Question

I have a some code below. My problem that i have a strange ending of encrypted and decrypted char array.

#include <iostream>
#include <openssl\evp.h>

#define AES_BLOCK_SIZE 16

static EVP_CIPHER_CTX Encrypt_Context, Decrypt_Context;

int aes_init(unsigned char *key_data, int key_data_len, unsigned char *salt, EVP_CIPHER_CTX *e_ctx, 
             EVP_CIPHER_CTX *d_ctx)
{
  int i, nrounds = 1;
  unsigned char key[16], iv[16];

  i = EVP_BytesToKey(EVP_aes_128_cbc(), EVP_sha1(), salt, key_data, key_data_len, nrounds, key, iv);
  if (i != 16) {
    printf("Key size is %d bits - should be 128 bits\n", i);
    return -1;
  }

  EVP_CIPHER_CTX_init(e_ctx);
  EVP_EncryptInit_ex(e_ctx, EVP_aes_128_cbc(), NULL, key, iv);
  EVP_CIPHER_CTX_init(d_ctx);
  EVP_DecryptInit_ex(d_ctx, EVP_aes_128_cbc(), NULL, key, iv);

  return 0;
}

unsigned char *aes_encrypt(EVP_CIPHER_CTX *e, unsigned char *plaintext, int *len)
{
  int c_len = *len + AES_BLOCK_SIZE, f_len = 0;
  unsigned char *ciphertext = (unsigned char *)malloc(c_len);
  EVP_EncryptInit_ex(e, NULL, NULL, NULL, NULL);
  EVP_EncryptUpdate(e, ciphertext, &c_len, plaintext, *len);
  EVP_EncryptFinal_ex(e, ciphertext+c_len, &f_len);
  *len = c_len + f_len;
  return ciphertext;
}

unsigned char *aes_decrypt(EVP_CIPHER_CTX *e, unsigned char *ciphertext, int *len)
{
  int p_len = *len, f_len = 0;
  unsigned char *plaintext = (unsigned char*)malloc(p_len);
  EVP_DecryptInit_ex(e, NULL, NULL, NULL, NULL);
  EVP_DecryptUpdate(e, plaintext, &p_len, ciphertext, *len);
  EVP_DecryptFinal_ex(e, plaintext+p_len, &f_len);
  *len = p_len + f_len;
  return plaintext;
}

void main()
{
    unsigned char * data_to_encrypt = (unsigned char*)"aaaaaaaaabbbbbbbbbbbbbbccccccccc";
    unsigned char *key_data = (unsigned char *)"aaaaaaaaaaaaaaaa";
    int key_data_len = strlen((const char*) key_data);
    char * ciphertext;
    char * plaintext;

    aes_init(key_data, 16, NULL, &Encrypt_Context, &Decrypt_Context);

    int length = strlen((const char*) data_to_encrypt)-1;

    printf("Clear text :%s\n", data_to_encrypt);

    ciphertext = (char *)aes_encrypt(&Encrypt_Context, data_to_encrypt, &length);
    printf("Crypted text :%s\n", ciphertext);

    plaintext = (char *)aes_decrypt(&Decrypt_Context, (unsigned char*)ciphertext, &length);
    printf("Decrypted text :%s\n", plaintext);
}

With this code i getting in console:

Clear text :aaaaaaaaabbbbbbbbbbbbbbccccccccc
Crypted text :°ЁТ☼#єз▬├^^,♦Ёфм:ЇъШ╙y╒КzukЩu@8¤═══════════════¤¤¤¤
Decrypted text :aaaaaaaaabbbbbbbbbbbbbbcccccccc☺¤¤¤¤

What is ═══════════════¤¤¤¤ in the end of crypted text and ☺¤¤¤¤ in the end of decrypted? What’s wrong with my code?

Thanks!

Answer 1

The encryption routines deal with binary data, not with C strings, the arrays they’re returning pointers to are not null-terminated, so when printing with printf (which expects a null-terminated string) you’re getting random garbage that happens to be located right after these arrays.

To solve this, ensure null-termination, use something like fwrite or iterate over the characters in order to print the arrays.