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Missing cipher algorithm

An encryption or decryption algorithm is not associated with the cipher context

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Description

This defect occurs when you do not assign a cipher algorithm when setting up your cipher context.

You can initialize your cipher context without an algorithm. However, before you encrypt or decrypt your data, you must associate the cipher context with a cipher algorithm.

Risk

A missing cipher algorithm can lead to run-time errors or at least, non-secure ciphertext.

Before encryption or decryption, you set up a cipher context that has the information required for encryption: the cipher algorithm and mode, an encryption or decryption key and an initialization vector (for modes that require initialization vectors).

ret = EVP_EncryptInit(&ctx, EVP_aes_128_cbc(), key, iv)
The function EVP_aes_128_cbc() specifies that the Advanced Encryption Standard (AES) algorithm must be used for encryption. The function also specifies a block size of 128 bits and the Cipher Bloch Chaining (CBC) mode.

Instead of specifying the algorithm, you can use NULL in the initialization step. However, before using the cipher context for encryption or decryption, you must perform an additional initialization that associates an algorithm with the context. Otherwise, the update steps for encryption or decryption can lead to run-time errors.

Fix

Before your encryption or decryption steps

 ret = EVP_EncryptUpdate(&ctx, out_buf, &out_len, src, len)
associate your cipher context ctx with an algorithm.
ret = EVP_EncryptInit(ctx, EVP_aes_128_cbc(), key, iv)

Examples

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

unsigned char key[SIZE16];
unsigned char iv[SIZE16];
void func(void) {
    EVP_CIPHER_CTX *ctx = EVP_CIPHER_CTX_new();       
    EVP_CIPHER_CTX_init(ctx);
    EVP_EncryptInit_ex(ctx, NULL, NULL, key, iv); 
}

In this example, an algorithm is not provided when the cipher context ctx is initialized.

Before you encrypt or decrypt your data, you have to provide a cipher algorithm. If you perform a second initialization to provide the algorithm, the cipher context is completely re-initialized. Therefore, the current initialization statement using EVP_EncryptInit_ex is redundant.

Correction — Provide Algorithm During Initialization

One possible correction is to provide an algorithm when you initialize the cipher context. In the corrected code below, the routine EVP_aes_128_cbc invokes the Advanced Encryption Standard (AES) algorithm. The routine also specifies a block size of 128 bits and the Cipher Block Chaining (CBC) mode for encryption.


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

unsigned char key[SIZE16];
unsigned char iv[SIZE16];
void func(unsigned char *src, int len, unsigned char *out_buf, int out_len) {
    EVP_CIPHER_CTX *ctx = EVP_CIPHER_CTX_new();       
    EVP_CIPHER_CTX_init(ctx);
    
    /* Initialization of cipher context */
    EVP_EncryptInit_ex(ctx, EVP_aes_128_cbc(), NULL, key, iv); 
    
    /* Update steps for encryption */
    EVP_EncryptUpdate(ctx, out_buf, &out_len, src, len);
}

Result Information

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

Version History

Introduced in R2017a

See Also

Topics