Mandatory parameter.

Value generated during the subscription to the payment gateway.

If the value is incorrect, the buyer will get an error message in their browser when making the payment.

It becomes impossible to make the payment and the transaction is definitively lost.

A warning e-mail is then sent to the shop administrator. It contains the form that the gateway was unable to process with the value of the signature.

Input and output field, returned in the response (IPN and Return URL).