/*

 * Copyright 1995-2025 The OpenSSL Project Authors. All Rights Reserved.

 *

 * Licensed under the Apache License 2.0 (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

 */

/*

 * DH low level APIs are deprecated for public use, but still ok for

 * internal use.

 */

#include "internal/deprecated.h"

#include <stdio.h>

#include "internal/cryptlib.h"

#include "dh_local.h"

#include "crypto/bn.h"

#include "crypto/dh.h"

#include "crypto/security_bits.h"

#ifdef FIPS_MODULE

#define MIN_STRENGTH 112

#else

#define MIN_STRENGTH 80

#endif

static int generate_key(DH *dh);

static int dh_bn_mod_exp(const DH *dh, BIGNUM *r,

    const BIGNUM *a, const BIGNUM *p,

    const BIGNUM *m, BN_CTX *ctx, BN_MONT_CTX *m_ctx);

static int dh_init(DH *dh);

static int dh_finish(DH *dh);

/*

 * See SP800-56Ar3 Section 5.7.1.1

 * Finite Field Cryptography Diffie-Hellman (FFC DH) Primitive

 */

int ossl_dh_compute_key(unsigned char *key, const BIGNUM *pub_key, DH *dh)

{

    BN_CTX *ctx = NULL;

    BN_MONT_CTX *mont = NULL;

    BIGNUM *z = NULL, *pminus1;

    int ret = -1;

    if (BN_num_bits(dh->params.p) > OPENSSL_DH_MAX_MODULUS_BITS) {

        ERR_raise(ERR_LIB_DH, DH_R_MODULUS_TOO_LARGE);

        goto err;

    }

    if (dh->params.q != NULL

        && BN_num_bits(dh->params.q) > OPENSSL_DH_MAX_MODULUS_BITS) {

        ERR_raise(ERR_LIB_DH, DH_R_Q_TOO_LARGE);

        goto err;

    }

    if (BN_num_bits(dh->params.p) < DH_MIN_MODULUS_BITS) {

        ERR_raise(ERR_LIB_DH, DH_R_MODULUS_TOO_SMALL);

        return 0;

    }

    ctx = BN_CTX_new_ex(dh->libctx);

    if (ctx == NULL)

        goto err;

    BN_CTX_start(ctx);

    pminus1 = BN_CTX_get(ctx);

    z = BN_CTX_get(ctx);

    if (z == NULL)

        goto err;

    if (dh->priv_key == NULL) {

        ERR_raise(ERR_LIB_DH, DH_R_NO_PRIVATE_VALUE);

        goto err;

    }

    if (dh->flags & DH_FLAG_CACHE_MONT_P) {

        mont = BN_MONT_CTX_set_locked(&dh->method_mont_p,

            dh->lock, dh->params.p, ctx);

        BN_set_flags(dh->priv_key, BN_FLG_CONSTTIME);

        if (!mont)

            goto err;

    }

    /* (Step 1) Z = pub_key^priv_key mod p */

    if (!dh->meth->bn_mod_exp(dh, z, pub_key, dh->priv_key, dh->params.p, ctx,

            mont)) {

        ERR_raise(ERR_LIB_DH, ERR_R_BN_LIB);

        goto err;

    }

    /* (Step 2) Error if z <= 1 or z = p - 1 */

    if (BN_copy(pminus1, dh->params.p) == NULL

        || !BN_sub_word(pminus1, 1)

        || BN_cmp(z, BN_value_one()) <= 0

        || BN_cmp(z, pminus1) == 0) {

        ERR_raise(ERR_LIB_DH, DH_R_INVALID_SECRET);

        goto err;

    }

    /* return the padded key, i.e. same number of bytes as the modulus */

    ret = BN_bn2binpad(z, key, BN_num_bytes(dh->params.p));

err:

    BN_clear(z); /* (Step 2) destroy intermediate values */

    BN_CTX_end(ctx);

    BN_CTX_free(ctx);

    return ret;

}

/*-

 * NB: This function is inherently not constant time due to the

 * RFC 5246 (8.1.2) padding style that strips leading zero bytes.

 */

int DH_compute_key(unsigned char *key, const BIGNUM *pub_key, DH *dh)

{

    int ret = 0, i;

    volatile size_t npad = 0, mask = 1;

    /* compute the key; ret is constant unless compute_key is external */

#ifdef FIPS_MODULE

    ret = ossl_dh_compute_key(key, pub_key, dh);

#else

    ret = dh->meth->compute_key(key, pub_key, dh);

#endif

    if (ret <= 0)

        return ret;

    /* count leading zero bytes, yet still touch all bytes */

    for (i = 0; i < ret; i++) {

        mask &= !key[i];

        npad += mask;

    }

    /* unpad key */

    ret -= npad;

    /* key-dependent memory access, potentially leaking npad / ret */

    memmove(key, key + npad, ret);

    /* key-dependent memory access, potentially leaking npad / ret */

    memset(key + ret, 0, npad);

    return ret;

}

int DH_compute_key_padded(unsigned char *key, const BIGNUM *pub_key, DH *dh)

{

    int rv, pad;

