n base 64

Examples:

Unlock the wallet for 30 seconds
> bitcoin-cli walletpassphrase "mypassphrase" 30

Create the signature
> bitcoin-cli signmessage "1D1ZrZNe3JUo7ZycKEYQQiQAWd9y54F4XX" "my message"

Verify the signature
> bitcoin-cli verifymessage "1D1ZrZNe3JUo7ZycKEYQQiQAWd9y54F4XX" "signature" "my message"

As a JSON-RPC call
> curl --user myusername --data-binary '{"jsonrpc": "1.0", "id": "curltest", "method": "signmessage", "params": ["1D1ZrZNe3JUo7ZycKEYQQiQAWd9y54F4XX", "my message"]}' -H 'content