Some systems, that operate over flaky bearers, need to send signed, encrypted messages that are entirely self contained - i.e. send a single 300 byte UDP rather than have a 10Kb, two-way, certificate conversation over TCP.
Assume the destination has a private key and already knows the source's public key, and the source has a private key and already knows the destination's public key.
Given this assumption, is there a standard for encrypting/signing a small, but arbitrary length, message?
(I'm well aware of the dangers of rolling my own encryption scheme.)
Would there be a minimum ciphertext size that is related to the public key size?
(I'm thinking plain PKI encryption for plaintexts shorter than the key, and a PKI encrypted AES key for longer plaintexts.)