    /* rv is constant unless compute_key is external */

#ifdef FIPS_MODULE

    rv = ossl_dh_compute_key(key, pub_key, dh);

#else

    rv = dh->meth->compute_key(key, pub_key, dh);

#endif

    if (rv <= 0)

        return rv;

    pad = BN_num_bytes(dh->params.p) - rv;

    /* pad is constant (zero) unless compute_key is external */

    if (pad > 0) {

        memmove(key + pad, key, rv);

        memset(key, 0, pad);

    }

    return rv + pad;

}

static DH_METHOD dh_ossl = {

    "OpenSSL DH Method",

    generate_key,

    ossl_dh_compute_key,

    dh_bn_mod_exp,

    dh_init,

    dh_finish,

    DH_FLAG_FIPS_METHOD,

    NULL,

    NULL

};

static const DH_METHOD *default_DH_method = &dh_ossl;

const DH_METHOD *DH_OpenSSL(void)

{

    return &dh_ossl;

}

const DH_METHOD *DH_get_default_method(void)

{

    return default_DH_method;

}

static int dh_bn_mod_exp(const DH *dh, BIGNUM *r,

    const BIGNUM *a, const BIGNUM *p,

    const BIGNUM *m, BN_CTX *ctx, BN_MONT_CTX *m_ctx)

{

#ifdef S390X_MOD_EXP

    return s390x_mod_exp(r, a, p, m, ctx, m_ctx);

#else

    return BN_mod_exp_mont(r, a, p, m, ctx, m_ctx);

#endif

}

static int dh_init(DH *dh)

{

    dh->flags |= DH_FLAG_CACHE_MONT_P;

    dh->dirty_cnt++;

    return 1;

}

static int dh_finish(DH *dh)

{

    BN_MONT_CTX_free(dh->method_mont_p);

    return 1;

}

#ifndef FIPS_MODULE

void DH_set_default_method(const DH_METHOD *meth)

{

    default_DH_method = meth;

}

#endif /* FIPS_MODULE */

int DH_generate_key(DH *dh)

{

#ifdef FIPS_MODULE

    return generate_key(dh);

#else

    return dh->meth->generate_key(dh);

#endif

}

int ossl_dh_generate_public_key(BN_CTX *ctx, const DH *dh,

    const BIGNUM *priv_key, BIGNUM *pub_key)

{

    int ret = 0;

    BIGNUM *prk = BN_new();

    BN_MONT_CTX *mont = NULL;

    if (prk == NULL)

        return 0;

    if (dh->flags & DH_FLAG_CACHE_MONT_P) {

        /*

         * We take the input DH as const, but we lie, because in some cases we

         * want to get a hold of its Montgomery context.

         *

         * We cast to remove the const qualifier in this case, it should be

         * fine...

         */

        BN_MONT_CTX **pmont = (BN_MONT_CTX **)&dh->method_mont_p;

        mont = BN_MONT_CTX_set_locked(pmont, dh->lock, dh->params.p, ctx);

        if (mont == NULL)

            goto err;

    }

    BN_with_flags(prk, priv_key, BN_FLG_CONSTTIME);

    /* pub_key = g^priv_key mod p */

    if (!dh->meth->bn_mod_exp(dh, pub_key, dh->params.g, prk, dh->params.p,

            ctx, mont))

        goto err;

    ret = 1;

err:

    BN_clear_free(prk);

    return ret;

}

static int generate_key(DH *dh)

{

    int ok = 0;

    int generate_new_key = 0;

#ifndef FIPS_MODULE

    int l;

#endif

    BN_CTX *ctx = NULL;

    BIGNUM *pub_key = NULL, *priv_key = NULL;

    if (BN_num_bits(dh->params.p) > OPENSSL_DH_MAX_MODULUS_BITS) {

        ERR_raise(ERR_LIB_DH, DH_R_MODULUS_TOO_LARGE);

        return 0;

    }

    if (dh->params.q != NULL

        && BN_num_bits(dh->params.q) > OPENSSL_DH_MAX_MODULUS_BITS) {

        ERR_raise(ERR_LIB_DH, DH_R_Q_TOO_LARGE);

        return 0;

    }

    if (BN_num_bits(dh->params.p) < DH_MIN_MODULUS_BITS) {

        ERR_raise(ERR_LIB_DH, DH_R_MODULUS_TOO_SMALL);

        return 0;

    }

    ctx = BN_CTX_new_ex(dh->libctx);

    if (ctx == NULL)

        goto err;

    if (dh->priv_key == NULL) {

        priv_key = BN_secure_new();

        if (priv_key == NULL)

            goto err;

        generate_new_key = 1;

    } else {

        priv_key = dh->priv_key;

    }

    if (dh->pub_key == NULL) {

        pub_key = BN_new();

        if (pub_key == NULL)

            goto err;

    } else {

        pub_key = dh->pub_key;

    }

    if (generate_new_key) {

        /* Is it an approved safe prime ?*/

        if (DH_get_nid(dh) != NID_undef) {

            int max_strength = ossl_ifc_ffc_compute_security_bits(BN_num_bits(dh->params.p));

            if (dh->params.q == NULL

                || dh->length > BN_num_bits(dh->params.q))

                goto err;

