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rsa_oaep.c

/* crypto/rsa/rsa_oaep.c */
/* Written by Ulf Moeller. This software is distributed on an "AS IS"
   basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. */

/* EME-OAEP as defined in RFC 2437 (PKCS #1 v2.0) */

/* See Victor Shoup, "OAEP reconsidered," Nov. 2000,
 * <URL: http://www.shoup.net/papers/oaep.ps.Z>
 * for problems with the security proof for the
 * original OAEP scheme, which EME-OAEP is based on.
 * 
 * A new proof can be found in E. Fujisaki, T. Okamoto,
 * D. Pointcheval, J. Stern, "RSA-OEAP is Still Alive!",
 * Dec. 2000, <URL: http://eprint.iacr.org/2000/061/>.
 * The new proof has stronger requirements for the
 * underlying permutation: "partial-one-wayness" instead
 * of one-wayness.  For the RSA function, this is
 * an equivalent notion.
 */


#if !defined(OPENSSL_NO_SHA) && !defined(OPENSSL_NO_SHA1)
#include <stdio.h>
#include "cryptlib.h"
#include <openssl/bn.h>
#include <openssl/rsa.h>
#include <openssl/evp.h>
#include <openssl/rand.h>
#include <openssl/sha.h>

int MGF1(unsigned char *mask, long len,
      const unsigned char *seed, long seedlen);

int RSA_padding_add_PKCS1_OAEP(unsigned char *to, int tlen,
      const unsigned char *from, int flen,
      const unsigned char *param, int plen)
      {
      int i, emlen = tlen - 1;
      unsigned char *db, *seed;
      unsigned char *dbmask, seedmask[SHA_DIGEST_LENGTH];

      if (flen > emlen - 2 * SHA_DIGEST_LENGTH - 1)
            {
            RSAerr(RSA_F_RSA_PADDING_ADD_PKCS1_OAEP,
               RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
            return 0;
            }

      if (emlen < 2 * SHA_DIGEST_LENGTH + 1)
            {
            RSAerr(RSA_F_RSA_PADDING_ADD_PKCS1_OAEP, RSA_R_KEY_SIZE_TOO_SMALL);
            return 0;
            }

      dbmask = OPENSSL_malloc(emlen - SHA_DIGEST_LENGTH);
      if (dbmask == NULL)
            {
            RSAerr(RSA_F_RSA_PADDING_ADD_PKCS1_OAEP, ERR_R_MALLOC_FAILURE);
            return 0;
            }

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

      EVP_Digest((void *)param, plen, db, NULL, EVP_sha1(), NULL);
      memset(db + SHA_DIGEST_LENGTH, 0,
            emlen - flen - 2 * SHA_DIGEST_LENGTH - 1);
      db[emlen - flen - SHA_DIGEST_LENGTH - 1] = 0x01;
      memcpy(db + emlen - flen - SHA_DIGEST_LENGTH, from, (unsigned int) flen);
      if (RAND_bytes(seed, SHA_DIGEST_LENGTH) <= 0)
            return 0;
#ifdef PKCS_TESTVECT
      memcpy(seed,
         "\xaa\xfd\x12\xf6\x59\xca\xe6\x34\x89\xb4\x79\xe5\x07\x6d\xde\xc2\xf0\x6c\xb5\x8f",
         20);
#endif

      MGF1(dbmask, emlen - SHA_DIGEST_LENGTH, seed, SHA_DIGEST_LENGTH);
      for (i = 0; i < emlen - SHA_DIGEST_LENGTH; i++)
            db[i] ^= dbmask[i];

      MGF1(seedmask, SHA_DIGEST_LENGTH, db, emlen - SHA_DIGEST_LENGTH);
      for (i = 0; i < SHA_DIGEST_LENGTH; i++)
            seed[i] ^= seedmask[i];

