/* $OpenBSD: dsa_ossl.c,v 1.57 2025/05/10 05:54:38 tb Exp $ */
/* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
 * All rights reserved.
 *
 * This package is an SSL implementation written
 * by Eric Young (eay@cryptsoft.com).
 * The implementation was written so as to conform with Netscapes SSL.
 *
 * This library is free for commercial and non-commercial use as long as
 * the following conditions are aheared to.  The following conditions
 * apply to all code found in this distribution, be it the RC4, RSA,
 * lhash, DES, etc., code; not just the SSL code.  The SSL documentation
 * included with this distribution is covered by the same copyright terms
 * except that the holder is Tim Hudson (tjh@cryptsoft.com).
 *
 * Copyright remains Eric Young's, and as such any Copyright notices in
 * the code are not to be removed.
 * If this package is used in a product, Eric Young should be given attribution
 * as the author of the parts of the library used.
 * This can be in the form of a textual message at program startup or
 * in documentation (online or textual) provided with the package.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *    "This product includes cryptographic software written by
 *     Eric Young (eay@cryptsoft.com)"
 *    The word 'cryptographic' can be left out if the rouines from the library
 *    being used are not cryptographic related :-).
 * 4. If you include any Windows specific code (or a derivative thereof) from
 *    the apps directory (application code) you must include an acknowledgement:
 *    "This product includes software written by Tim Hudson (tjh@cryptsoft.com)"
 *
 * THIS SOFTWARE IS PROVIDED BY ERIC YOUNG ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 * The licence and distribution terms for any publically available version or
 * derivative of this code cannot be changed.  i.e. this code cannot simply be
 * copied and put under another distribution licence
 * [including the GNU Public Licence.]
 */

/* Original version from Steven Schoch <schoch@sheba.arc.nasa.gov> */

#include <stdio.h>

#include <openssl/asn1.h>
#include <openssl/bn.h>
#include <openssl/dsa.h>
#include <openssl/sha.h>

#include "bn_local.h"
#include "dsa_local.h"
#include "err_local.h"

/*
 * Since DSA parameters are entirely arbitrary and checking them to be
 * consistent is very expensive, we cannot do so on every sign operation.
 * Instead, cap the number of retries so we do not loop indefinitely if
 * the generator of the multiplicative group happens to be nilpotent.
 * The probability of needing a retry with valid parameters is negligible,
 * so trying 32 times is amply enough.
 */
#define DSA_MAX_SIGN_ITERATIONS         32

static DSA_SIG *
dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa)
{
        BIGNUM *b = NULL, *bm = NULL, *bxr = NULL, *binv = NULL, *m = NULL;
        BIGNUM *kinv = NULL, *r = NULL, *s = NULL;
        BN_CTX *ctx = NULL;
        int reason = ERR_R_BN_LIB;
        DSA_SIG *ret = NULL;
        int attempts = 0;
        int noredo = 0;

        if (!dsa_check_key(dsa)) {
                reason = DSA_R_INVALID_PARAMETERS;
                goto err;
        }

        if ((s = BN_new()) == NULL)
                goto err;

        if ((ctx = BN_CTX_new()) == NULL)
                goto err;

        BN_CTX_start(ctx);

        if ((b = BN_CTX_get(ctx)) == NULL)
                goto err;
        if ((binv = BN_CTX_get(ctx)) == NULL)
                goto err;
        if ((bm = BN_CTX_get(ctx)) == NULL)
                goto err;
        if ((bxr = BN_CTX_get(ctx)) == NULL)
                goto err;
        if ((m = BN_CTX_get(ctx)) == NULL)
                goto err;

        /*
         * If the digest length is greater than N (the bit length of q), the
         * leftmost N bits of the digest shall be used, see FIPS 186-3, 4.2.
         * In this case the digest length is given in bytes.
         */
        if (dlen > BN_num_bytes(dsa->q))
                dlen = BN_num_bytes(dsa->q);
        if (BN_bin2bn(dgst, dlen, m) == NULL)
                goto err;

 redo:
        if (dsa->kinv == NULL || dsa->r == NULL) {
                if (!DSA_sign_setup(dsa, ctx, &kinv, &r))
                        goto err;
        } else {
                kinv = dsa->kinv;
                dsa->kinv = NULL;
                r = dsa->r;
                dsa->r = NULL;
                noredo = 1;
        }

