Updated the javadoc of the PRT field. Now, the public key is directly used as 'id' of the participant.
Removed the id attribute since now the public key works as 'id'.
Changed the usage of the participant since now the public key is used as id (there is not participant id anymore).
Changed the format of the url a little. Now it's inchat:ServerPublicKeyAsHex.ClientPublicKeyAsHex. It's just simpler that way.
The cipher now uses sha-256 as kdf digest, aes-256 instead of aes-128 and 256 bit to key the hmac (which also uses sha-256).