At the Berlin Crypto this month we had a talk by Kevin about a shared-secret service they developed at Syselevn. After experimenting with PGP and deciding that it doesn’t do what they needed, they decided to go with a very simple, custom encrypt-then-mac scheme. You can find details here. When someone says they built their own encryption scheme and message format I get obviously curious. In this post I want to summarize the scheme, design decisions, compare it standard authenticated encryption schemes, and ponder the question of the right security definitions.