RSA公钥可以从证书和公钥文件，RSA私钥可以从私钥文件中提取。OpenSSL使用了一种BIO抽象IO机制读写所用文件，可以打开文件相关联的BIO，通过BIO读写文件内容。
代码示例如下：
// 证书、私钥、公钥都是PEM格式文件
// 编译命令：gcc -o test test_pubkey_pem.c -lcrypto -std=c99
#include <openssl/x509.h>
#include <openssl/pem.h>
#include<stdio.h>
#include<string.h>
// 文件路径
char cert_filename[] = "/root/test.crt";
char pubkey_filename[] = "/root/testpub.pem";
char prikey_filename[] = "/root/testpri.pem";
int main()
{
EVP_PKEY *pkey;
BIO *pubkey_bio;
BIO *prikey_bio;
BIO *cert;
/************ 从证书中提取公钥 ****************/
// 打开证书文件
cert = BIO_new_file(cert_filename, "r");
// 读入X509证书
X509 * x_cert = PEM_read_bio_X509(cert, NULL, NULL, NULL);
BIO_free(cert);
// 提取出密钥EVP_PKEY结构
pkey = X509_get_pubkey(x_cert);
// 提取出RSA结构的公钥
RSA* rsa_from_cert = EVP_PKEY_get1_RSA(pkey);
X509_free(x_cert);
EVP_PKEY_free(pkey);
// 打印公钥的值
BIO * print_out=BIO_new(BIO_s_file());
BIO_set_fp(print_out,stdout,BIO_NOCLOSE);
RSA_print(print_out, rsa_from_cert, 0);
int ret;
/************ 从公钥文件中提取公钥 ****************/
//导入公钥文件
char rsa_in[] = "testing";
int rsa_inlen = strlen(rsa_in);
pubkey_bio=BIO_new_file(pubkey_filename,"r");
// 提取出密钥EVP_PKEY结构
pkey=PEM_read_bio_PUBKEY(pubkey_bio, NULL, NULL, NULL);
// 提取出公钥RSA结构
RSA* rsa = EVP_PKEY_get1_RSA(pkey);
RSA_print(print_out, rsa, 0);
/************ 从公钥文件中提取公钥 ****************/
prikey_bio=BIO_new_file(prikey_filename,"r");
pkey=PEM_read_bio_PrivateKey(prikey_bio, NULL, NULL, NULL);
RSA *pri_rsa = EVP_PKEY_get1_RSA(pkey);
/*********** 预分配控件 ******************/
// 根据RSA公钥长度分配RSA加密输出空间
int keysize = RSA_size(rsa);
printf("keysize:%d\n", keysize);
unsigned char *rsa_out = OPENSSL_malloc(keysize);
unsigned char *rsa_out_cert = OPENSSL_malloc(keysize);
unsigned char *dec_out = OPENSSL_malloc(keysize);
unsigned char *sign_out = OPENSSL_malloc(keysize);
unsigned char *verify_out = OPENSSL_malloc(keysize);
// 使用RKCS#1填充标准
int pad = RSA_PKCS1_PADDING;
/*********** RSA公钥加密 ******************/
int rsa_outlen_cert = RSA_public_encrypt(rsa_inlen, rsa_in, rsa_out_cert, rsa_from_cert, pad);
int rsa_outlen = RSA_public_encrypt(rsa_inlen, rsa_in, rsa_out, rsa, pad);
printf("rsa_outlen is: %d\n", rsa_outlen);
for(int i=0; i<rsa_outlen; i++){
printf("%02x", rsa_out[i]);
if((i+1)%16 == 0)
printf("\n");
}
printf("\n");
printf("rsa_outlen_cert is: %d\n", rsa_outlen_cert);
for(int i=0; i<rsa_outlen_cert; i++){
printf("%02x", rsa_out_cert[i]);
if((i+1)%16 == 0)
printf("\n");
}
printf("\n");
/*********** RSA私钥解密 ******************/
int dec_len = RSA_private_decrypt(rsa_outlen_cert, rsa_out_cert, dec_out, pri_rsa, pad);
if(!memcmp(rsa_in, dec_out, dec_len)){
printf("decrypt success!\n");
}else{
printf("decrypt fail!\n");
}
printf("dec_len is %d\n", dec_len);
/*********** RSA私钥签名 *******************/
int sign_len = RSA_private_encrypt(rsa_inlen, rsa_in, sign_out, pri_rsa, pad);
printf("sign_len is: %d\n", sign_len);
for(int i=0; i<sign_len; i++){
printf("%02x", sign_out[i]);
if((i+1)%16 == 0)
printf("\n");
}
printf("\n");
/********** RSA公钥验签 *******************/
int verify_len = RSA_public_decrypt(sign_len, sign_out, verify_out, rsa, pad);
printf("verify_len is %d\n", verify_len);
if(!memcmp(rsa_in, verify_out, verify_len)){
printf("verify success!\n");
}else{
printf("verify fail!\n");
}
/***********释放变量清理空间 ****************/
OPENSSL_free(rsa_out);
OPENSSL_free(dec_out);
OPENSSL_free(rsa_out_cert);
OPENSSL_free(sign_out);
OPENSSL_free(verify_out);
BIO_free(pubkey_bio);
BIO_free(prikey_bio);
EVP_PKEY_free(pkey);
RSA_free(rsa);
RSA_free(pri_rsa);
RSA_free(rsa_from_cert);
return 0;
}
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