santoniu/xor-cipher ? reverse-engineered prompt

Build me a small C project that implements a simple XOR cipher using a circular byte buffer.

I want it to compile cleanly with make, and it should support encrypting and decrypting data with XOR in a way that wraps around the key buffer as needed. If it makes sense, include support for choosing a starting offset and a length, so only part of the input gets processed.

Keep it simple and practical, with a small command line tool or sample usage that shows it working on files or input data. Make sure the code is easy to read and the build works on a normal system, and if you need to check anything recent about cross compilation or compiler flags, go ahead and look it up.

Please include a clear README style explanation of how to compile and use it, plus any small example that proves the cipher is working.