Add messages for crypto operations

**Describe the solution you'd like**
It is not possible to perform common crypto oprations.

- [ ] Encrypt / decrypt key value
- [ ] Retrieve and sign identities
- [ ] Generate [ECDH](https://en.wikipedia.org/wiki/Elliptic_curve_Diffie-Hellman) session key
- [ ] Collective signature (a.k.a [CoSi](http://arxiv.org/abs/1602.06997))

**Additional context**
Related to #79

Start by reading issue #79 and the repository's protocol buffer schemas to understand the related protocol context. The requested scope covers encryption/decryption, identity retrieval and signing, ECDH session keys, and CoSi, so completion would require agreed message definitions for these operations.

Written by the indexing model from the issue text.