I receive emails from my bank with password protected PDF files, and they provide two passwords - either xxxx or yyyy. How is this possible?


Not sure if this is how it is done, but it is one way to do it.

Key A is derived from the first password, and key B is derived from the second password. A random key X is encrypted twice, once with key A and once with key B. Both are stored. The PDF is encrypted with key X.

When the user enters a password, it derives a key from it and tries to decrypt both verisons of X with it. Once X is known, it can be used to decrypt the entire PDF.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.