Please suggest a simple 8-bit cipher

From: Milan VXdgsvt (really_at_not.needed)
Date: 07/29/04 

Date: Thu, 29 Jul 2004 09:02:07 +0000 (UTC)

Hello, people.

We need to encrypt short messages (100 bytes or less) with only a very
little computing power and memory (let's say, 50 bytes of memory and
some 1000 8-bit arithmetic operations per byte of message).
We don't really need it to be unbreakable, just it should look
encrypted so nobody tries it.

I understand that given the restrictions, it will be probably easy to
brute-force.

Some technical details:
1) The algorithm nor the keys will not be published. The keys will be
fixed.
2) The messages transmitted will all have the following format:
  +9999;9999,99 99,99
perhaps a little longer but similar.
3) Because the messages have a simple and fixed format, xoring it with
a constant stream would make it really easy to break. So I thought of
dividing the password into 'fixed' and 'variable' part, generating the
variable part randomly and transmitting it with the message.
4) Streaming cipher is preferrable. Block mode is acceptable only if
the block is short, let's say 32 bits.
5) The most probable attack is that the attacker knows a few (not much
more than 1000) message-ciphertext pairs and wants to encrypt a new
message without using our device.

It there some full-featured algorithm that can be cut down to 32 or 8
bits? All the algorithms I could find use way too much memory and time.
I could basically design something simple myself, but I always end up
needing a 'secure' 32->32 bit hashing function.

TIA for any suggestions or help,

  Milan Vandrovec

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