text: /* $OpenBSD: rsa_oaep.c,v 1.26 2017/01/29 17:49:23 beck Exp $ */
  /* 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.
   */
  
  #include <stdio.h>
  #include <stdlib.h>
  #include <string.h>
  
  #include <openssl/opensslconf.h>
  
  #if !defined(OPENSSL_NO_SHA) && !defined(OPENSSL_NO_SHA1)
  
  #include <openssl/bn.h>
  #include <openssl/err.h>
  #include <openssl/evp.h>
  #include <openssl/rsa.h>
  #include <openssl/sha.h>
  
  static 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) {
  		RSAerror(RSA_R_DATA_TOO_LARGE_FOR_KEY_SIZE);
  		return 0;
  	}
  
  	if (emlen < 2 * SHA_DIGEST_LENGTH + 1) {
  		RSAerror(RSA_R_KEY_SIZE_TOO_SMALL);
  		return 0;
  	}
  
  	to[0] = 0;
  	seed = to + 1;
  	db = to + SHA_DIGEST_LENGTH + 1;
  
  	if (!EVP_Digest((void *)param, plen, db, NULL, EVP_sha1(), NULL))
  		return 0;
  	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, flen);
  	arc4random_buf(seed, SHA_DIGEST_LENGTH);
  
  	dbmask = malloc(emlen - SHA_DIGEST_LENGTH);
  	if (dbmask == NULL) {
  		RSAerror(ERR_R_MALLOC_FAILURE);
  		return 0;
  	}
  
  	if (MGF1(dbmask, emlen - SHA_DIGEST_LENGTH, seed,
  	    SHA_DIGEST_LENGTH) < 0)
  		return 0;
  	for (i = 0; i < emlen - SHA_DIGEST_LENGTH; i++)
  		db[i] ^= dbmask[i];
  
  	if (MGF1(seedmask, SHA_DIGEST_LENGTH, db,
  	    emlen - SHA_DIGEST_LENGTH) < 0)
  		return 0;
  	for (i = 0; i < SHA_DIGEST_LENGTH; i++)
  		seed[i] ^= seedmask[i];
  
  	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;
  	unsigned char seed[SHA_DIGEST_LENGTH], phash[SHA_DIGEST_LENGTH];
  	unsigned char *padded_from;
  	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) {
  		/*
  		 * 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;
  		flen = num; /* don't overflow the memcpy to padded_from */
  	}
  
  	dblen = num - SHA_DIGEST_LENGTH;
  	db = malloc(dblen + num);
  	if (db == NULL) {
  		RSAerror(ERR_R_MALLOC_FAILURE);
  		return -1;
  	}
  
  	/*
  	 * Always do this zero-padding copy (even when lzero == 0)
  	 * to avoid leaking timing info about the value of lzero.
  	 */
  	padded_from = db + dblen;
  	memset(padded_from, 0, lzero);
  	memcpy(padded_from + lzero, from, flen);
  
  	maskeddb = padded_from + SHA_DIGEST_LENGTH;
  
  	if (MGF1(seed, SHA_DIGEST_LENGTH, maskeddb, dblen))
  		return -1;
  	for (i = 0; i < SHA_DIGEST_LENGTH; i++)
  		seed[i] ^= padded_from[i];
  
  	if (MGF1(db, dblen, seed, SHA_DIGEST_LENGTH))
  		return -1;
  	for (i = 0; i < dblen; i++)
  		db[i] ^= maskeddb[i];
  
  	if (!EVP_Digest((void *)param, plen, phash, NULL, EVP_sha1(), NULL))
  		return -1;
  
  	if (timingsafe_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 (i == dblen || db[i] != 0x01)
  			goto decoding_err;
  		else {
  			/* everything looks OK */
  
  			mlen = dblen - ++i;
  			if (tlen < mlen) {
  				RSAerror(RSA_R_DATA_TOO_LARGE);
  				mlen = -1;
  			} else
  				memcpy(to, db + i, mlen);
  		}
  	}
  	free(db);
  	return mlen;
  
  decoding_err:
  	/*
  	 * To avoid chosen ciphertext attacks, the error message should not
  	 * reveal which kind of decoding error happened
  	 */
  	RSAerror(RSA_R_OAEP_DECODING_ERROR);
  	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;
  	int rv = -1;
  
  	EVP_MD_CTX_init(&c);
  	mdlen = EVP_MD_size(dgst);
  	if (mdlen < 0)
  		goto err;
  	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);
  		if (!EVP_DigestInit_ex(&c, dgst, NULL) ||
  		    !EVP_DigestUpdate(&c, seed, seedlen) ||
  		    !EVP_DigestUpdate(&c, cnt, 4))
  			goto err;
  		if (outlen + mdlen <= len) {
  			if (!EVP_DigestFinal_ex(&c, mask + outlen, NULL))
  				goto err;
  			outlen += mdlen;
  		} else {
  			if (!EVP_DigestFinal_ex(&c, md, NULL))
  				goto err;
  			memcpy(mask + outlen, md, len - outlen);
  			outlen = len;
  		}
  	}
  	rv = 0;
  err:
  	EVP_MD_CTX_cleanup(&c);
  	return rv;
  }
  
  static int
  MGF1(unsigned char *mask, long len, const unsigned char *seed, long seedlen)
  {
  	return PKCS1_MGF1(mask, len, seed, seedlen, EVP_sha1());
  }
  #endif
  