Why AES can't Decrypt , PHP. Openssl - php

Question

I'm having problem with decrypting
I have
$key="Gwu078980";
$cipher="aes-128-gcm";
$iv=md5($cipher);
$text="yaw0";
$tag="";
echo $encrypted=openssl_encrypt($text, $cipher, $key, 0, $iv, $tag);
echo $de_ciphertext=openssl_decrypt($encrypted, $cipher, $key, 0, $iv, $tag);
Output
ELRmWQ==
yaw0
So the raw text is yaw0 and the encrypted is ELRmWQ== and the decrypted is yaw0 so perfect.
But when I manually copy the encrypted text and use it as
$encrypted ="ELRmWQ==";
And I run the decryption
the decryption returns null.
Thanks in advance for anyone to help me out.

Answer 1

Your openssl_encrypt message modifies $tag by reference since you're using aes-128-gcm.
That parameter is required for openssl_decrypt aswell (when using AEAD - Authenticated Encryption and Decryption) and is probably an empty string in your case when you omit the openssl_encrypt call.
See Example 1 in the docs:
The comment about storing $cipher, $iv, and $tag is the important part:
<?php
//$key should have been previously generated in a cryptographically safe way, like openssl_random_pseudo_bytes
$plaintext = "message to be encrypted";
$cipher = "aes-128-gcm";
if (in_array($cipher, openssl_get_cipher_methods()))
{
$ivlen = openssl_cipher_iv_length($cipher);
$iv = openssl_random_pseudo_bytes($ivlen);
$ciphertext = openssl_encrypt($plaintext, $cipher, $key, $options=0, $iv, $tag);
//store $cipher, $iv, and $tag for decryption later
$original_plaintext = openssl_decrypt($ciphertext, $cipher, $key, $options=0, $iv, $tag);
echo $original_plaintext."\n";
}
?>