How to determine the encryption scheme used when given a cipher text and the key?

My answer would be there is nothing wrong with trying various different algorithms out and seeing what works.

My answer would be there is nothing wrong with trying various different algorithms out and seeing what works. Cryptanalysis is like solving a puzzle, not a step by step process. You try things, you see what works, what you think gets you closer.It is absolutely trial and error based on knowledge of the potential algorithms, patterns and techniques and the reasons for them.

Differential cryptanalysis, a modern technique, basically amounts to trying various combinations of keys and plaintexts within an algorithm and looking at the differences to see if you can find patterns. From your comments, I think you're facing a vigenere cipher or some similar variant thereof.In this case, the key is important because essentially a vigenere cipher is a set of caesar ciphers and the length of the key determines the number of these ciphers. Now, the rules of the scheme in question will tell you exactly what cipher it is, but that's the basis of it.

I was really confused about what you were trying to get at, at first. – BlueRaja - Danny Pflughoeft Feb 18 at 17:54 I think you're right, edited, thanks. – Ninefingers Feb 18 at 20:44.

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