.net - How do I encrypt a string and get a equal length encrypted string?

Question

My problem is the following:

In an existing database I want to encrypt data in a couple of columns. The columns contains strings of different lengths.

I don't want to change the size of the columns so the encryption need to produce an equal length text representation of the input text.

The strength of the encryption algorithm is of secondary interest but of course I want it to be as strong as it can be. Otherwise I wouldn't need to encrypt the data. But the most important thing is the size of the output.

Is this possible? If so how would I do it?

I'm interested in doing it in .NET. No database-level encryption.

Answer 1

Within your constrants, I would use AES in CFB mode, which turns it into a stream cipher and the output length will be the same as the input length. Unless you're storing the strings in blobs, you'll need to hex or base64 encode the output to make it char friendly, which will be a 100% or 33% increase in length.

One .NET implementation is here.