Signature Generation

In order to prevent against malicious attacks and hijacking of browser sessions, Certegy Ezi-Pay implements a signing mechanism based on HMAC-SHA256. This section provides information on how you can use HMAC-SHA256 for signing and verification purposes.

As mentioned, Certegy Ezi-Pay uses HMAC-SHA256 for purposes of signing and verifying requests.

Below is an example that demonstrates how you can go about implementing a method in a shopping platform that is based on PHP to generate the signature.

PHP Example

Below is a PHP code snippet that demonstrates how a signature might be generated in the context of Certegy Ezi-Pay:

    function certegy_ezipay_sign($query, $api_key )
        $clear_text = '';
        foreach ($query as $key => $value) {
            if (substr($key, 0, 2) === "x_") {
                $clear_text .= $key . $value;
        $hash = hash_hmac( "sha256", $clear_text, $api_key);
        return str_replace('-', '', $hash);

First note that the method expects two parameters and they are $query and $api_key. The $query represents the various key-value pairs that form your HTTP request POST and vary depending on the information that is entered as part of the checkout process on your shopping cart.

The parameter $api_key represents the API Key that is unique for every merchant. It should only change once the API key has been changed on the Certegy Ezi-Pay side.

Having received the two parameters, the certegy_ezipay_sign method will then perform an alphabetical sorting of the various key-value pairs based on the key but still maintaining the correlation between the keys and their respective values.

The method will then examine the $query variable for the various key-value pairs by checking for the x_ prefix and would then append them together.

The method then computes the keyed hash value using the hash_hmac method.