使用AES限制加密结果

Question

I need to encrypt a text (16 chars), preferably using AES, and I need limit the length of the result encrypted text (14 or 16 characteres). The encrypted has to be only chars and numbers (not '=', '?', ...) Is it possible?

I'll need to get back the original text from the cipher text(encrypted).

Is there a way to do this using RijndaelManaged (System.Security.Cryptography )?

Answer 1

The cypher-text needs to be at least as long(in an entropic sense) as plain-text. You can't losslessly compress arbitrary texts. So if you limit your output to log2(10+2*26)*16=95 bits the input can't have any more entropy than that. This has nothing to do with AES, it's a mathematical limitation that applies to all lossless encodings.

What's a character? A byte a char or a unicode-codepoint?

AES has the additional problem that it's a block cypher, the minimum output size is equal to the blocksize, 128bits. And since the output appears random it can't be compressed after encrypting. And that already exceeds your limit. And most encryption modes add a bit of additional padding.

There are functions which map arbitrary length input to constant length output. They are called hash-functions. But following the pidgeon-hole-principle they map multiple inputs to an output. So you can't get back the input for all possible inputs.