AES_128_CBC OpenSSL Decryption

Question

In implementing AES_128_CBC with OpenSSL, I have been given a key, IV, and some plaintext/cipher text pairs to see if my code is working as it should. Currently, when I decrypt I am getting the expected cipher text result. However, when the code attempts to decrypt cipher text back to plaintext, I am not getting the original plaintext. Instead, I am getting: ���S@�3��6�y�y�.

 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <openssl/aes.h>

 #define BUFSIZE 1024

 /* AES key for Encryption and Decryption */
 const static unsigned char aes_key[]= . {0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0xFF};
 unsigned char iv[] = {0x98,0x76,0x54,0x32,0x10,0xFE,0xDC,0xBA,0x98,0x76,0x54,0x32,0x10,0xFE,0xDC,0xBA};
 unsigned char tmpIv[sizeof(iv)]; 
 /* Print Encrypted and Decrypted data packets */
 void print_data(const char *title, const void* data, int len);


int main( ) {
    /* Input data to encrypt */
    unsigned char aes_input[BUFSIZE] = {0};
    strcpy(aes_input, "Testing");

    fprintf(stderr, "%s\n", aes_input);

    print_data("Init IV: ", iv, sizeof(iv));


    /* Init vector */

    memcpy(tmpIv, iv, sizeof(iv));

    /* Buffers for Encryption and Decryption */
    unsigned char enc_out[sizeof(aes_input)];
    unsigned char dec_out[sizeof(aes_input)];
    unsigned char *e = enc_out;

    /* AES-128 bit CBC Encryption */
    AES_KEY enc_key, dec_key;
    AES_set_encrypt_key(aes_key, sizeof(aes_key)*8, &enc_key);
    AES_cbc_encrypt(aes_input, enc_out, strlen(aes_input), &enc_key, iv, AES_ENCRYPT);
    fprintf(stderr, "Encrypted:");
    while (*e) printf(" [%02x]", *e++);
    printf("\n");

    /* AES-128 bit CBC Decryption */
    memset(tmpIv, 0x00, sizeof(iv));
    memcpy(tmpIv, iv, sizeof(iv)); 
    AES_set_decrypt_key(aes_key, sizeof(aes_key)*8, &dec_key); 
    AES_cbc_encrypt(enc_out, dec_out, strlen(aes_input), &dec_key, tmpIv, AES_DECRYPT);
    fprintf(stderr, "The Plaintext After Decryption is: %s\n", dec_out);



    return 0;
}

Answer 1

The problem is, that the iv is modified in aes_cbc_encrypt() for the next round of encryption.

Here, you overwrite the iv for decryption with the already modified vector:

/* AES-128 bit CBC Decryption */
memset(tmpIv, 0x00, sizeof(iv));
memcpy(tmpIv, iv, sizeof(iv)); 
AES_set_decrypt_key(aes_key, sizeof(aes_key)*8, &dec_key); 

Since you already copy the correct vector above

/* Init vector */
memcpy(tmpIv, iv, sizeof(iv));

it should suffice to remove the memset() / memcpy() below to get a correct result.