Is there a 8 bit block sized Public-Private key encryption algorithm?

Question

I checked out TripleDES. It's block size is of 64 bits.

Is there any algorithm for 8 bits block size?

Thanks

EDIT : I intend not to use this for perfect protection, but for a just-in-case situation where one who sees the code should not find the plaintext. So 8 bit is kinda okay for me.

Answer 1

RSA with 8-bit key :)

Seriously though, the block-based cyphers are stateless - the ciphertext of a block depends only on the cleartext of the block, not on the previous blocks (otherwise it would be a stream cypher). A block cypher that acts on 8-bit blocks can be brute-forced easily, so there's no point.

Answer 2

A block cipher with 8-bit blocks means that each input block can be encrypted into 256 possible values -- which means that an attacker has a 1/256 chance of guessing the input value. It turns out to be very difficult to use such an algorithm securely. Nevertheless it is possible to define a block cipher over 8-bit blocks, and to do it "perfectly"; just do not expect it to be generally useful.

There also are "block-less" ciphers, known as "stream ciphers" which encrypt data "byte by byte" (or even "bit by bit"); most are just pseudo-random generators which produce an arbitrary amount of bytes from a key. That generated stream is just to be combined with the data to encrypt with a XOR. The traditional stream cipher is RC4; but newer and better stream ciphers have been designed.

A block cipher, by itself, is a mathematical tool. In order to actually encrypt data, the block cipher must be used properly. The keywords are chaining and padding. Chaining is about defining what actually goes into the block cipher and what to do with the output. Padding is about adding some bytes to the data, in a reversible way, so that the padded message length is appropriate for the chosen chaining mode. The traditional chaining mode is called CBC. A newer (and arguably better) chaining mode is CTR (same link), which has the added bonus of avoiding the need for padding (CTR just turns a block cipher into a stream cipher).

As for block ciphers, you should use AES instead of TripleDES. It is faster, more secure, and the current American standard.