implement signature support for get/put data

[thomasdelaet]

If I put a value, it should be signed with my own public key. If we enable encryption on the transport protocol of KAD, other hosts can reject all values that are being PUT from a connection that doesnt match the signature of the value that is being PUT. In addition: - If the key is the public Key itself, we can also reject any signature that doesnt match the public key (only relevant for publishing connectionInfo, not for lookup of email addresses).

probably makes sense to include nonce

[thomasdelaet]

Assume that a PUT contains key => value mapping where value is a dictionary with the following keys:

• payload: actual data
• nonce: expiration time
• signature: elliptic curve signature
• signID: public key of signer

When you do a get and have a result returned you verify:

• that signature is valid
• that expiration time is not yet reached
• potentially using access control list you can check boxId by conerting signId to boxID