openssl实现了标准的x509v3数字证书，其源码在crypto/x509和crypto/x509v3中。其中x509目录实现了数字证书以及证书申请相关的各种函数，包括了X509和X509_REQ结构的设置、读取、打印和比较；数字证书的验证、摘要；各种公钥的导入导出等功能。x509v3目录主要实现了数字证书扩展项相关的函数。

在进行身份认证时，首先要对发送给服务器进行认证的x509证书有效性进行验证，在Openssl中，可以用一个API接口可以实现：int X509_verify_cert(X509_STORE_CTX *ctx);
接口中形参是X509_STORE_CTX（X509证书库上下文）类型，在X509证书库上下文中，存在一个X509证书库和一个待验证的X509证书，可以加入信任的证书链，也可以加入CRL证书链（证书撤销列表）~
对X509证书有效性进行验证可以由以下几个函数来完成~

X509_STORE_CTX *ctx; //证书上下文  
X509_STORE *cert_store; //证书库，存在证书链  
X509* x509; //待验证X509证书  
ctx = X509_STORE_CTX_new();  

X509_STORE_CTX_init(ctx,cert_store,x509,NULL);  

X509_verify_cert(ctx);//根据返回值可以确认X509证书是否有效，也可以根据X509_STORE_CTX_get_error和X509_verify_cert_error_string函数来确认无效原因   

示例程序如下：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#include <openssl/evp.h>
#include <openssl/x509.h>

#define CERT_PATH       "/home/dengaj/Desktop/openssl"
#define ROOT_CERT       "RootCert.crt"
#define CARD_CERT       "CardCert.crt"
#define GET_DEFAULT_CA_CERT(str) sprintf(str, "%s/%s", CERT_PATH, ROOT_CERT)
#define GET_CUSTOM_CERT(str, path, name) sprintf(str, "%s/%s", path, name)

#define MAX_LEGTH 4096


int my_load_cert(unsigned char *str, unsigned long *str_len,
              const char *verify_cert, const unsigned int cert_len)
{
    FILE *fp;
    fp = fopen(verify_cert, "rb");
    if ( NULL == fp)
    {
        fprintf(stderr, "fopen fail\n");
        return -1;
    }

    *str_len = fread(str, 1, cert_len, fp);
    fclose(fp);
    return 0;
}

X509 *der_to_x509(const unsigned char *der_str, unsigned int der_str_len)
{
    X509 *x509;
    x509 = d2i_X509(NULL, &der_str, der_str_len);
    if ( NULL == x509 )
    {
        fprintf(stderr, "d2i_X509 fail\n");

        return NULL;
    }
    return x509;
}
int x509_verify()
{
    int ret;
    char cert[MAX_LEGTH];

    unsigned char user_der[MAX_LEGTH];
    unsigned long user_der_len;
    X509 *user = NULL;

    unsigned char ca_der[MAX_LEGTH];
    unsigned long ca_der_len;
    X509 *ca = NULL;

    X509_STORE *ca_store = NULL;
    X509_STORE_CTX *ctx = NULL;
    STACK_OF(X509) *ca_stack = NULL;

    /* x509初始化 */
    ca_store = X509_STORE_new();
    ctx = X509_STORE_CTX_new();

    /* root ca*/
    GET_DEFAULT_CA_CERT(cert);
    /* 从文件中读取 */
    my_load_cert(ca_der, &ca_der_len, cert, MAX_LEGTH);
    /* DER编码转X509结构 */
    ca = der_to_x509(ca_der, ca_der_len);
    /* 加入证书存储区 */
    ret = X509_STORE_add_cert(ca_store, ca);
    if ( ret != 1 )
    {
        fprintf(stderr, "X509_STORE_add_cert fail, ret = %d\n", ret);
        goto EXIT;
    }

    /* 需要校验的证书 */
    GET_CUSTOM_CERT(cert, CERT_PATH, CARD_CERT);
    my_load_cert(user_der, &user_der_len, cert, MAX_LEGTH);
    user = der_to_x509(user_der, user_der_len);

    ret = X509_STORE_CTX_init(ctx, ca_store, user, ca_stack);
    if ( ret != 1 )
    {
        fprintf(stderr, "X509_STORE_CTX_init fail, ret = %d\n", ret);
        goto EXIT;
    }

    //openssl-1.0.1c/crypto/x509/x509_vfy.h
    ret = X509_verify_cert(ctx);
    if ( ret != 1 )
    {
        fprintf(stderr, "X509_verify_cert fail, ret = %d, error id = %d, %s\n",
                ret, ctx->error, X509_verify_cert_error_string(ctx->error));
        goto EXIT;
    }
    fprintf(stdout, "X509_verify_cert successful\n");
EXIT:
    X509_free(user);
    X509_free(ca);

    X509_STORE_CTX_cleanup(ctx);
    X509_STORE_CTX_free(ctx);

    X509_STORE_free(ca_store);

    return ret == 1 ? 0 : -1;
}

int main()
{
    OpenSSL_add_all_algorithms();
    x509_verify();
    return 0;
}

第二种方法使用

int X509_verify(X509 * x509, EVP_PKEY * pkey);

  X509 * root;
   X509 * mycert;

//Get root certificate into root
//Get mycert into mycert.

//Get the public key.
EVP_PKEY * pubkey = X509_get_pubkey(root);

//verify. result less than or 0 means not verified or some error.
int result = X509_verify(mycert, pubkey);

//free the public key.
EVP_PKEY_free(pubkey);