Swap Kdf subkey_size and subkey_id arguments.

README.md

@@ -131,11 +131,11 @@ output = d.hexdigest
 ```crystal
 kdf = Cox::Kdf.new
 
-# kdf.derive(8_byte_context, subkey_size, subkey_id)
-subkey1 = kdf.derive "context1", 16, 0
-subkey2 = kdf.derive "context1", 16, 1
-subkey3 = kdf.derive "context2", 32, 0
-subkey4 = kdf.derive "context2", 64, 1
+# kdf.derive(8_byte_context, subkey_id, subkey_size)
+subkey1 = kdf.derive "context1", 0, 16
+subkey2 = kdf.derive "context1", 1, 16
+subkey3 = kdf.derive "context2", 0, 32
+subkey4 = kdf.derive "context2", 1, 64
 ```
 
 ### Password Hashing

spec/cox/kdf_spec.cr

@@ -10,15 +10,15 @@ describe Cox::Kdf do
     kdf2 = Cox::Kdf.from_base64 kdf1.to_base64
 
     # verify generated subkey's are the same after loading
-    key1_s1 = kdf1.derive CONTEXT, 16, 0
-    key2_s1 = kdf2.derive CONTEXT, 16, 0
+    key1_s1 = kdf1.derive CONTEXT, 0, 16
+    key2_s1 = kdf2.derive CONTEXT, 0, 16
     key1_s1.should eq key2_s1
   end
 
   it "generates different keys" do
     kdf1 = Cox::Kdf.new
-    subkey1 = kdf1.derive CONTEXT, 16, 0
-    subkey2 = kdf1.derive CONTEXT, 16, 1
+    subkey1 = kdf1.derive CONTEXT, 0, 16
+    subkey2 = kdf1.derive CONTEXT, 1, 16
     subkey1.should_not eq subkey2
   end
 

src/cox/kdf.cr

@@ -16,7 +16,7 @@ module Cox
 
     # context must be 8 bytes
     # subkey_size must be 16..64 bytes as of libsodium 1.0.17
-    def derive(context, subkey_size, subkey_id = 0)
+    def derive(context, subkey_id, subkey_size)
       if context.bytesize != LibSodium::KDF_CONTEXT_SIZE
         raise ArgumentError.new("context must be #{LibSodium::KDF_CONTEXT_SIZE}, got #{context.bytesize}")
       end