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Predictable cipher key

Encryption or decryption key is generated from a weak random number generator

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Description

This defect occurs when you use a weak random number generator for the encryption or decryption key.

Risk

If you use a weak random number generator for the encryption or decryption key, an attacker can retrieve your key easily.

You use a key to encrypt and later decrypt your data. If a key is easily retrieved, data encrypted using that key is not secure.

Fix

Use a strong pseudo-random number generator (PRNG) for the key. For instance:

  • Use an OS-level PRNG such as /dev/random on UNIX® or CryptGenRandom() on Windows®

  • Use an application-level PRNG such as Advanced Encryption Standard (AES) in Counter (CTR) mode, HMAC-SHA1, etc.

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

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

In this example, the function RAND_pseudo_bytes declared in openssl/rand.h produces the cipher key. However, the byte sequences that RAND_pseudo_bytes generates are not necessarily unpredictable.

Correction — Use Strong Random Number Generator

One possible correction is to use a strong random number generator to produce the cipher key. 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

int func(EVP_CIPHER_CTX *ctx, unsigned char *iv){
    unsigned char key[SIZE16];
    RAND_bytes(key, 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_PREDICTABLE_KEY
Impact: Medium

Version History

Introduced in R2017a

See Also

Topics