openssl开源程序dh算法解析之dh_gen.c

/*----------------实现了生成 DH 密钥参数------------*/
//DH_generate_parameters_ex

#include <stdio.h>
#include "cryptlib.h"
#include <openssl/bn.h>
#include <openssl/dh.h>

#ifdef OPENSSL_FIPS
# include <openssl/fips.h>
#endif

static int dh_builtin_genparams(DH *ret, int prime_len, int generator,
                                BN_GENCB *cb);

int DH_generate_parameters_ex(DH *ret, int prime_len, int generator,  //产生dh参数，保存到dh结构体
                              BN_GENCB *cb)
{
#ifdef OPENSSL_FIPS  //FIPS 联邦信息处理标准是一套描述文件处理、加密算法和其他信息技术标准的标准。

    if (FIPS_mode() && !(ret->meth->flags & DH_FLAG_FIPS_METHOD)
        && !(ret->flags & DH_FLAG_NON_FIPS_ALLOW)) {
        DHerr(DH_F_DH_GENERATE_PARAMETERS_EX, DH_R_NON_FIPS_METHOD);
        return 0;
    }
#endif
    if (ret->meth->generate_params)       //generate_params是生成DH密钥参数的回调函数
        return ret->meth->generate_params(ret, prime_len, generator, cb);//产生dh参数
#ifdef OPENSSL_FIPS
    if (FIPS_mode())
        return FIPS_dh_generate_parameters_ex(ret, prime_len, generator, cb);
#endif
    return dh_builtin_genparams(ret, prime_len, generator, cb);
}

/*-
 * We generate DH parameters as follows
 * find a prime q which is prime_len/2 bits long.
 * p=(2*q)+1 or (p-1)/2 = q
 * For this case, g is a generator if
 * g^((p-1)/q) mod p != 1 for values of q which are the factors of p-1.
 * Since the factors of p-1 are q and 2, we just need to check
 * g^2 mod p != 1 and g^q mod p != 1.
 *
 * Having said all that,
 * there is another special case method for the generators 2, 3 and 5.
 * for 2, p mod 24 == 11
 * for 3, p mod 12 == 5  <<<<< does not work for safe primes.
 * for 5, p mod 10 == 3 or 7
 *
 * Thanks to Phil Karn <karn@qualcomm.com> for the pointers about the
 * special generators and for answering some of my questions.
 *
 * I‘ve implemented the second simple method :-).
 * Since DH should be using a safe prime (both p and q are prime),
 * this generator function can take a very very long time to run.
 */
/*
 * Actually there is no reason to insist that ‘generator‘ be a generator.
 * It‘s just as OK (and in some sense better) to use a generator of the
 * order-q subgroup.
 */
static int dh_builtin_genparams(DH *ret, int prime_len, int generator,
                                BN_GENCB *cb)
{
    BIGNUM *t1, *t2;
    int g, ok = -1;
    BN_CTX *ctx = NULL;

    ctx = BN_CTX_new();         //申请一个新的上下文结构
    if (ctx == NULL)
        goto err;
    BN_CTX_start(ctx);
    t1 = BN_CTX_get(ctx);  //新建一个BIGNUM结构
    t2 = BN_CTX_get(ctx);
    if (t1 == NULL || t2 == NULL)
        goto err;

    /* Make sure ‘ret‘ has the necessary elements */
    if (!ret->p && ((ret->p = BN_new()) == NULL))//新生成一个BIGNUM结构 
        goto err;
    if (!ret->g && ((ret->g = BN_new()) == NULL))
        goto err;

    if (generator <= 1) {  //g>1
        DHerr(DH_F_DH_BUILTIN_GENPARAMS, DH_R_BAD_GENERATOR);
        goto err;
    }
    if (generator == DH_GENERATOR_2) {
        //将BIGNUM结构的值置为unsigned long int类型整数的值
        if (!BN_set_word(t1, 24))
            goto err;
        if (!BN_set_word(t2, 11))
            goto err;
        g = 2;
    }
#if 0                           /* does not work for safe primes */
    else if (generator == DH_GENERATOR_3) {
        if (!BN_set_word(t1, 12))
            goto err;
        if (!BN_set_word(t2, 5))
            goto err;
        g = 3;
    }
#endif
    else if (generator == DH_GENERATOR_5) {  //g=DH_GENERATOR_5
        if (!BN_set_word(t1, 10))
            goto err;
        if (!BN_set_word(t2, 3))
            goto err;
        /*
         * BN_set_word(t3,7); just have to miss out on these ones 
         */
        g = 5;
    } else {
        /*
         * in the general case, don‘t worry if ‘generator‘ is a generator or
         * not: since we are using safe primes, it will generate either an
         * order-q or an order-2q group, which both is OK
         */
        if (!BN_set_word(t1, 2))
            goto err;
        if (!BN_set_word(t2, 1))
            goto err;
        g = generator;
    }

    if (!BN_generate_prime_ex(ret->p, prime_len, 1, t1, t2, cb))
        goto err;
    if (!BN_GENCB_call(cb, 3, 0))
        goto err;
    if (!BN_set_word(ret->g, g))  //设置值
        goto err;
    ok = 1;
 err:
    if (ok == -1) {
        DHerr(DH_F_DH_BUILTIN_GENPARAMS, ERR_R_BN_LIB);
        ok = 0;
    }

    if (ctx != NULL) {
        BN_CTX_end(ctx);
        BN_CTX_free(ctx);  //释放结构
    }
    return ok;
}