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Constant block cipher initialization vector

Initialization vector is constant instead of randomized

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Description

This defect occurs when you use a constant for the initialization vector (IV) during encryption.

Risk

Using a constant IV is equivalent to not using an IV. Your encrypted data is vulnerable to dictionary 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 constant IV to encrypt multiple data streams that have a common beginning, your data becomes vulnerable to dictionary attacks.

Fix

Produce a random IV by using a strong random number generator.

For a list of random number generators that are cryptographically weak, see Vulnerable pseudo-random number generator.

Examples

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#include <openssl/evp.h>
#include <stdlib.h>
#define SIZE16 16

/* Using the cryptographic routines */

int func(EVP_CIPHER_CTX *ctx, unsigned char *key){
    unsigned char iv[SIZE16] = {'1', '2', '3', '4','5','6','b','8','9',
                                 '1','2','3','4','5','6','7'};
    return EVP_CipherInit_ex(ctx, EVP_aes_128_cbc(), NULL, key, iv, 1); 
}

In this example, the initialization vector iv has constants only. The constant initialization vector makes your cipher vulnerable to dictionary attacks.

Correction — Use Random Initialization Vector

One possible correction is to 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 SIZE16 16

/* Using the cryptographic routines */

int func(EVP_CIPHER_CTX *ctx, unsigned char *key){
    unsigned char iv[SIZE16];
    RAND_bytes(iv, 16);
    return EVP_CipherInit_ex(ctx, EVP_aes_128_cbc(), NULL, key, iv, 1); 
}

Result Information

Group: Cryptography
Language: C | C++
Default: Off
Command-Line Syntax: CRYPTO_CIPHER_CONSTANT_IV
Impact: Medium

Version History

Introduced in R2017a

See Also

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