Symmetric key authentication protocol

Does anybody know some simple authentication and data transfer protocol based 开发者_如何学Pythonon symmetric keys only? Due to memory constraints (kilobytes RAM and ROM) we cant afford asymmetric cryptography and due to closed environment asymmetric cryptography does not increase security of any way.

I am looking for simple symmetric cryptography protocol that can be kept in head and written on one paper sheet. I was looking in EAP-PSK https://www.rfc-editor.org/rfc/rfc4764#page-4 but still think that 2^6 pages is way to much for something simple and secure.

Does anybody know some useful url, paper or idea?

For secrecy, use AES-CBC. For message authentication, use HMAC-SHA256. Use a different key for each.

In both cases, use an existing, validated, timing-attack-free implementation of the cryptographic primitives.

I think you're looking for the Diffie-Hellman key exchange: only requires bignum integer arithmetic (powers, multiplication, and modulus only, at that): http://en.wikipedia.org/wiki/Diffie–Hellman_key_exchange