, dblen, mgf1md)) {
	goto err;
	}
	for (i = 0; i < mdlen; i++) {
	seed[i] ^= maskedseed[i];
	}

	if (!PKCS1_MGF1(db, dblen, seed, mdlen, mgf1md)) {
	goto err;
	}
	for (i = 0; i < dblen; i++) {
	db[i] ^= maskeddb[i];
	}

	if (!EVP_Digest(param, param_len, phash, NULL, md, NULL)) {
	goto err;
	}

	bad = ~constant_time_is_zero(CRYPTO_memcmp(db, phash, mdlen));
	bad \|= ~constant_time_is_zero(from[0]);

	looking_for_one_byte = ~0u;
	for (i = mdlen; i < dblen; i++) {
	unsigned equals1 = constant_time_eq(db[i], 1);
	unsigned equals0 = constant_time_eq(db[i], 0);
	one_index = constant_time_select(looking_for_one_byte & equals1, i,
	one_index);
	looking_for_one_byte =
	constant_time_select(equals1, 0, looking_for_one_byte);
	bad \|= looking_for_one_byte & ~equals0;
	}

	bad \|= looking_for_one_byte;

	if (bad) {
	goto decoding_err;
	}

	one_index++;
	mlen = dblen - one_index;
	if (to_len < mlen) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_LARGE);
	mlen = -1;
	} else {
	OPENSSL_memcpy(to, db + one_index, mlen);
	}

	OPENSSL_free(db);
	return mlen;

	decoding_err:
	/* to avoid chosen ciphertext attacks, the error message should not reveal
	* which kind of decoding error happened */
	OPENSSL_PUT_ERROR(RSA, RSA_R_OAEP_DECODING_ERROR);
	err:
	OPENSSL_free(db);
	return -1;
	}

	static const unsigned char zeroes[] = {0,0,0,0,0,0,0,0};

	int RSA_verify_PKCS1_PSS_mgf1(RSA rsa, const uint8_t mHash,
	const EVP_MD Hash, const EVP_MD mgf1Hash,
	const uint8_t *EM, int sLen) {
	int i;
	int ret = 0;
	int maskedDBLen, MSBits, emLen;
	size_t hLen;
	const uint8_t *H;
	uint8_t *DB = NULL;
	EVP_MD_CTX ctx;
	uint8_t H_[EVP_MAX_MD_SIZE];
	EVP_MD_CTX_init(&ctx);

	if (mgf1Hash == NULL) {
	mgf1Hash = Hash;
	}

	hLen = EVP_MD_size(Hash);

	/* Negative sLen has special meanings:
	* -1 sLen == hLen
	* -2 salt length is autorecovered from signature
	* -N reserved */
	if (sLen == -1) {
	sLen = hLen;
	} else if (sLen == -2) {
	sLen = -2;
	} else if (sLen < -2) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_SLEN_CHECK_FAILED);
	goto err;
	}

	MSBits = (BN_num_bits(rsa->n) - 1) & 0x7;
	emLen = RSA_size(rsa);
	if (EM[0] & (0xFF << MSBits)) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_FIRST_OCTET_INVALID);
	goto err;
	}
	if (MSBits == 0) {
	EM++;
	emLen--;
	}
	if (emLen < ((int)hLen + sLen + 2)) {
	/* sLen can be small negative */
	OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_LARGE);
	goto err;
	}
	if (EM[emLen - 1] != 0xbc) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_LAST_OCTET_INVALID);
	goto err;
	}
	maskedDBLen = emLen - hLen - 1;
	H = EM + maskedDBLen;
	DB = OPENSSL_malloc(maskedDBLen);
	if (!DB) {
	OPENSSL_PUT_ERROR(RSA, ERR_R_MALLOC_FAILURE);
	goto err;
	}
	if (!PKCS1_MGF1(DB, maskedDBLen, H, hLen, mgf1Hash)) {
	goto err;
	}
	for (i = 0; i < maskedDBLen; i++) {
	DB[i] ^= EM[i];
	}
	if (MSBits) {
	DB[0] &= 0xFF >> (8 - MSBits);
	}
	for (i = 0; DB[i] == 0 && i < (maskedDBLen - 1); i++) {
	;
	}
	if (DB[i++] != 0x1) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_SLEN_RECOVERY_FAILED);
	goto err;
	}
	if (sLen >= 0 && (maskedDBLen - i) != sLen) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_SLEN_CHECK_FAILED);
	goto err;
	}
	if (!EVP_DigestInit_ex(&ctx, Hash, NULL) \|\|
	!EVP_DigestUpdate(&ctx, zeroes, sizeof zeroes) \|\|
	!EVP_DigestUpdate(&ctx, mHash, hLen)) {
	goto err;
	}
	if (maskedDBLen - i) {
	if (!EVP_DigestUpdate(&ctx, DB + i, maskedDBLen - i)) {
	goto err;
	}
	}
	if (!EVP_DigestFinal_ex(&ctx, H_, NULL)) {
	goto err;
	}
	if (OPENSSL_memcmp(H_, H, hLen)) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_BAD_SIGNATURE);
	ret = 0;
	} else {
	ret = 1;
	}

