RSA functions

Encryption and decryption, singing and verification.
Verification function depends on fixed message length.
Also checks hash, nonce.

rsa.outgoing() and rsa.incomming() are the only functions that should need to be directly called.  Outgoing generates ctxt according to protocol (hashed, signed, encrypted).  Still need nonce generation.  Incomming decrypts and verifies.  Returns None if verification fails, ptxt otherwise.