Which testing method is most appropriate to ensure thorough evaluation of a payment system's vulnerabilities when confidentiality is crucial?

Hiring an independent firm for grey box testing is the most appropriate method to ensure a thorough evaluation of a payment system's vulnerabilities, especially when confidentiality is crucial. Grey box testing combines elements of both black box and white box testing; testers have some knowledge of the internal workings of the system but not full access to its code. This balance allows for a more in-depth examination of potentially vulnerable areas while simulating an outsider's perspective.

Using an independent firm adds an extra layer of objectivity and expertise to the testing process. These firms are often experienced in critical payment systems and are familiar with the latest vulnerabilities, tools, and techniques used by attackers. Their knowledge and fresh perspective can lead to the discovery of issues that internal teams might overlook due to familiarity or bias.

Other options, while they may have their merits, could present limitations. Conducting a public penetration test might expose sensitive data or system details to potential threats. Using internal resources could introduce biases or a lack of comprehensive evaluation, as staff may not approach the testing with the same critical viewpoint as an outside expert. Performing a code review internally may not adequately simulate real-world attack scenarios and might miss vulnerabilities that can be exploited without direct access to the codebase. Therefore, grey box testing by an independent firm