I understand that Nxt uses Curve25519 to generate pubkeys for user accounts, however, I can't find a reference that states what which signing algorithm/process is used for transactions and blocks. (Sorry, I also don't have access to the source at the moment.)

How does Nxt sign and validate transactions and blocks? Note, I'm not interested in discussing POS. I'm strictly interested in how Nxt signs and verifies its data.