/*

 * Copyright 1995-2019 The OpenSSL Project Authors. All Rights Reserved.

 *

 * Licensed under the OpenSSL license (the "License").  You may not use

 * this file except in compliance with the License.  You can obtain a copy

 * in the file LICENSE in the source distribution or at

 * https://www.openssl.org/source/license.html

 */

#include <stdio.h>

#include "internal/cryptlib.h"

#include "crypto/bn.h"

#include <openssl/bn.h>

#include <openssl/sha.h>

#include "dsa_local.h"

#include <openssl/asn1.h>

#ifdef OPENSSL_FIPS

# include <openssl/fips.h>

#endif

static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa);

static int dsa_sign_setup_no_digest(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp,

                                    BIGNUM **rp);

static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp,

                          BIGNUM **rp, const unsigned char *dgst, int dlen);

static int dsa_do_verify(const unsigned char *dgst, int dgst_len,

                         DSA_SIG *sig, DSA *dsa);

static int dsa_init(DSA *dsa);

static int dsa_finish(DSA *dsa);

static BIGNUM *dsa_mod_inverse_fermat(const BIGNUM *k, const BIGNUM *q,

                                      BN_CTX *ctx);

static DSA_METHOD openssl_dsa_meth = {

    "OpenSSL DSA method",

    dsa_do_sign,

    dsa_sign_setup_no_digest,

    dsa_do_verify,

    NULL,                       /* dsa_mod_exp, */

    NULL,                       /* dsa_bn_mod_exp, */

    dsa_init,

    dsa_finish,

    DSA_FLAG_FIPS_METHOD,

    NULL,

    NULL,

    NULL

};

static const DSA_METHOD *default_DSA_method = &openssl_dsa_meth;

void DSA_set_default_method(const DSA_METHOD *meth)

{

    default_DSA_method = meth;

}

const DSA_METHOD *DSA_get_default_method(void)

{

    return default_DSA_method;

}

const DSA_METHOD *DSA_OpenSSL(void)

{

    return &openssl_dsa_meth;

}

static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa)

{

    BIGNUM *kinv = NULL;

    BIGNUM *m, *blind, *blindm, *tmp;

    BN_CTX *ctx = NULL;

    int reason = ERR_R_BN_LIB;

    DSA_SIG *ret = NULL;

    int rv = 0;

    if (dsa->p == NULL || dsa->q == NULL || dsa->g == NULL) {

        reason = DSA_R_MISSING_PARAMETERS;

        goto err;

    }

    if (dsa->priv_key == NULL) {

        reason = DSA_R_MISSING_PRIVATE_KEY;

        goto err;

    }

#ifdef OPENSSL_FIPS

    if (FIPS_selftest_failed()) {

        FIPSerr(FIPS_F_DSA_DO_SIGN, FIPS_R_FIPS_SELFTEST_FAILED);

        return NULL;

    }

    if (FIPS_mode() && !(dsa->flags & DSA_FLAG_NON_FIPS_ALLOW)

        && (BN_num_bits(dsa->p) < OPENSSL_DSA_FIPS_MIN_MODULUS_BITS)) {

        DSAerr(DSA_F_DSA_DO_SIGN, DSA_R_KEY_SIZE_TOO_SMALL);

        return NULL;

    }

#endif

    ret = DSA_SIG_new();

    if (ret == NULL)

        goto err;

    ret->r = BN_new();

    ret->s = BN_new();

    if (ret->r == NULL || ret->s == NULL)

        goto err;

    ctx = BN_CTX_new();

    if (ctx == NULL)

        goto err;

    m = BN_CTX_get(ctx);

    blind = BN_CTX_get(ctx);

    blindm = BN_CTX_get(ctx);

    tmp = BN_CTX_get(ctx);

    if (tmp == NULL)

        goto err;

 redo:

    if (!dsa_sign_setup(dsa, ctx, &kinv, &ret->r, dgst, dlen))

        goto err;

    if (dlen > BN_num_bytes(dsa->q))

        /*

         * if the digest length is greater than the size of q use the

         * BN_num_bits(dsa->q) leftmost bits of the digest, see fips 186-3,

         * 4.2

         */

        dlen = BN_num_bytes(dsa->q);

    if (BN_bin2bn(dgst, dlen, m) == NULL)

        goto err;