      OPENSSL_free(dbmask);
      return 1;
      }

int RSA_padding_check_PKCS1_OAEP(unsigned char *to, int tlen,
      const unsigned char *from, int flen, int num,
      const unsigned char *param, int plen)
      {
      int i, dblen, mlen = -1;
      const unsigned char *maskeddb;
      int lzero;
      unsigned char *db = NULL, seed[SHA_DIGEST_LENGTH], phash[SHA_DIGEST_LENGTH];
      int bad = 0;

      if (--num < 2 * SHA_DIGEST_LENGTH + 1)
            /* 'num' is the length of the modulus, i.e. does not depend on the
             * particular ciphertext. */
            goto decoding_err;

      lzero = num - flen;
      if (lzero < 0)
            {
            /* lzero == -1 */

            /* signalling this error immediately after detection might allow
             * for side-channel attacks (e.g. timing if 'plen' is huge
             * -- cf. James H. Manger, "A Chosen Ciphertext Attack on RSA Optimal
             * Asymmetric Encryption Padding (OAEP) [...]", CRYPTO 2001),
             * so we use a 'bad' flag */
            bad = 1;
            lzero = 0;
            }
      maskeddb = from - lzero + SHA_DIGEST_LENGTH;

      dblen = num - SHA_DIGEST_LENGTH;
      db = OPENSSL_malloc(dblen);
      if (db == NULL)
            {
            RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_OAEP, ERR_R_MALLOC_FAILURE);
            return -1;
            }

      MGF1(seed, SHA_DIGEST_LENGTH, maskeddb, dblen);
      for (i = lzero; i < SHA_DIGEST_LENGTH; i++)
            seed[i] ^= from[i - lzero];
  
      MGF1(db, dblen, seed, SHA_DIGEST_LENGTH);
      for (i = 0; i < dblen; i++)
            db[i] ^= maskeddb[i];

      EVP_Digest((void *)param, plen, phash, NULL, EVP_sha1(), NULL);

      if (memcmp(db, phash, SHA_DIGEST_LENGTH) != 0 || bad)
            goto decoding_err;
      else
            {
            for (i = SHA_DIGEST_LENGTH; i < dblen; i++)
                  if (db[i] != 0x00)
                        break;
            if (db[i] != 0x01 || i++ >= dblen)
                  goto decoding_err;
            else
                  {
                  /* everything looks OK */

                  mlen = dblen - i;
                  if (tlen < mlen)
                        {
                        RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_OAEP, RSA_R_DATA_TOO_LARGE);
                        mlen = -1;
                        }
                  else
                        memcpy(to, db + i, 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 */
      RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_OAEP, RSA_R_OAEP_DECODING_ERROR);
      if (db != NULL) OPENSSL_free(db);
      return -1;
      }

int PKCS1_MGF1(unsigned char *mask, long len,
      const unsigned char *seed, long seedlen, const EVP_MD *dgst)
      {
      long i, outlen = 0;
      unsigned char cnt[4];
      EVP_MD_CTX c;
      unsigned char md[EVP_MAX_MD_SIZE];
      int mdlen;

      EVP_MD_CTX_init(&c);
      mdlen = EVP_MD_size(dgst);
      for (i = 0; outlen < len; i++)
            {
            cnt[0] = (unsigned char)((i >> 24) & 255);
            cnt[1] = (unsigned char)((i >> 16) & 255);
            cnt[2] = (unsigned char)((i >> 8)) & 255;
            cnt[3] = (unsigned char)(i & 255);
            EVP_DigestInit_ex(&c,dgst, NULL);
            EVP_DigestUpdate(&c, seed, seedlen);
            EVP_DigestUpdate(&c, cnt, 4);
            if (outlen + mdlen <= len)
                  {
                  EVP_DigestFinal_ex(&c, mask + outlen, NULL);
                  outlen += mdlen;
                  }
            else
                  {
                  EVP_DigestFinal_ex(&c, md, NULL);
                  memcpy(mask + outlen, md, len - outlen);
                  outlen = len;
                  }
            }
      EVP_MD_CTX_cleanup(&c);
      return 0;
      }

int MGF1(unsigned char *mask, long len, const unsigned char *seed, long seedlen)
      {
      return PKCS1_MGF1(mask, len, seed, seedlen, EVP_sha1());
      }
#endif

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