Use openssl to decrypt rsa private key in c language

The maximum length of single-time encrypted data (block_len) is related to the RSA key module length RSA_size() and the padding mode.

  填充模式：RSA_PKCS1_PADDING， block_len=RSA_size() - 11
  填充模式：RSA_PKCS1_OAEP_PADDING，block_len=RSA_size() - 41
  填充模式：RSA_NO_PADDING（不填充），block_len=RSA_size()

This example is padded with RSA_PKCS1_PADDING, so 117 bytes are encrypted at a time and 128 bytes are decrypted at a time. Exceeding characters need to be decrypted in a loop.

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<openssl/rsa.h>
#include<openssl/pem.h>
#include<openssl/err.h>
#include<openssl/bio.h>
#include<openssl/evp.h>

char* prikey="-----BEGIN RSA PRIVATE KEY-----\n\
MIICdwIBADANBgkqhkiG9w0BAQEFAASCAmEwggJdAgEAAoGB\
ALr1s+g6od9C0Jj/a+oEhSUGLOoM8QwAJlTkVTLeKrvEtZCP\
4AQRvsmH7akOo86ja2o33qK+p54AFoKr0T0GaQP6NfsklipK\
N0azBIiwo/tTlwOJUjW3fLQMMNRX/9ZtIYMGSPIOL4FdYJhT\
oJC/dPZ1khaJvf7Oi0rxPuMNVekZAgMBAAECgYBWi9smi83X\
TlTGHqdUA5enFf7AshD73y3fanvOOm5J0bvDGfc3qwdIv7XS\
5sYJqDjXSNm1ONDbIWtKPkN0Hn2J3wDMKzECvvlc+Jb9n1Sm\
7HblKI5PqB3578lcYUQ0M1fIa4AbUBPruInAkAOwuUWvfzmt\
gVky19BXDZNnS+UmAQJBAP389aYz/NfKMSvEq7zht8IQsqXA\
VQinoRLWGTg8uWTd6OZktuBxm9qQDSugIHQlS99PZrSL2xXf\
45bfYKWq7nkCQQC8cNJ+MBr787VmaHdaUK7K32S5NtgJ3g1O\
Nyuzuwi3fj/+PDxB2SMOSWMX5j10N2iGSQ9ctVpXlfaVtQPw\
LaehAkEAvyyU9enxqbct9aTOI5i+SNtqrGotVxtdmSD/0J2c\
ajjB4hDyudOpnQSKdUjMxL+B8W8Bfmo6mKdx1hEC3Q/g+QJB\
AKAlvi5yEazInQTwiWafJT3TBxYFTytrpKExcBmqPak8SYR2\
irtnn0xYADhiex3WrSVa2AxxL9s53ruygCtRkaECQCUkMs3i\
e2OUKa8MQBhup7cbhXiSunO354Xnc4ddjbgPLwW7tGoWEezi\
jNsiJ6XBSMcx6zifs1ybCbeDvdaT/Jo=\
\n-----END RSA PRIVATE KEY-----\n";

typedef struct{
    
    
	unsigned char *data; //解密数据
	int len;            //解密数据长度
}decrypt_data_t;


static void crypt_rsa_decrypt(unsigned char *endata,int enlen,decrypt_data_t *out_data){
    
    
	RSA *rsa;
	BIO *bio;
	if((bio = BIO_new_mem_buf(prikey, -1)) == NULL)       //从字符串读取RSA公钥
    {
    
    
    	printf("BIO_new_mem_buf failed!\n");
    }
	if((rsa=PEM_read_bio_RSAPrivateKey(bio,NULL,NULL,NULL))==NULL)
	{
    
    
		printf("PEM_read_bio_RSAPrivateKey failed!");
    }

	int key_len = RSA_size(rsa);
	int ret=0,pos=0;
	int blocklen = 128;
	out_data->data = (unsigned char *)malloc(enlen+1);
	memset(out_data->data,0,enlen+1);
	unsigned char *sub_str = (unsigned char *)malloc(key_len+1);
	unsigned char *out_str = (unsigned char *)malloc(key_len+1);
	memset(sub_str,0,key_len+1);
	memset(out_str,0,key_len+1);
	while(enlen-pos >0){
    
    
		if(enlen-pos >blocklen){
    
    
			memset(sub_str,0,key_len+1);
			memcpy(sub_str,endata+pos,blocklen);
			ret = RSA_private_decrypt(blocklen,(unsigned char *)sub_str,(unsigned char*)out_str,rsa,RSA_PKCS1_PADDING);
			printf("RSA_private_decrypt ret:%d\n",ret);
			pos +=blocklen;
			if(ret >0){
    
    								
				memcpy(out_data->data+out_data->len,out_str,ret);
				out_data->len += ret;
			}
		}else{
    
    				
			memset(sub_str,0,key_len+1);
			memcpy(sub_str,endata+pos,enlen-pos);
			ret = RSA_private_decrypt(enlen-pos,(unsigned char *)sub_str,(unsigned char*)out_str,rsa,RSA_PKCS1_PADDING);
			printf("RSA_private_decrypt ret:%d\n",ret);
			pos = enlen;
			if(ret >0){
    
    								
				memcpy(out_data->data+out_data->len,out_str,ret);
				out_data->len += ret;
			}		
		}
	}
	
	if(sub_str){
    
    
		free(sub_str);
		sub_str = NULL;
	}
	if(out_str){
    
    
		free(out_str);
		out_str = NULL;
	}
	RSA_free(rsa);
	BIO_free_all(bio);
	
}