OpenSSL和AES

Question

我正在使用OpenSSL lib并使用AES加密/解密获得非常奇怪的效果:如果我将更改加密消息中的某个字节并对其进行解密,我将看到原始消息的一部分,这不是假设的.这是源代码:

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

int do_crypt(void) 
{  
 int outlen, inlen;  
 FILE *in, *out;  
 in = fopen("in.txt", "r"); 
 out = fopen("out.txt", "w"); 
 unsigned char key[32];
 strcpy(key, "10000000000000000000000000000002"); 
 unsigned char iv[8];
 unsigned char inbuf[BUFSIZE], outbuf[BUFSIZE];  
 EVP_CIPHER_CTX ctx;  
 const EVP_CIPHER * cipher;    


 EVP_CIPHER_CTX_init(&ctx);   
 cipher = EVP_aes_256_cfb(); 
 EVP_EncryptInit(&ctx, cipher, key, 0); 

while(1) {                    
 inlen = fread(inbuf, 1, BUFSIZE, in);  
 if(inlen <= 0) break;  
 if(!EVP_EncryptUpdate(&ctx, outbuf, &outlen, inbuf, inlen)) return 0;  
 fwrite(outbuf, 1, outlen, out);  
 }    

if(!EVP_EncryptFinal(&ctx, outbuf, &outlen)) return 0;  
 fwrite(outbuf, 1, outlen, out);  
 EVP_CIPHER_CTX_cleanup(&ctx);  
 return 1;  
} 

int do_decrypt(char *infile) 
{  
 int outlen, inlen;  
 FILE *in, *out;  
 in = fopen("out.txt", "r"); 
 out = fopen("out2.txt", "w"); 
 unsigned char key[32];
 strcpy(key, "10000000000000000000000000000002"); 
 unsigned char iv[8];  
 unsigned char inbuf[BUFSIZE], outbuf[BUFSIZE];  
 EVP_CIPHER_CTX ctx;  

 EVP_CIPHER_CTX_init(&ctx);  
 EVP_DecryptInit(&ctx, EVP_aes_256_cfb(), key, 0);  

 while(1) {  
 inlen = fread(inbuf, 1, BUFSIZE, in);  
 if(inlen <= 0) break;  
 if(!EVP_DecryptUpdate(&ctx, outbuf, &outlen, inbuf, inlen)) return 0; 
 fwrite(outbuf, 1, outlen, out);  
 }  

 if(!EVP_DecryptFinal(&ctx, outbuf, &outlen)) return 0;  
 fwrite(outbuf, 1, outlen, out);  
 EVP_CIPHER_CTX_cleanup(&ctx);  
 return 1;  
} 

main(int argc, char **argv){ 
if(atoi(argv[1]) == 1) 
    do_crypt(0); 
if(atoi(argv[1]) == 2) 
    do_decrypt(0);    
} 

可能有什么不对？

Answer 1

你期望整个消息因单个字节被改变而变得不可读,这是错误的.

消息的哪些部分变得不可读取决于所选的加密模式.你正在使用CFB.这意味着如果更改密文中的单个字节,则相应的字节和之后的块会被破坏,然后密码将从错误中恢复.

错误发生后PCBC将损坏所有输出.但它仍然没有检测到错误.

我建议添加身份验证(MAC或具有集成身份验证的模式,如AES-GCM).