        /*
         * Compute:
         *
         *  s = inv(k)(m + xr) mod q
         *
         * In order to reduce the possibility of a side-channel attack, the
         * following is calculated using a blinding value:
         *
         *  s = inv(b)(bm + bxr)inv(k) mod q
         *
         * Where b is a random value in the range [1, q).
         */
        if (!bn_rand_interval(b, 1, dsa->q))
                goto err;
        if (BN_mod_inverse_ct(binv, b, dsa->q, ctx) == NULL)
                goto err;

        if (!BN_mod_mul(bxr, b, dsa->priv_key, dsa->q, ctx))    /* bx */
                goto err;
        if (!BN_mod_mul(bxr, bxr, r, dsa->q, ctx))      /* bxr */
                goto err;
        if (!BN_mod_mul(bm, b, m, dsa->q, ctx))         /* bm */
                goto err;
        if (!BN_mod_add(s, bxr, bm, dsa->q, ctx))       /* s = bm + bxr */
                goto err;
        if (!BN_mod_mul(s, s, kinv, dsa->q, ctx))       /* s = b(m + xr)k^-1 */
                goto err;
        if (!BN_mod_mul(s, s, binv, dsa->q, ctx))       /* s = (m + xr)k^-1 */
                goto err;

        /*
         * Redo if r or s is zero as required by FIPS 186-3: this is very
         * unlikely.
         */
        if (BN_is_zero(r) || BN_is_zero(s)) {
                if (noredo) {
                        reason = DSA_R_NEED_NEW_SETUP_VALUES;
                        goto err;
                }
                if (++attempts > DSA_MAX_SIGN_ITERATIONS) {
                        reason = DSA_R_INVALID_PARAMETERS;
                        goto err;
                }
                goto redo;
        }

        if ((ret = DSA_SIG_new()) == NULL) {
                reason = ERR_R_MALLOC_FAILURE;
                goto err;
        }
        ret->r = r;
        ret->s = s;

 err:
        if (!ret) {
                DSAerror(reason);
                BN_free(r);
                BN_free(s);
        }
        BN_CTX_end(ctx);
        BN_CTX_free(ctx);
        BN_free(kinv);

        return ret;
}

static int
dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp, BIGNUM **rp)
{
        BIGNUM *k = NULL, *l = NULL, *m = NULL, *kinv = NULL, *r = NULL;
        BN_CTX *ctx = NULL;
        int q_bits;
        int ret = 0;

        if (!dsa_check_key(dsa))
                goto err;

        if ((r = BN_new()) == NULL)
                goto err;

        if ((ctx = ctx_in) == NULL)
                ctx = BN_CTX_new();
        if (ctx == NULL)
                goto err;

        BN_CTX_start(ctx);

        if ((k = BN_CTX_get(ctx)) == NULL)
                goto err;
        if ((l = BN_CTX_get(ctx)) == NULL)
                goto err;
        if ((m = BN_CTX_get(ctx)) == NULL)
                goto err;

        /* Preallocate space */
        q_bits = BN_num_bits(dsa->q);
        if (!BN_set_bit(k, q_bits) ||
            !BN_set_bit(l, q_bits) ||
            !BN_set_bit(m, q_bits))
                goto err;

        if (!bn_rand_interval(k, 1, dsa->q))
                goto err;

        BN_set_flags(k, BN_FLG_CONSTTIME);

        if (dsa->flags & DSA_FLAG_CACHE_MONT_P) {
                if (!BN_MONT_CTX_set_locked(&dsa->method_mont_p,
                    CRYPTO_LOCK_DSA, 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 exponent 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.
         *
         * TODO: revisit the bn_copy aiming for a memory access agnostic
         * conditional copy.
         */

        if (!BN_add(l, k, dsa->q) ||
            !BN_add(m, l, dsa->q) ||
            !bn_copy(k, BN_num_bits(l) > q_bits ? l : m))
                goto err;

        if (!BN_mod_exp_mont_ct(r, dsa->g, k, dsa->p, ctx, dsa->method_mont_p))
                goto err;

        if (!BN_mod_ct(r, r, dsa->q, ctx))
                goto err;

        /* Compute  part of 's = inv(k) (m + xr) mod q' */
        if ((kinv = BN_mod_inverse_ct(NULL, k, dsa->q, ctx)) == NULL)
                goto err;

