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.
 */

#include "constant_time_locl.h"

#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>

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)
		{
		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;
		}

	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, (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

	dbmask = OPENSSL_malloc(emlen - SHA_DIGEST_LENGTH);
	if (dbmask == NULL)
		{
		RSAerr(RSA_F_RSA_PADDING_ADD_PKCS1_OAEP, 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];

	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, one_index = 0, msg_index;
	unsigned int good, found_one_byte;
	const unsigned char *maskedseed, *maskeddb;
	/* |em| is the encoded message, zero-padded to exactly |num| bytes:
	 * em = Y || maskedSeed || maskedDB */
	unsigned char *db = NULL, *em = NULL, seed[EVP_MAX_MD_SIZE],
		phash[EVP_MAX_MD_SIZE];

        if (tlen <= 0 || flen <= 0)
		return -1;

	/*
	 * |num| is the length of the modulus; |flen| is the length of the
	 * encoded message. Therefore, for any |from| that was obtained by
	 * decrypting a ciphertext, we must have |flen| <= |num|. Similarly,
	 * num < 2 * SHA_DIGEST_LENGTH + 2 must hold for the modulus
	 * irrespective of the ciphertext, see PKCS #1 v2.2, section 7.1.2.
	 * This does not leak any side-channel information.
	 */
	if (num < flen || num < 2 * SHA_DIGEST_LENGTH + 2)
		goto decoding_err;

	dblen = num - SHA_DIGEST_LENGTH - 1;
	db = OPENSSL_malloc(dblen);
	em = OPENSSL_malloc(num);
	if (db == NULL || em == NULL)
		{
		RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_OAEP, ERR_R_MALLOC_FAILURE);
		goto cleanup;
		}

	/*
	 * Always do this zero-padding copy (even when num == flen) to avoid
	 * leaking that information. The copy still leaks some side-channel
	 * information, but it's impossible to have a fixed  memory access
	 * pattern since we can't read out of the bounds of |from|.
	 *
	 * TODO(emilia): Consider porting BN_bn2bin_padded from BoringSSL.
	 */
	memset(em, 0, num);
	memcpy(em + num - flen, from, flen);

	/*
	 * The first byte must be zero, however we must not leak if this is
	 * true. See James H. Manger, "A Chosen Ciphertext  Attack on RSA
	 * Optimal Asymmetric Encryption Padding (OAEP) [...]", CRYPTO 2001).
	 */
	good = constant_time_is_zero(em[0]);

	maskedseed = em + 1;
	maskeddb = em + 1 + SHA_DIGEST_LENGTH;

	if (MGF1(seed, SHA_DIGEST_LENGTH, maskeddb, dblen))
		goto cleanup;
	for (i = 0; i < SHA_DIGEST_LENGTH; i++)
		seed[i] ^= maskedseed[i];

	if (MGF1(db, dblen, seed, SHA_DIGEST_LENGTH))
		goto cleanup;
	for (i = 0; i < dblen; i++)
		db[i] ^= maskeddb[i];

	if (!EVP_Digest((void *)param, plen, phash, NULL, EVP_sha1(), NULL))
		goto cleanup;

	good &= constant_time_is_zero(CRYPTO_memcmp(db, phash, SHA_DIGEST_LENGTH));

	found_one_byte = 0;
	for (i = SHA_DIGEST_LENGTH; i < dblen; i++)
		{
		/* Padding consists of a number of 0-bytes, followed by a 1. */
		unsigned int equals1 = constant_time_eq(db[i], 1);
		unsigned int equals0 = constant_time_is_zero(db[i]);
		one_index = constant_time_select_int(~found_one_byte & equals1,
			i, one_index);
		found_one_byte |= equals1;
		good &= (found_one_byte | equals0);
		}

	good &= found_one_byte;

	/*
	 * At this point |good| is zero unless the plaintext was valid,
	 * so plaintext-awareness ensures timing side-channels are no longer a
	 * concern.
	 */
	if (!good)
		goto decoding_err;

	msg_index = one_index + 1;
	mlen = dblen - msg_index;

	if (tlen < mlen)
		{
		RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_OAEP, RSA_R_DATA_TOO_LARGE);
		mlen = -1;
		}
	else
		{
		memcpy(to, db + msg_index, mlen);
		goto cleanup;
		}

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);
cleanup:
	if (db != NULL) OPENSSL_free(db);
	if (em != NULL) OPENSSL_free(em);
	return mlen;
	}

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