openssl开源程序dh算法解析之dh_key.c

/*实现openssl 提供的默认的DH_METHOD，实现了根据密钥参数生成DH公私
钥，以及根据DH 公钥(一方)以及DH 私钥(另一方)来生成一个共享密钥，用于密
钥交换*/

#include <stdio.h>
#include "cryptlib.h"
#include <openssl/bn.h>
#include <openssl/rand.h>  //实现了伪随机数生成，支持用户自定义随机数生成
#include <openssl/dh.h>

static int generate_key(DH *dh);
static int compute_key(unsigned char *key, const BIGNUM *pub_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); //r=a^p % m
static int dh_init(DH *dh);
static int dh_finish(DH *dh);

int DH_generate_key(DH *dh)      //生成公私钥
{
#ifdef OPENSSL_FIPS
    if (FIPS_mode() && !(dh->meth->flags & DH_FLAG_FIPS_METHOD)
        && !(dh->flags & DH_FLAG_NON_FIPS_ALLOW)) {
        DHerr(DH_F_DH_GENERATE_KEY, DH_R_NON_FIPS_METHOD);
        return 0;
    }
#endif
    return dh->meth->generate_key(dh); //生成公私钥，存放于dh结构体的公私钥属性中
}
//根据对方公钥和己方DH 密钥来生成共享密钥的函数
int DH_compute_key(unsigned char *key, const BIGNUM *pub_key, DH *dh)
{
#ifdef OPENSSL_FIPS
    if (FIPS_mode() && !(dh->meth->flags & DH_FLAG_FIPS_METHOD)
        && !(dh->flags & DH_FLAG_NON_FIPS_ALLOW)) {
        DHerr(DH_F_DH_COMPUTE_KEY, DH_R_NON_FIPS_METHOD);
        return 0;
    }
#endif
    return dh->meth->compute_key(key, pub_key, dh);   //结果保存在key中
}

int DH_compute_key_padded(unsigned char *key, const BIGNUM *pub_key, DH *dh)
{
    int rv, pad;
    rv = dh->meth->compute_key(key, pub_key, dh);
    if (rv <= 0)
        return rv;
    pad = BN_num_bytes(dh->p) - rv;  //返回dh->p的字节数
    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,
    compute_key,
    dh_bn_mod_exp,
    dh_init,
    dh_finish,
    0,
    NULL,
    NULL
};

const DH_METHOD *DH_OpenSSL(void)
{
    return &dh_ossl;
}

static int generate_key(DH *dh)   //被DH_generate_key调用，这里具体实现
{
    int ok = 0;
    int generate_new_key = 0;
    unsigned l;
    BN_CTX *ctx;  //新建上下文结构
    BN_MONT_CTX *mont = NULL;
    BIGNUM *pub_key = NULL, *priv_key = NULL;

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

    if (dh->priv_key == NULL) {
        priv_key = BN_new();  //获取私钥
        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();  //获取公钥，暂时的
        if (pub_key == NULL)
            goto err;
    } else
        pub_key = dh->pub_key;

    if (dh->flags & DH_FLAG_CACHE_MONT_P) {
        mont = BN_MONT_CTX_set_locked(&dh->method_mont_p,
                                      CRYPTO_LOCK_DH, dh->p, ctx);
        if (!mont)
            goto err;
    }

    if (generate_new_key) {
        if (dh->q) {
            do {
                if (!BN_rand_range(priv_key, dh->q))//确保priv_key<dh->q
                    goto err;
            }
            while (BN_is_zero(priv_key) || BN_is_one(priv_key));
        } else {
            /* secret exponent length */
            l = dh->length ? dh->length : BN_num_bits(dh->p) - 1;
            if (!BN_rand(priv_key, l, 0, 0))
                goto err;
        }
    }

    {
        BIGNUM local_prk;
        BIGNUM *prk;

        if ((dh->flags & DH_FLAG_NO_EXP_CONSTTIME) == 0) {
            BN_init(&local_prk);
            prk = &local_prk;
            BN_with_flags(prk, priv_key, BN_FLG_CONSTTIME);
        } else
            prk = priv_key;
        //真正产生公钥
        if (!dh->meth->bn_mod_exp(dh, pub_key, dh->g, prk, dh->p, ctx, mont))  
            goto err;
    }

    dh->pub_key = pub_key;   //将公私钥赋值到dh结构
    dh->priv_key = priv_key;
    ok = 1;
 err:
    if (ok != 1)
        DHerr(DH_F_GENERATE_KEY, ERR_R_BN_LIB);

    if ((pub_key != NULL) && (dh->pub_key == NULL))
        BN_free(pub_key);
    if ((priv_key != NULL) && (dh->priv_key == NULL))
        BN_free(priv_key);
    BN_CTX_free(ctx);
    return (ok);
}
//被DH_compute_key调用，这里具体实现
static int compute_key(unsigned char *key, const BIGNUM *pub_key, DH *dh)  
{
    BN_CTX *ctx = NULL;
    BN_MONT_CTX *mont = NULL;
    BIGNUM *tmp;
    int ret = -1;
    int check_result;

    if (BN_num_bits(dh->p) > OPENSSL_DH_MAX_MODULUS_BITS) {
        DHerr(DH_F_COMPUTE_KEY, DH_R_MODULUS_TOO_LARGE);
        goto err;
    }

    ctx = BN_CTX_new();  //新建上下文结构
    if (ctx == NULL)
        goto err;
    BN_CTX_start(ctx);
    tmp = BN_CTX_get(ctx);

    if (dh->priv_key == NULL) {
        DHerr(DH_F_COMPUTE_KEY, 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,
                                      CRYPTO_LOCK_DH, dh->p, ctx);
        if ((dh->flags & DH_FLAG_NO_EXP_CONSTTIME) == 0) {
            /* XXX */
            BN_set_flags(dh->priv_key, BN_FLG_CONSTTIME);
        }
        if (!mont)
            goto err;
    }

    if (!DH_check_pub_key(dh, pub_key, &check_result) || check_result) {//检查公钥合理性
        DHerr(DH_F_COMPUTE_KEY, DH_R_INVALID_PUBKEY);
        goto err;
    }

    if (!dh->
        //tep=pub_key ^ dh->priv_key % dh->p，tmp就是key
        meth->bn_mod_exp(dh, tmp, pub_key, dh->priv_key, dh->p, ctx, mont)) { 
        DHerr(DH_F_COMPUTE_KEY, ERR_R_BN_LIB);
        goto err;
    }

    ret = BN_bn2bin(tmp, key); //转换为字节存储方式：大端法，存入key中
 err:
    if (ctx != NULL) {
        BN_CTX_end(ctx);
        BN_CTX_free(ctx);
    }
    return (ret);
}

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)
{
    /*
     * If a is only one word long and constant time is false, use the faster
     * exponenentiation function.
     */
    if (a->top == 1 && ((dh->flags & DH_FLAG_NO_EXP_CONSTTIME) != 0)) {
        BN_ULONG A = a->d[0];
        return BN_mod_exp_mont_word(r, A, p, m, ctx, m_ctx);
    } else
        return BN_mod_exp_mont(r, a, p, m, ctx, m_ctx);
}

static int dh_init(DH *dh) //初始化函数
{
    dh->flags |= DH_FLAG_CACHE_MONT_P;
    return (1);
}

static int dh_finish(DH *dh)  //结束函数
{
    if (dh->method_mont_p)
        BN_MONT_CTX_free(dh->method_mont_p);
    return (1);
}