    /*

     * The normal signature calculation is:

     *

     *   s := k^-1 * (m + r * priv_key) mod q

     *

     * We will blind this to protect against side channel attacks

     *

     *   s := blind^-1 * k^-1 * (blind * m + blind * r * priv_key) mod q

     */

    /* Generate a blinding value */

    do {

        if (!BN_priv_rand(blind, BN_num_bits(dsa->q) - 1,

                          BN_RAND_TOP_ANY, BN_RAND_BOTTOM_ANY))

            goto err;

    } while (BN_is_zero(blind));

    BN_set_flags(blind, BN_FLG_CONSTTIME);

    BN_set_flags(blindm, BN_FLG_CONSTTIME);

    BN_set_flags(tmp, BN_FLG_CONSTTIME);

    /* tmp := blind * priv_key * r mod q */

    if (!BN_mod_mul(tmp, blind, dsa->priv_key, dsa->q, ctx))

        goto err;

    if (!BN_mod_mul(tmp, tmp, ret->r, dsa->q, ctx))

        goto err;

    /* blindm := blind * m mod q */

    if (!BN_mod_mul(blindm, blind, m, dsa->q, ctx))

        goto err;

    /* s : = (blind * priv_key * r) + (blind * m) mod q */

    if (!BN_mod_add_quick(ret->s, tmp, blindm, dsa->q))

        goto err;

    /* s := s * k^-1 mod q */

    if (!BN_mod_mul(ret->s, ret->s, kinv, dsa->q, ctx))

        goto err;

    /* s:= s * blind^-1 mod q */

    if (BN_mod_inverse(blind, blind, dsa->q, ctx) == NULL)

        goto err;

    if (!BN_mod_mul(ret->s, ret->s, blind, dsa->q, ctx))

        goto err;

    /*

     * Redo if r or s is zero as required by FIPS 186-3: this is very

     * unlikely.

     */

    if (BN_is_zero(ret->r) || BN_is_zero(ret->s))

        goto redo;

    rv = 1;

 err:

    if (rv == 0) {

        DSAerr(DSA_F_DSA_DO_SIGN, reason);

        DSA_SIG_free(ret);

        ret = NULL;

    }

    BN_CTX_free(ctx);

    BN_clear_free(kinv);

    return ret;

}

static int dsa_sign_setup_no_digest(DSA *dsa, BN_CTX *ctx_in,

                                    BIGNUM **kinvp, BIGNUM **rp)

{

    return dsa_sign_setup(dsa, ctx_in, kinvp, rp, NULL, 0);

}

static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,

                          BIGNUM **kinvp, BIGNUM **rp,

                          const unsigned char *dgst, int dlen)

{

    BN_CTX *ctx = NULL;

    BIGNUM *k, *kinv = NULL, *r = *rp;

    BIGNUM *l;

    int ret = 0;

    int q_bits, q_words;

    if (!dsa->p || !dsa->q || !dsa->g) {

        DSAerr(DSA_F_DSA_SIGN_SETUP, DSA_R_MISSING_PARAMETERS);

        return 0;

    }

    /* Reject obviously invalid parameters */

    if (BN_is_zero(dsa->p) || BN_is_zero(dsa->q) || BN_is_zero(dsa->g)) {

        DSAerr(DSA_F_DSA_SIGN_SETUP, DSA_R_INVALID_PARAMETERS);

        return 0;

    }

    if (dsa->priv_key == NULL) {

        DSAerr(DSA_F_DSA_SIGN_SETUP, DSA_R_MISSING_PRIVATE_KEY);

        return 0;

    }

    k = BN_new();

    l = BN_new();

    if (k == NULL || l == NULL)

        goto err;

    if (ctx_in == NULL) {

        if ((ctx = BN_CTX_new()) == NULL)

            goto err;

    } else

        ctx = ctx_in;

    /* Preallocate space */

    q_bits = BN_num_bits(dsa->q);

    q_words = bn_get_top(dsa->q);

    if (!bn_wexpand(k, q_words + 2)

        || !bn_wexpand(l, q_words + 2))

        goto err;

    /* Get random k */

    do {

        if (dgst != NULL) {

            /*

             * We calculate k from SHA512(private_key + H(message) + random).

