Java Snippet #01
This challenge covers the review of a snippet of code written in Java
In this challenge, participants are presented with a Java code snippet that includes methods for building a payment redirect URL and signing payment information. The build_redirect method generates a transaction ID using SecureRandom and constructs payment parameters, including a cryptographic signature generated by the sign_for_payment method. This method concatenates a secret with the payment information, hashes the result using SHA-256, and Base64 encodes the hash.
The video explains that the primary issue lies in the use of plain SHA-256 for creating the hash, rather than a more secure HMAC (Hash-based Message Authentication Code). This makes the code vulnerable to length extension attacks. An attacker who knows one hash can create a new hash derived from the initial hash without knowing the secret, by adding data to the digest. This vulnerability could potentially allow unauthorized modifications to the payment information.
To mitigate this issue, the code should use HMAC instead of a basic hash function. HMAC provides better security by using a secret key in the hashing process, which prevents length extension attacks and ensures the integrity and authenticity of the signed data.