dh_check.c

/*
 * Copyright 1995-2016 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 <openssl/bn.h>
#include "dh_locl.h"

/*-
 * Check that p is a safe prime and
 * if g is 2, 3 or 5, check that it is a suitable generator
 * where
 * for 2, p mod 24 == 11
 * for 3, p mod 12 == 5
 * for 5, p mod 10 == 3 or 7
 * should hold.
 */

int DH_check(const DH *dh, int *ret)
{
    int ok = 0;
    BN_CTX *ctx = NULL;
    BN_ULONG l;
    BIGNUM *t1 = NULL, *t2 = NULL;

    *ret = 0;
    ctx = BN_CTX_new();
    if (ctx == NULL)
        goto err;
    BN_CTX_start(ctx);
    t1 = BN_CTX_get(ctx);
    if (t1 == NULL)
        goto err;
    t2 = BN_CTX_get(ctx);
    if (t2 == NULL)
        goto err;

    if (dh->q) {
        if (BN_cmp(dh->g, BN_value_one()) <= 0)
            *ret |= DH_NOT_SUITABLE_GENERATOR;
        else if (BN_cmp(dh->g, dh->p) >= 0)
            *ret |= DH_NOT_SUITABLE_GENERATOR;
        else {
            /* Check g^q == 1 mod p */
            if (!BN_mod_exp(t1, dh->g, dh->q, dh->p, ctx))
                goto err;
            if (!BN_is_one(t1))
                *ret |= DH_NOT_SUITABLE_GENERATOR;
        }
        if (!BN_is_prime_ex(dh->q, BN_prime_checks, ctx, NULL))
            *ret |= DH_CHECK_Q_NOT_PRIME;
        /* Check p == 1 mod q  i.e. q divides p - 1 */
        if (!BN_div(t1, t2, dh->p, dh->q, ctx))
            goto err;
        if (!BN_is_one(t2))
            *ret |= DH_CHECK_INVALID_Q_VALUE;
        if (dh->j && BN_cmp(dh->j, t1))
            *ret |= DH_CHECK_INVALID_J_VALUE;

    } else if (BN_is_word(dh->g, DH_GENERATOR_2)) {
        l = BN_mod_word(dh->p, 24);
        if (l != 11)
            *ret |= DH_NOT_SUITABLE_GENERATOR;
    } else if (BN_is_word(dh->g, DH_GENERATOR_5)) {
        l = BN_mod_word(dh->p, 10);
        if ((l != 3) && (l != 7))
            *ret |= DH_NOT_SUITABLE_GENERATOR;
    } else
        *ret |= DH_UNABLE_TO_CHECK_GENERATOR;

    if (!BN_is_prime_ex(dh->p, BN_prime_checks, ctx, NULL))
        *ret |= DH_CHECK_P_NOT_PRIME;
    else if (!dh->q) {
        if (!BN_rshift1(t1, dh->p))
            goto err;
        if (!BN_is_prime_ex(t1, BN_prime_checks, ctx, NULL))
            *ret |= DH_CHECK_P_NOT_SAFE_PRIME;
    }
    ok = 1;
 err:
    if (ctx != NULL) {
        BN_CTX_end(ctx);
        BN_CTX_free(ctx);
    }
    return (ok);
}

int DH_check_pub_key(const DH *dh, const BIGNUM *pub_key, int *ret)
{
    int ok = 0;
    BIGNUM *tmp = NULL;
    BN_CTX *ctx = NULL;

    *ret = 0;
    ctx = BN_CTX_new();
    if (ctx == NULL)
        goto err;
    BN_CTX_start(ctx);
    tmp = BN_CTX_get(ctx);
    if (tmp == NULL || !BN_set_word(tmp, 1))
        goto err;
    if (BN_cmp(pub_key, tmp) <= 0)
        *ret |= DH_CHECK_PUBKEY_TOO_SMALL;
    if (BN_copy(tmp, dh->p) == NULL || !BN_sub_word(tmp, 1))
        goto err;
    if (BN_cmp(pub_key, tmp) >= 0)
        *ret |= DH_CHECK_PUBKEY_TOO_LARGE;

    if (dh->q != NULL) {
        /* Check pub_key^q == 1 mod p */
        if (!BN_mod_exp(tmp, pub_key, dh->q, dh->p, ctx))
            goto err;
        if (!BN_is_one(tmp))
            *ret |= DH_CHECK_PUBKEY_INVALID;
    }

    ok = 1;
 err:
    if (ctx != NULL) {
        BN_CTX_end(ctx);
        BN_CTX_free(ctx);
    }
    return (ok);
}