             * This protects the private key from a weak PRNG.

             */

            if (!BN_generate_dsa_nonce(k, dsa->q, dsa->priv_key, dgst,

                                       dlen, ctx))

                goto err;

        } else if (!BN_priv_rand_range(k, dsa->q))

            goto err;

    } while (BN_is_zero(k));

    BN_set_flags(k, BN_FLG_CONSTTIME);

    BN_set_flags(l, BN_FLG_CONSTTIME);

    if (dsa->flags & DSA_FLAG_CACHE_MONT_P) {

        if (!BN_MONT_CTX_set_locked(&dsa->method_mont_p,

                                    dsa->lock, dsa->p, ctx))

            goto err;

    }

    /* Compute r = (g^k mod p) mod q */

    /*

     * We do not want timing information to leak the length of k, so we

     * compute G^k using an equivalent scalar of fixed bit-length.

     *

     * We unconditionally perform both of these additions to prevent a

     * small timing information leakage.  We then choose the sum that is

     * one bit longer than the modulus.

     *

     * There are some concerns about the efficacy of doing this.  More

     * specifically refer to the discussion starting with:

     *     https://github.com/openssl/openssl/pull/7486#discussion_r228323705

     * The fix is to rework BN so these gymnastics aren't required.

     */

    if (!BN_add(l, k, dsa->q)

        || !BN_add(k, l, dsa->q))

        goto err;

    BN_consttime_swap(BN_is_bit_set(l, q_bits), k, l, q_words + 2);

    if ((dsa)->meth->bn_mod_exp != NULL) {

            if (!dsa->meth->bn_mod_exp(dsa, r, dsa->g, k, dsa->p, ctx,

                                       dsa->method_mont_p))

                goto err;

    } else {

            if (!BN_mod_exp_mont(r, dsa->g, k, dsa->p, ctx, dsa->method_mont_p))

                goto err;

    }

    if (!BN_mod(r, r, dsa->q, ctx))

        goto err;

    /* Compute part of 's = inv(k) (m + xr) mod q' */

    if ((kinv = dsa_mod_inverse_fermat(k, dsa->q, ctx)) == NULL)

        goto err;

    BN_clear_free(*kinvp);

    *kinvp = kinv;

    kinv = NULL;

    ret = 1;

 err:

    if (!ret)

        DSAerr(DSA_F_DSA_SIGN_SETUP, ERR_R_BN_LIB);

    if (ctx != ctx_in)

        BN_CTX_free(ctx);

    BN_clear_free(k);

    BN_clear_free(l);

    return ret;

}

static int dsa_do_verify(const unsigned char *dgst, int dgst_len,

                         DSA_SIG *sig, DSA *dsa)

{

    BN_CTX *ctx;

    BIGNUM *u1, *u2, *t1;

    BN_MONT_CTX *mont = NULL;

    const BIGNUM *r, *s;

    int ret = -1, i;

    if (!dsa->p || !dsa->q || !dsa->g) {

        DSAerr(DSA_F_DSA_DO_VERIFY, DSA_R_MISSING_PARAMETERS);

        return -1;

    }

    i = BN_num_bits(dsa->q);

    /* fips 186-3 allows only different sizes for q */

    if (i != 160 && i != 224 && i != 256) {

        DSAerr(DSA_F_DSA_DO_VERIFY, DSA_R_BAD_Q_VALUE);

        return -1;

    }

#ifdef OPENSSL_FIPS

    if (FIPS_selftest_failed()) {

        FIPSerr(FIPS_F_DSA_DO_VERIFY, FIPS_R_FIPS_SELFTEST_FAILED);

        return -1;

    }

    if (FIPS_mode() && !(dsa->flags & DSA_FLAG_NON_FIPS_ALLOW)

        && (BN_num_bits(dsa->p) < OPENSSL_DSA_FIPS_MIN_MODULUS_BITS)) {

        DSAerr(DSA_F_DSA_DO_VERIFY, DSA_R_KEY_SIZE_TOO_SMALL);

        return -1;

