Symmetric encryption algorithm for embedded system

All the tables in Rijndael are defined as simplish operations in GF2. As such, I'd be tempted to say it's possible to write eg. 128-bit AES in 1k.

See also: https://electronics.stackexchange.com/questions/13275/smallest-aes-implementation-for-microcontrollers

But any chosen algorithm is a minor factor in security eg. if the key is distributed with the binary. In that case, the simplest mechanism to unveil the original binary is to run the code through emulator and store the memory.

A very simple encryption algorithm that I saw being used in the embedded world is XXTEA