Secure Electronic Transaction (SET) protocol is a security standard developed to protect financial transactions conducted over open networks, especially the internet. It was jointly designed by Visa and MasterCard to ensure confidentiality, integrity, and authentication during online credit card payments. The protocol aims to provide a secure framework where cardholders, merchants, and payment gateways can exchange sensitive information safely. SET uses a combination of encryption techniques, digital certificates, and authentication mechanisms to reduce fraud and unauthorized access. In the context of web application testing and audit, SET is important because it demonstrates how layered security architecture can be implemented to protect financial data during web-based transactions. SET protocol operates on public key infrastructure (PKI), which provides digital certificates to all participating entities. These digital certificates are issued by trusted certificate authorities and verify the identities of cardholders and merchants. Authentication is a critical feature of SET because it ensures that the merchant is legitimate and the customer is authorized to use the payment card. During a transaction, both parties verify each other’s certificates before any sensitive information is exchanged. This two-way authentication reduces the chances of fraudulent activities and builds trust in electronic commerce platforms. The working of SET begins when a customer decides to purchase goods or services from an online merchant. The customer first selects the products and proceeds to the checkout page. The merchant’s web application sends its digital certificate to the customer’s browser, allowing the customer to verify the authenticity of the merchant. Once verified, the customer