crypto/rsa/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>

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;

    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

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

    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, 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;

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

    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);

    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;

    EVP_MD_CTX_init(&c);
    mdlen = M_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