x509证书验证示例

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;
}