            /* dh->length = maximum bit length of generated private key */

            if (!ossl_ffc_generate_private_key(ctx, &dh->params, dh->length,

                    max_strength, priv_key))

                goto err;

        } else {

#ifdef FIPS_MODULE

            if (dh->params.q == NULL)

                goto err;

#else

            if (dh->params.q == NULL) {

                /* secret exponent length, must satisfy 2^l < (p-1)/2 */

                l = BN_num_bits(dh->params.p);

                if (dh->length >= l)

                    goto err;

                l -= 2;

                if (dh->length != 0 && dh->length < l)

                    l = dh->length;

                if (!BN_priv_rand_ex(priv_key, l, BN_RAND_TOP_ONE,

                        BN_RAND_BOTTOM_ANY, 0, ctx))

                    goto err;

                /*

                 * We handle just one known case where g is a quadratic non-residue:

                 * for g = 2: p % 8 == 3

                 */

                if (BN_is_word(dh->params.g, DH_GENERATOR_2)

                    && !BN_is_bit_set(dh->params.p, 2)) {

                    /* clear bit 0, since it won't be a secret anyway */

                    if (!BN_clear_bit(priv_key, 0))

                        goto err;

                }

            } else

#endif

            {

                /* Do a partial check for invalid p, q, g */

                if (!ossl_ffc_params_simple_validate(dh->libctx, &dh->params,

                        FFC_PARAM_TYPE_DH, NULL))

                    goto err;

                /*

                 * For FFC FIPS 186-4 keygen

                 * security strength s = 112,

                 * Max Private key size N = len(q)

                 */

                if (!ossl_ffc_generate_private_key(ctx, &dh->params,

                        BN_num_bits(dh->params.q),

                        MIN_STRENGTH,

                        priv_key))

                    goto err;

            }

        }

    }

    if (!ossl_dh_generate_public_key(ctx, dh, priv_key, pub_key))

        goto err;

    dh->pub_key = pub_key;

    dh->priv_key = priv_key;

    dh->dirty_cnt++;

    ok = 1;

err:

    if (ok != 1)

        ERR_raise(ERR_LIB_DH, ERR_R_BN_LIB);

    if (pub_key != dh->pub_key)

        BN_free(pub_key);

    if (priv_key != dh->priv_key)

        BN_free(priv_key);

    BN_CTX_free(ctx);

    return ok;

}

int ossl_dh_buf2key(DH *dh, const unsigned char *buf, size_t len)

{

    int err_reason = DH_R_BN_ERROR;

    BIGNUM *pubkey = NULL;

    const BIGNUM *p;

    int ret;

    if ((pubkey = BN_bin2bn(buf, len, NULL)) == NULL)

        goto err;

    DH_get0_pqg(dh, &p, NULL, NULL);

    if (p == NULL || BN_num_bytes(p) == 0) {

        err_reason = DH_R_NO_PARAMETERS_SET;

        goto err;

    }

    /* Prevent small subgroup attacks per RFC 8446 Section 4.2.8.1 */

    if (!ossl_dh_check_pub_key_partial(dh, pubkey, &ret)) {

        err_reason = DH_R_INVALID_PUBKEY;

        goto err;

    }

    if (DH_set0_key(dh, pubkey, NULL) != 1)

        goto err;

    return 1;

err:

    ERR_raise(ERR_LIB_DH, err_reason);

    BN_free(pubkey);

    return 0;

}

size_t ossl_dh_key2buf(const DH *dh, unsigned char **pbuf_out, size_t size,

    int alloc)

{

    const BIGNUM *pubkey;

    unsigned char *pbuf = NULL;

    const BIGNUM *p;

    int p_size;

    DH_get0_pqg(dh, &p, NULL, NULL);

    DH_get0_key(dh, &pubkey, NULL);

    if (p == NULL || pubkey == NULL

        || (p_size = BN_num_bytes(p)) == 0

        || BN_num_bytes(pubkey) == 0) {

        ERR_raise(ERR_LIB_DH, DH_R_INVALID_PUBKEY);

        return 0;

    }

    if (pbuf_out != NULL && (alloc || *pbuf_out != NULL)) {

        if (!alloc) {

            if (size >= (size_t)p_size)

                pbuf = *pbuf_out;

            if (pbuf == NULL)

                ERR_raise(ERR_LIB_DH, DH_R_INVALID_SIZE);

        } else {

            pbuf = OPENSSL_malloc(p_size);

        }

        /* Errors raised above */

        if (pbuf == NULL)

            return 0;

        /*

         * As per Section 4.2.8.1 of RFC 8446 left pad public

         * key with zeros to the size of p

         */

        if (BN_bn2binpad(pubkey, pbuf, p_size) < 0) {

            if (alloc)

                OPENSSL_free(pbuf);

            ERR_raise(ERR_LIB_DH, DH_R_BN_ERROR);

            return 0;

        }

        *pbuf_out = pbuf;

    }

    return p_size;

}