openssl實現了標準的x509v3數字證書，其源碼在crypto/x509和crypto/x509v3中。其中x509目錄實現了數字證書以及證書申請相關的各種函數，包括了X509和X509_REQ結構的設置、讀取、打印和比較；數字證書的驗證、摘要；各種公鑰的導入導出等功能。x509v3目錄主要實現了數字證書擴展項相關的函數。

在進行身份認證時，首先要對發送給服務器進行認證的x509證書有效性進行驗證，在Openssl中，可以用一個API接口可以實現：int X509_verify_cert(X509_STORE_CTX *ctx);
接口中形參是X509_STORE_CTX（X509證書庫上下文）類型，在X509證書庫上下文中，存在一個X509證書庫和一個待驗證的X509證書，可以加入信任的證書鏈，也可以加入CRL證書鏈（證書撤銷列表）~
對X509證書有效性進行驗證可以由以下幾個函數來完成~

X509_STORE_CTX *ctx; //證書上下文  
X509_STORE *cert_store; //證書庫，存在證書鏈  
X509* x509; //待驗證X509證書  
ctx = X509_STORE_CTX_new();  X509_STORE_CTX_init(ctx,cert_store,x509,NULL);  X509_verify_cert(ctx);//根據返回值可以確認X509證書是否有效，也可以根據X509_STORE_CTX_get_error和X509_verify_cert_error_string函數來確認無效原因   

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 4096int 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);/* 從文件中讀取 */my_load_cert(ca_der, &ca_der_len, cert, MAX_LEGTH);/* DER編碼轉X509結構 */ca = der_to_x509(ca_der, ca_der_len);/* 加入證書存儲區 */ret = X509_STORE_add_cert(ca_store, ca);if ( ret != 1 ){fprintf(stderr, "X509_STORE_add_cert fail, ret = %d\n", ret);goto EXIT;}/* 需要校驗的證書 */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.hret = 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;
}

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第二種方法使用

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);

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