Continue #8

spec/sodium/sign/secret_key_spec.cr

@@ -76,6 +76,16 @@ describe Sodium::Sign::SecretKey do
     message = "foo"
     sskey = Sodium::Sign::SecretKey.new
     cskey = sskey.to_curve25519
+
+    spkey = sskey.public_key
+    cpkey = spkey.to_curve25519
+
+    data = "foo".to_slice
+    cskey.box cpkey do |box|
+      enc, nonce = box.encrypt data
+      dec = box.decrypt enc, nonce: nonce
+      dec.should eq data
+    end
   end
 
   it "RbNaCl detached test vectors" do

src/sodium/sign/public_key.cr

@@ -35,6 +35,12 @@ module Sodium
       end
     end
 
+    def to_curve25519 : CryptoBox::PublicKey
+      pk = CryptoBox::PublicKey.new
+      LibSodium.crypto_sign_ed25519_pk_to_curve25519 pk.to_slice, @bytes
+      pk
+    end
+
     module SerializeConverter
       def self.to_json(value : PublicKey, json : JSON::Builder)
         json.string Base64.strict_encode(value.to_slice)