Node.js AES-256-CBC encryption issue - php

Question

I'm trying to create an encrypted string according to one sample code which is given in PHP, my main problem is Node.js crypto module doesn't accept keys with more than 32 bytes length but PHP openssl_encrypt does, it seems that's why I'm getting Invalid key size error.
here is my js code:
let iv = sha1(await HelpersService.makeRandomNumber(null, null, 16));
iv = iv.substr(0, 16);
const text = bundledData;
const password = sha1(this.credentials.appSecret);
let salt = sha1(await HelpersService.makeRandomNumber(null, null, 4));
salt = salt.substr(0, 4);
const key = crypto.createHash('sha256').update(password + salt).digest('hex');
const cipher = crypto.createCipheriv('aes-256-cbc', key, iv);
let encoded = cipher.update(text, 'utf8', 'hex');
encoded += cipher.final('hex');
and this is the PHP sample:
function generateCashOutAPIHashKey($app_secret ){
//remove plus(+) sign from gsm number.
$data = 'text';
$iv = substr(sha1(mt_rand()), 0, 16);
$password = sha1($app_secret);
$salt = substr(sha1(mt_rand()), 0, 4);
$saltWithPassword = hash('sha256', $password . $salt);
$encrypted = openssl_encrypt("$data", 'aes-256-cbc', "$saltWithPassword", null, $iv );
return $encrypted;
}

Answer 1

The key generated in the PHP and NodeJS code has a length of 64 bytes. The used encryption AES-256 needs a 32 bytes key. In the PHP code, openssl_encrypt() implicitly shortens the 64 bytes key to 32 bytes. In the NodeJS code this has to be done explicitly:
const key = crypto.createHash('sha256').update(password + salt).digest('hex').substr(0, 32);
Also, openssl_encrypt() returns the ciphertext Base64 encoded by default. In the NodeJS code the result is returned hex encoded. Here you have to change the output encoding from 'hex' to 'base64' in the update() and final() call:
let encoded = cipher.update(text, 'utf8', 'base64');
encoded += cipher.final('base64');
Please note that the PHP reference code has a number of vulnerabilities:
mt_rand() is not a cryptographically secure pseudorandom number generator (CSPRNG). PHP provides cryptographically secure methods for deriving a random IV / salt, e.g. random_bytes() or random_int().
The key is inferred using SHA-256. It is more secure to use a reliable key derivation function such as PBKDF2. A 4 bytes salt is generally too small.
SHA1 is considered insecure in most contexts and must no longer be used.

Answer 2

Anyone have a problem with the following ?
crypto.createHash('sha256').update(password +
salt).digest('hex').substr(0, 32);
I understand the intent of needing a 32 byte value produced by sha256 but if you hex encode and then take the first 32 characters of the string you have SIGNIFICANTLY reduced the value of this. You now have a string 32 characters long that has only 16 possible values (0-9a-f) for each character. You really want a 32 byte value that has 256 possible values for each byte. You have changed your key security of your encryption key from the intended 256^32 (1.1E77) to 16^32 (3.4E38).
crypto.createHash('sha256').update('').digest('hex');
'e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855'
crypto.createHash('sha256').update('').digest('hex').substr(0,32);
'e3b0c44298fc1c149afbf4c8996fb924'
Find a way to use the byte values produced by SHA256.