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What you're looking for can be done using existing schemes for format preserving encryption (FPE). In general, FPE schemes convert an existing strong algorithm like AES into a block cipher that operates on a set of any size. For instance, FPE can encrypt 15 digit integers to other 15 digit integers (eg for credit card numbers, one of the common reasons for ...

There is a technique called "format preserving encryption", which could be called an "arbitrary-size block cipher". This would allow to map your set of 5-character strings onto itself. Of course, this can't really get too secure, as it has still the limitations of ECB mode: encrypting the same string with the same key always gives the same ciphertext. Your ...

There is a generic construction, by Granboulan and myself, which shows that it can be done "perfectly": if you have a seekable pseudo-random stream (which you can get with a conventional block cipher in CTR mode), then you can have a pseudo-random permutation over a domain of arbitrary size N, such that evaluating that permutation over a given input uses ...

The Vigenère cipher has many weaknesses, but perhaps the most obvious ones are: An attacker, who knows (or can guess) as many consecutive characters of any plaintext message as there are in the key, can trivially recover the key and thus decrypt all messages. (In fact, the characters need not even be consecutive, they just need to cover the entire key, or ...

Yes, this is possible (conditionally). It sounds like you want Format Preserving Encryption. FPE works by encrypting from an arbitrary domain $X$ onto $X$. Consequentially, if plaintext $M \in X$ is encrypted to ciphertext $C \in X$, any decryption of $C$ - even with the wrong key - will yield a decrypted message inside of $X$. Thus an attacker doesn't know ...

Your problem, the way I read it, could be described as follows: You are currently using password encryption for protecting the confidentiality of files on a known format. You have concerns regarding the long term confidentiality of those files, given that you don't know what computers will be able to do in the future. Ideally, you want the confidentiality to ...

The real security of Vigenere is difficult to quantify. A million character plaintext with a 10 character password is easy to break. But a 10 character plaintext with a 10 character randomly chosen password is essentially a one-time-pad and theoretically unbreakable. Given the data you've told us (plaintext: 100 to 5000 characters; password: 30 to 100 ...

Eek! The Vigenere cipher is completely and totally insecure. You should never use it. Instead, use a modern authenticated encryption scheme. If you are protecting data in transit, I recommend using TLS (or SSL). If you are protecting data in storage, I recommend encrypting it with GPG (or PGP). This is the simplest, easiest way to get well-vetted ...

Selective format-compliant JPEG encryption as you are trying to do it is a great idea, but it won't work... not like this. To keep the reasons short and simple: JPEG uses lossy compression (and even lossier recompression). If you really want to create a format-compliant implementation, you'll have to take care that you're independent of any ...

For such short messages, you're not gaining much (if anything) from using a CBC, CFB, etc., that require an initialization vector (these modes of operation are to ensure against a block of output being repeated when a block of input is repeated). For your situation (input that's shorter than one block), using ECB shouldn't pose a major problem. For a block ...