        BN_free(*kinvp);
        *kinvp = kinv;
        kinv = NULL;

        BN_free(*rp);
        *rp = r;

        ret = 1;

 err:
        if (!ret) {
                DSAerror(ERR_R_BN_LIB);
                BN_free(r);
        }
        BN_CTX_end(ctx);
        if (ctx != ctx_in)
                BN_CTX_free(ctx);

        return ret;
}

static int
dsa_do_verify(const unsigned char *dgst, int dgst_len, DSA_SIG *sig, DSA *dsa)
{
        BIGNUM *u1 = NULL, *u2 = NULL, *t1 = NULL;
        BN_CTX *ctx = NULL;
        BN_MONT_CTX *mont = NULL;
        int qbits;
        int ret = -1;

        if (!dsa_check_key(dsa))
                goto err;

        if ((ctx = BN_CTX_new()) == NULL)
                goto err;

        BN_CTX_start(ctx);

        if ((u1 = BN_CTX_get(ctx)) == NULL)
                goto err;
        if ((u2 = BN_CTX_get(ctx)) == NULL)
                goto err;
        if ((t1 = BN_CTX_get(ctx)) == NULL)
                goto err;

        if (BN_is_zero(sig->r) || BN_is_negative(sig->r) ||
            BN_ucmp(sig->r, dsa->q) >= 0) {
                ret = 0;
                goto err;
        }
        if (BN_is_zero(sig->s) || BN_is_negative(sig->s) ||
            BN_ucmp(sig->s, dsa->q) >= 0) {
                ret = 0;
                goto err;
        }

        /* Calculate w = inv(s) mod q, saving w in u2. */
        if ((BN_mod_inverse_ct(u2, sig->s, dsa->q, ctx)) == NULL)
                goto err;

        /*
         * 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 186-4, 4.2.
         */
        qbits = BN_num_bits(dsa->q);
        if (dgst_len > (qbits >> 3))
                dgst_len = (qbits >> 3);

        /* Save m in u1. */
        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, sig->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,
                    CRYPTO_LOCK_DSA, dsa->p, ctx);
                if (!mont)
                        goto err;
        }

        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_ct(u1, t1, dsa->q, ctx))
                goto err;

        /* v is in u1 - if the signature is correct, it will be equal to r. */
        ret = BN_ucmp(u1, sig->r) == 0;

 err:
        if (ret < 0)
                DSAerror(ERR_R_BN_LIB);
        BN_CTX_end(ctx);
        BN_CTX_free(ctx);

        return ret;
}

static int
dsa_init(DSA *dsa)
{
        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;
}

static const DSA_METHOD openssl_dsa_meth = {
        .name = "OpenSSL DSA method",
        .dsa_do_sign = dsa_do_sign,
        .dsa_sign_setup = dsa_sign_setup,
        .dsa_do_verify = dsa_do_verify,
        .init = dsa_init,
        .finish = dsa_finish,
};

const DSA_METHOD *
DSA_OpenSSL(void)
{
        return &openssl_dsa_meth;
}
LCRYPTO_ALIAS(DSA_OpenSSL);

DSA_SIG *
DSA_SIG_new(void)
{
        return calloc(1, sizeof(DSA_SIG));
}
LCRYPTO_ALIAS(DSA_SIG_new);

void
DSA_SIG_free(DSA_SIG *sig)
{
        if (sig == NULL)
                return;

        BN_free(sig->r);
        BN_free(sig->s);
        free(sig);
}
LCRYPTO_ALIAS(DSA_SIG_free);

int
DSA_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp, BIGNUM **rp)
{
        return dsa->meth->dsa_sign_setup(dsa, ctx_in, kinvp, rp);
}
LCRYPTO_ALIAS(DSA_sign_setup);

DSA_SIG *
DSA_do_sign(const unsigned char *dgst, int dlen, DSA *dsa)
{
        return dsa->meth->dsa_do_sign(dgst, dlen, dsa);
}
LCRYPTO_ALIAS(DSA_do_sign);

int
DSA_do_verify(const unsigned char *dgst, int dgst_len, DSA_SIG *sig, DSA *dsa)
{
        return dsa->meth->dsa_do_verify(dgst, dgst_len, sig, dsa);
}
LCRYPTO_ALIAS(DSA_do_verify);