Not Using a Random IV with CBC Mode

Background

CBC is the most commonly used mode of operation for a block cipher. It solves electronic code book's dictionary problems by XORing the ciphertext with plaintext. If it used to encrypt multiple data streams, dictionary attacks are possible, provided that the streams have a common beginning sequence.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

In the following examples, CBC mode is used when encrypting data:

EVP_CIPHER_CTX ctx;
char key[EVP_MAX_KEY_LENGTH];
char iv[EVP_MAX_IV_LENGTH];
RAND_bytes(key, b);
memset(iv,0,EVP_MAX_IV_LENGTH);
EVP_EncryptInit(&ctx,EVP_bf_cbc(), key,iv);

public class SymmetricCipherTest {

  public static void main() {


    byte[] text ="Secret".getBytes();
    byte[] iv ={
      0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00
    };
    KeyGenerator kg = KeyGenerator.getInstance("DES");
    kg.init(56);
    SecretKey key = kg.generateKey();
    Cipher cipher = Cipher.getInstance("DES/CBC/PKCS5Padding");
    IvParameterSpec ips = new IvParameterSpec(iv);
    cipher.init(Cipher.ENCRYPT_MODE, key, ips);
    return cipher.doFinal(inpBytes);

  }

}

In both of these examples, the initialization vector (IV) is always a block of zeros. This makes the resulting cipher text much more predictable and susceptible to a dictionary attack.

See Also