Extract event signing into its own class

lib/nostr/crypto.rb

@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+module Nostr
+  # Performs cryptographic operations on a +Nostr::Event+.
+  class Crypto
+    # Uses the private key to generate an event id and sign the event
+    #
+    # @api public
+    #
+    # @example Signing an event
+    #   crypto = Nostr::Crypto.new
+    #   crypto.sign(event, private_key)
+    #   event.id # => an id
+    #   event.sig # => a signature
+    #
+    # @param event [Event] The event to be signed
+    # @param private_key [String] 32-bytes hex-encoded private key.
+    #
+    # @return [Event] An unsigned event.
+    #
+    def sign_event(event, private_key)
+      event_digest = hash_event(event)
+
+      hex_private_key = Array(private_key).pack('H*')
+      hex_message     = Array(event_digest).pack('H*')
+      event_signature = Schnorr.sign(hex_message, hex_private_key).encode.unpack1('H*')
+
+      event.id = event_digest
+      event.sig = event_signature
+
+      event
+    end
+
+    private
+
+    # Generates a SHA256 hash of a +Nostr::Event+
+    #
+    # @api private
+    #
+    # @param event [Event] The event to be hashed
+    #
+    # @return [String] A SHA256 digest of the event
+    #
+    def hash_event(event)
+      Digest::SHA256.hexdigest(JSON.dump(event.serialize))
+    end
+  end
+end