	err:
	OPENSSL_free(DB);
	EVP_MD_CTX_cleanup(&ctx);

	return ret;
	}

	int RSA_padding_add_PKCS1_PSS_mgf1(RSA rsa, unsigned char EM,
	const unsigned char *mHash,
	const EVP_MD Hash, const EVP_MD mgf1Hash,
	int sLenRequested) {
	int ret = 0;
	size_t maskedDBLen, MSBits, emLen;
	size_t hLen;
	unsigned char H, salt = NULL, *p;
	EVP_MD_CTX ctx;

	if (mgf1Hash == NULL) {
	mgf1Hash = Hash;
	}

	hLen = EVP_MD_size(Hash);

	if (BN_is_zero(rsa->n)) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_EMPTY_PUBLIC_KEY);
	goto err;
	}

	MSBits = (BN_num_bits(rsa->n) - 1) & 0x7;
	emLen = RSA_size(rsa);
	if (MSBits == 0) {
	assert(emLen >= 1);
	*EM++ = 0;
	emLen--;
	}

	if (emLen < hLen + 2) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
	goto err;
	}

	/* Negative sLenRequested has special meanings:
	* -1 sLen == hLen
	* -2 salt length is maximized
	* -N reserved */
	size_t sLen;
	if (sLenRequested == -1) {
	sLen = hLen;
	} else if (sLenRequested == -2) {
	sLen = emLen - hLen - 2;
	} else if (sLenRequested < 0) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_SLEN_CHECK_FAILED);
	goto err;
	} else {
	sLen = (size_t)sLenRequested;
	}

	if (emLen - hLen - 2 < sLen) {
	OPENSSL_PUT_ERROR(RSA, RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
	goto err;
	}

	if (sLen > 0) {
	salt = OPENSSL_malloc(sLen);
	if (!salt) {
	OPENSSL_PUT_ERROR(RSA, ERR_R_MALLOC_FAILURE);
	goto err;
	}
	if (!RAND_bytes(salt, sLen)) {
	goto err;
	}
	}
	maskedDBLen = emLen - hLen - 1;
	H = EM + maskedDBLen;
	EVP_MD_CTX_init(&ctx);
	if (!EVP_DigestInit_ex(&ctx, Hash, NULL) \|\|
	!EVP_DigestUpdate(&ctx, zeroes, sizeof zeroes) \|\|
	!EVP_DigestUpdate(&ctx, mHash, hLen)) {
	goto err;
	}
	if (sLen && !EVP_DigestUpdate(&ctx, salt, sLen)) {
	goto err;
	}
	if (!EVP_DigestFinal_ex(&ctx, H, NULL)) {
	goto err;
	}
	EVP_MD_CTX_cleanup(&ctx);

	/* Generate dbMask in place then perform XOR on it */
	if (!PKCS1_MGF1(EM, maskedDBLen, H, hLen, mgf1Hash)) {
	goto err;
	}

	p = EM;

	/* Initial PS XORs with all zeroes which is a NOP so just update
	* pointer. Note from a test above this value is guaranteed to
	* be non-negative. */
	p += emLen - sLen - hLen - 2;
	*p++ ^= 0x1;
	if (sLen > 0) {
	for (size_t i = 0; i < sLen; i++) {
	*p++ ^= salt[i];
	}
	}
	if (MSBits) {
	EM[0] &= 0xFF >> (8 - MSBits);
	}

	/* H is already in place so just set final 0xbc */

	EM[emLen - 1] = 0xbc;

	ret = 1;

	err:
	OPENSSL_free(salt);

	return ret;
	}