Security of generating keys for AES Encryption - php

Question

I found this following code while researching about AES encryption on the internet. In this code I found that the key and the iv are generated using hash function and uses sha256. I would like to know whether this method is safe for encryption of text using PHP.
The code which I found online is given below,
<?php
function encrypt_decrypt($action, $string) {
$output = false;
$encrypt_method = "AES-256-CBC";
$secret_key = 'This is my secret key';
$secret_iv = 'This is my secret iv';
// hash
$key = hash('sha256', $secret_key);
// iv - encrypt method AES-256-CBC expects 16 bytes - else you will get a warning
$iv = substr(hash('sha256', $secret_iv), 0, 16);
if ( $action == 'encrypt' ) {
$output = openssl_encrypt($string, $encrypt_method, $key, 0, $iv);
$output = base64_encode($output);
} else if( $action == 'decrypt' ) {
$output = openssl_decrypt(base64_decode($string), $encrypt_method, $key, 0, $iv);
}
return $output;
}
$plain_txt = "This is my plain text";
echo "Plain Text =" .$plain_txt. "\n";
$encrypted_txt = encrypt_decrypt('encrypt', $plain_txt);
echo "Encrypted Text = " .$encrypted_txt. "\n";
$decrypted_txt = encrypt_decrypt('decrypt', $encrypted_txt);
echo "Decrypted Text =" .$decrypted_txt. "\n";
if ( $plain_txt === $decrypted_txt ) echo "SUCCESS";
else echo "FAILED";
echo "\n";
?>

Answer 1

No it is not a secure method to derive an encryption key from a password.
Just using a hash function is not sufficient and just adding a salt does little to improve the security. Instead iterate over an HMAC with a random salt for about a 100ms duration and save the salt with the hash. Use a function such as PBKDF2, Rfc2898DeriveBytes, Bcrypt or similar functions. The point is to make the attacker spend a lot of time finding passwords by brute force.
The generally accepted method is PBKDF2 (Password Key Derivation Function 2), sometimes referred to as Rfc2898DeriveBytes in some implementations.
Note: One generally accepted way to handle the IV is to prefix the encrypted message with the IV for use in decryption. The IV does not need to be secret, it does need to be different for each encryption with the same key, this is generally achieved by using a random byte array from a CSPRNG (Cryptographically Secure PseudoRandom Number Generator).

Answer 2

The IV isn't a secret, but should be unique to make two strings encrypted with the same password have different encrypted values. So this is a bad way to generate it. Use http://php.net/manual/kr/function.openssl-random-pseudo-bytes.php or a similar function to generate an unique IV for each time you encrypt some data and store the IV with the data.
#zaph have already commented on the issues with how the key is derived from the password