Is it difficult to recognize successful decryption?

Question

When I hear about methods of breaking encryption algorithms, I notice that the emphasis is often on how to quickly decrypt and how to reduce the search space. However, I always wonder how you can recognize successful decryption and why this is not a bottleneck. Or is it often assumed that an encrypted / decrypted pair is known?

Answer 1

From Cryptonomicon :

There is a compromise between the two extremes, on the one hand, not knowing any plaintext, and, on the other hand, knowing all this. In Cryptonomicon, which falls under the cheat sheet title. A cheat sheet is about what words or phrases may be present in a message. For example, if you were decrypting German messages from the world of War II, you might guess that the plaintext included the phrase "HElL HITLER" or "SIEG HElL." You can choose to print a sequence of ten characters in random and say: "Suppose it represented the HEIL HITLER. If this is what it would mean the rest of the message?"

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Sitting in his office with a fresh Arethus intercepting, he work using FUNERAL as a crib: if this group of seven letters decrypts FUNERAL, then what does the rest of the message look like? Gibberish? Okay, what about this seven letter group?

Answer 2

Typically, you have an idea of ​​the file format that you expect to receive from decryption, and most formats provide an easy way to identify them. For example, almost all binary formats, such as images, documents, zip files, etc., have easily identifiable headers, while text files will contain only ASCII or only valid UTF-8 sequences.

Answer 3

In asymmetric cryptography, you usually have access to the public key. Therefore, any decryption of the encrypted ciphertext can be re-encrypted using the public key and compared with the original ciphertext, thus showing that the decryption was successful.

The same is true for symmetric encryption. If you think you have decrypted the cipher, you should also think that you have found the key. Therefore, you can use this key to encrypt your supposedly correct decrypted text and see if the encrypted result matches the original encrypted text.

Answer 4

For symmetric encryption, where the key length is shorter than the length of the encoder, you are guaranteed not to be able to create all possible text files. You can probably guess what form of your plain text will take, to some extent, you probably know whether it is an image or XML, or if you don’t even know that much, then you can assume that you can run file and do not get the "data". You should hope that there are only a few keys that will give you even vaguely reasonable decryption and only one that matches the shape you are looking for.

If you have a sample of plain text (or partial text), it becomes much easier.