Missing block cipher initialization vector

Context used for encryption or decryption is associated with NULL initialization vector or not associated with an initialization vector

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Description

This defect occurs when you encrypt or decrypt data using a NULL initialization vector (IV).

Note

You can initialize your cipher context with a NULL initialization vector (IV). However, if your algorithm requires an IV, before the encryption or decryption step, you must associate the cipher context with a non-NULL IV.

Risk

Many block cipher modes use an initialization vector (IV) to prevent dictionary attacks. If you use a NULL IV, your encrypted data is vulnerable to such attacks.

Block ciphers break your data into blocks of fixed size. Block cipher modes such as CBC (Cipher Block Chaining) protect against dictionary attacks by XOR-ing each block with the encrypted output from the previous block. To protect the first block, these modes use a random initialization vector (IV). If you use a NULL IV, you get the same ciphertext when encrypting the same plaintext. Your data becomes vulnerable to dictionary attacks.

Fix

Before your encryption or decryption steps

 ret = EVP_EncryptUpdate(&ctx, out_buf, &out_len, src, len)

associate your cipher context ctx with a non-NULL initialization vector.

ret = EVP_EncryptInit_ex(ctx, EVP_aes_128_cbc(), NULL, key, iv)

Examples

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NULL Initialization Vector Used for Encryption


#include <openssl/evp.h>
#include <stdlib.h>
#define fatal_error() abort()

unsigned char *out_buf;
int out_len;

int func(EVP_CIPHER_CTX *ctx, unsigned char *key, unsigned char *src, int len){
    if (key == NULL)
        fatal_error();
    
    /* Last argument is initialization vector */
    EVP_EncryptInit_ex(ctx, EVP_aes_128_cbc(), NULL, key, NULL); 
    
    /* Update step with NULL initialization vector */
    return EVP_EncryptUpdate(ctx, out_buf, &out_len, src, len);
}

In this example, the initialization vector associated with the cipher context ctx is NULL. If you use this context to encrypt your data, your data is vulnerable to dictionary attacks.

Correction — Use Random Initialization Vector

Use a strong random number generator to produce the initialization vector. The corrected code here uses the function RAND_bytes declared in openssl/rand.h.


#include <openssl/evp.h>
#include <openssl/rand.h>
#include <stdlib.h>
#define fatal_error() abort()
#define SIZE16 16

unsigned char *out_buf;
int out_len;

int func(EVP_CIPHER_CTX *ctx, unsigned char *key, unsigned char *src, int len){
    if (key == NULL)
        fatal_error();
    unsigned char iv[SIZE16];
    RAND_bytes(iv, 16);
    
    /* Last argument is initialization vector */
    EVP_EncryptInit_ex(ctx, EVP_aes_128_cbc(), NULL, key, iv); 
    
    /* Update step with non-NULL initialization vector */
    return EVP_EncryptUpdate(ctx, out_buf, &out_len, src, len);
}

Result Information

Group: Cryptography

Language: C | C++

Default: Off

Command-Line Syntax: CRYPTO_CIPHER_NO_IV

Impact: Medium

Version History

Introduced in R2017a