Having trouble encrypting text using AES-128-CBC

Here's the brief from the payment gateway (Elavon):

The string must be encrypted using AES (block size 128-bit) in CBC mode with PKCS#5 padding. Use the provided password as both the key and initialisation vector and encode the result in hex (making sure the letters are in upper case). Prepend the ‘@’ sign to the beginning of the encoded result.

Here's my code:

    function encryptAesTest($string, $key)
        {
            $cipher = "aes-128-cbc";
            $crypt = openssl_encrypt($string, $cipher, $key, 0, $key);
            return "@" . strtoupper(bin2hex($crypt));
        }

The question is, does this code satisfy the brief from the payment gateway ?

I am attempting to convert from mcrypt_encrypt -> openssl_encrypt

My PHP version is 8.2.13

I think I might be going wrong with PKCS#5 padding as I understand that openssl_encrypt uses PKCS#7 padding

The error message from Elavon (SagePay) is unhelpful:

5080 : Form transaction registration failed.

Any help would be greatly appreciated

Solution

The document Sage Pay Form Integration and Protocol Guidelines 3.00 (see sec. A1.2 Example Crypt Field, p. 38) contains test data (in sec. A1.1 The Crypt field you will also find the specification that corresponds to yours):

plaintext:   VendorTxCode=TxCode-1310917599-223087284&Amount=36.95&Currency=GBP&Description=description&CustomerName=Fname Surname&CustomerEMail=customer@example.com&BillingSurname=Surname&BillingFirstnames=Fname&BillingAddress1=BillAddress Line 1&BillingCity=BillCity&BillingPostCode=W1A 1BL&BillingCountry=GB&BillingPhone=447933000000&DeliveryFirstnames=Fname&DeliverySurname=Surname&DeliveryAddress1=BillAddress Line 1&DeliveryCity=BillCity&DeliveryPostCode=W1A 1BL&DeliveryCountry=GB&DeliveryPhone=447933000000&SuccessURL=https://example.com/success&FailureURL=https://example.com/failure
password:    55a51621a6648525
ciphertext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

If in the posted code the default Base64 encoding in openssl_encrypt() is disabled with OPENSSL_RAW_DATA, the code works. Using the above password and plaintext results in the expected ciphertext!

Fixed PHP code:

<?php
$user_string = "VendorTxCode=TxCode-1310917599-223087284&Amount=36.95&Currency=GBP&Description=description&CustomerName=Fname Surname&CustomerEMail=customer@example.com&BillingSurname=Surname&BillingFirstnames=Fname&BillingAddress1=BillAddress Line 1&BillingCity=BillCity&BillingPostCode=W1A 1BL&BillingCountry=GB&BillingPhone=447933000000&DeliveryFirstnames=Fname&DeliverySurname=Surname&DeliveryAddress1=BillAddress Line 1&DeliveryCity=BillCity&DeliveryPostCode=W1A 1BL&DeliveryCountry=GB&DeliveryPhone=447933000000&SuccessURL=https://example.com/success&FailureURL=https://example.com/failure";
$key = '55a51621a6648525';
$iv = '55a51621a6648525';
function encryptAesTest($string, $key)
{
    $cipher = "aes-128-cbc";
    $crypt = openssl_encrypt($string, $cipher, $key, OPENSSL_RAW_DATA, $key); // Fix: disable Base64 encoding 
    return "@" . strtoupper(bin2hex($crypt));
}
print(encryptAesTest($user_string, $key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
?>

Security: The protocol has the following vulnerabilities:
The key seems to be a hex encoded value, which limits the range of values per key byte from 256 to 16 (so effectively only an 8 byte key is used instead of a 16 byte key), which is a serious vulnerability.
In addition, the use of the key as IV is a vulnerability since this results in the reuse of key/IV pairs.