Add the crypto format spec

We would like to split the current encrypt/decrypt specification into three different parts:

- Message Format (which we already have)
- Crypto Format
- Client APIs

Start by locating the current encrypt/decrypt specification and reviewing the existing Message Format section. Determine how the proposed Crypto Format and Client APIs parts should be separated and what material belongs in each. Done means the specification is organized into the three named parts with clear boundaries and complete content.

Written by the indexing model from the issue text.