/*

 * Copyright 2019-2021 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

 */

#include "internal/ffc.h"

/*

 * SP800-56Ar3 5.6.1.1.4 Key pair generation by testing candidates.

 * Generates a private key in the interval [1, min(2 ^ N - 1, q - 1)].

 *

 * ctx must be set up with a libctx (for fips mode).

 * params contains the FFC domain parameters p, q and g (for DH or DSA).

 * N is the maximum bit length of the generated private key,

 * s is the security strength.

 * priv_key is the returned private key,

 */

int ossl_ffc_generate_private_key(BN_CTX *ctx, const FFC_PARAMS *params,

                                  int N, int s, BIGNUM *priv)

{

    int ret = 0, qbits = BN_num_bits(params->q);

    BIGNUM *m, *two_powN = NULL;

    /* Deal with the edge cases where the value of N and/or s is not set */

    if (s == 0)

        goto err;

    if (N == 0)

        N = params->keylength ? params->keylength : 2 * s;

    /* Step (2) : check range of N */

    if (N < 2 * s || N > qbits)

        return 0;

    two_powN = BN_new();

    /* 2^N */

    if (two_powN == NULL || !BN_lshift(two_powN, BN_value_one(), N))

        goto err;

    /* Step (5) : M = min(2 ^ N, q) */

    m = (BN_cmp(two_powN, params->q) > 0) ? params->q : two_powN;

    do {

        /* Steps (3, 4 & 7) :  c + 1 = 1 + random[0..2^N - 1] */

        if (!BN_priv_rand_range_ex(priv, two_powN, 0, ctx)

            || !BN_add_word(priv, 1))

            goto err;

        /* Step (6) : loop if c > M - 2 (i.e. c + 1 >= M) */

        if (BN_cmp(priv, m) < 0)

            break;

    } while (1);

    ret = 1;

err:

    BN_free(two_powN);

    return ret;

}