I've been working on an open source project called CoinAuth: https://github.com/dangero/CoinAuth

The goal of the project is to allow a user to authenticate their ownership of a Bitcoin address. I've hit a usability snag that I wanted to talk about. In order for the authentication to work, a user needs to be able to sign a message with one of their given Bitcoin addresses' private keys. Is there currently any communication spec between wallet apps?

I'm thinking there are several commands that would be interesting. The wallet app would confirm before sending any information. Here are some of the commands I'm thinking:

1. Sign Message (Message, optional bitcoin address)
2. Get All Private Keys
3. Get Private Key (bitcoin address)

It would also be interesting for 2 apps to be able to share a list of private keys so that when one app adds a new private key the other app gets it immediately.

Does anything like this currently exist?