    }

#endif

    if (BN_num_bits(dsa->p) > OPENSSL_DSA_MAX_MODULUS_BITS) {

        DSAerr(DSA_F_DSA_DO_VERIFY, DSA_R_MODULUS_TOO_LARGE);

        return -1;

    }

    u1 = BN_new();

    u2 = BN_new();

    t1 = BN_new();

    ctx = BN_CTX_new();

    if (u1 == NULL || u2 == NULL || t1 == NULL || ctx == NULL)

        goto err;

    DSA_SIG_get0(sig, &r, &s);

    if (BN_is_zero(r) || BN_is_negative(r) ||

        BN_ucmp(r, dsa->q) >= 0) {

        ret = 0;

        goto err;

    }

    if (BN_is_zero(s) || BN_is_negative(s) ||

        BN_ucmp(s, dsa->q) >= 0) {

        ret = 0;

        goto err;

    }

    /*

     * Calculate W = inv(S) mod Q save W in u2

     */

    if ((BN_mod_inverse(u2, s, dsa->q, ctx)) == NULL)

        goto err;

    /* save M in u1 */

    if (dgst_len > (i >> 3))

        /*

         * if the digest length is greater than the size of q use the

         * BN_num_bits(dsa->q) leftmost bits of the digest, see fips 186-3,

         * 4.2

         */

        dgst_len = (i >> 3);

    if (BN_bin2bn(dgst, dgst_len, u1) == NULL)

        goto err;

    /* u1 = M * w mod q */

    if (!BN_mod_mul(u1, u1, u2, dsa->q, ctx))

        goto err;

    /* u2 = r * w mod q */

    if (!BN_mod_mul(u2, r, u2, dsa->q, ctx))

        goto err;

    if (dsa->flags & DSA_FLAG_CACHE_MONT_P) {

        mont = BN_MONT_CTX_set_locked(&dsa->method_mont_p,

                                      dsa->lock, dsa->p, ctx);

        if (!mont)

            goto err;

    }

    if (dsa->meth->dsa_mod_exp != NULL) {

        if (!dsa->meth->dsa_mod_exp(dsa, t1, dsa->g, u1, dsa->pub_key, u2,

                                    dsa->p, ctx, mont))

            goto err;

    } else {

        if (!BN_mod_exp2_mont(t1, dsa->g, u1, dsa->pub_key, u2, dsa->p, ctx,

                              mont))

            goto err;

    }

    /* let u1 = u1 mod q */

    if (!BN_mod(u1, t1, dsa->q, ctx))

        goto err;

    /*

     * V is now in u1.  If the signature is correct, it will be equal to R.

     */

    ret = (BN_ucmp(u1, r) == 0);

 err:

    if (ret < 0)

        DSAerr(DSA_F_DSA_DO_VERIFY, ERR_R_BN_LIB);

    BN_CTX_free(ctx);

    BN_free(u1);

    BN_free(u2);

    BN_free(t1);

    return ret;

}

static int dsa_init(DSA *dsa)

{

#ifdef OPENSSL_FIPS

    FIPS_selftest_check();

#endif

    dsa->flags |= DSA_FLAG_CACHE_MONT_P;

    return 1;

}

static int dsa_finish(DSA *dsa)

{

    BN_MONT_CTX_free(dsa->method_mont_p);

    return 1;

}

/*

 * Compute the inverse of k modulo q.

 * Since q is prime, Fermat's Little Theorem applies, which reduces this to

 * mod-exp operation.  Both the exponent and modulus are public information

 * so a mod-exp that doesn't leak the base is sufficient.  A newly allocated

 * BIGNUM is returned which the caller must free.

 */

static BIGNUM *dsa_mod_inverse_fermat(const BIGNUM *k, const BIGNUM *q,

                                      BN_CTX *ctx)

{

    BIGNUM *res = NULL;

    BIGNUM *r, *e;

    if ((r = BN_new()) == NULL)

        return NULL;

    BN_CTX_start(ctx);

    if ((e = BN_CTX_get(ctx)) != NULL

            && BN_set_word(r, 2)

            && BN_sub(e, q, r)

            && BN_mod_exp_mont(r, k, e, q, ctx, NULL))

        res = r;

    else

        BN_free(r);

    BN_CTX_end(ctx);

    return res;

}