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Missing cipher key

Context used for encryption or decryption is associated with NULL key or not associated with a key

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Description

This defect occurs when you encrypt or decrypt data using a NULL encryption or decryption key.

Note

You can initialize your cipher context with a NULL key. However, before you encrypt or decrypt your data, you must associate the cipher context with a non-NULL key.

Risk

Encryption or decryption with a NULL key can lead to run-time errors or at least, non-secure ciphertext.

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

Sometimes, you initialize your cipher context with a non-NULL key

ret = EVP_EncryptInit_ex(&ctx, cipher_algo_1, NULL, key, iv)
but change the cipher algorithm later. When you change the cipher algorithm, you use a NULL key.
 ret = EVP_EncryptInit_ex(&ctx, cipher_algo_2, NULL, NULL, NULL)
The second statement reinitializes the cipher context completely but with a NULL key. To avoid this issue, every time you initialize a cipher context with an algorithm, associate it with a key.

Examples

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#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 *iv, unsigned char *src, int len){
    if (iv == NULL)
        fatal_error();
    
    /* Fourth argument is cipher key */
    EVP_EncryptInit_ex(ctx, EVP_aes_128_cbc(), NULL, NULL, iv); 
    
    /* Update step with NULL key */
    return EVP_EncryptUpdate(ctx, out_buf, &out_len, src, len);
}

In this example, the cipher key associated with the context ctx is NULL. When you use this context to encrypt your data, you can encounter run-time errors.

Correction — Use Random Cipher Key

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 fatal_error() abort()
#define SIZE16 16

unsigned char *out_buf;
int out_len;

int func(EVP_CIPHER_CTX *ctx, unsigned char *iv, unsigned char *src, int len){
    if (iv == NULL)
        fatal_error();
    unsigned char key[SIZE16];
    RAND_bytes(key, 16);
    
    /* Fourth argument is cipher key */
    EVP_EncryptInit_ex(ctx, EVP_aes_128_cbc(), NULL, key, iv); 
    
    /* Update step with non-NULL cipher key */
    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_KEY
Impact: Medium

Version History

Introduced in R2017a

See Also

Topics