Abstraction layer for crypto primitives

[peacekeeper]

Currently, we store private keys inside XDI graphs. They are persisted in the graph store. We need some abstraction layer here that allows private keys to be handled more securely. Similar considerations apply to the handling of secret tokens.

There is a certain abstraction mechanism for some crypto operations already in place. For example, in order to validate signatures on an incoming XDI message, the corresponding public key could be obtained using XDI discovery, it could be hardcoded in a configuration file, or it could come from other sources. This abstraction is a good start, but needs to be improved.

See also:

https://github.com/projectdanube/xdi2/wiki/AuthenticationSignatureInterceptor
https://github.com/projectdanube/xdi2/wiki/AuthenticationSecretTokenInterceptor