Generation of Weak Initialization Vector (IV)

Description

By design, some cryptographic primitives (such as block ciphers) require that IVs must have certain properties for the uniqueness and/or unpredictability of an IV. Primitives may vary in how important these properties are. If these properties are not maintained, e.g. by a bug in the code, then the cryptography may be weakened or broken by attacking the IVs themselves.

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.

Example Two

The Wired Equivalent Privacy (WEP) protocol used in the 802.11 wireless standard only supported 40-bit keys, and the IVs were only 24 bits, increasing the chances that the same IV would be reused for multiple messages. The IV was included in plaintext as part of the packet, making it directly observable to attackers. Only 5000 messages are needed before a collision occurs due to the "birthday paradox" [REF-1176]. Some implementations would reuse the same IV for each packet. This IV reuse made it much easier for attackers to recover plaintext from two packets with the same IV, using well-understood attacks, especially if the plaintext was known for one of the packets [REF-1175].

See Also