1 files changed, 49 insertions, 0 deletions

diff --git a/Crypto/XEd25519/FieldElement.hs b/Crypto/XEd25519/FieldElement.hs
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+++ b/Crypto/XEd25519/FieldElement.hs
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+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE TypeOperators #-}
+module Crypto.XEd25519.FieldElement where
+
+import Crypto.Error
+import qualified Crypto.PubKey.Curve25519 as X25519
+import qualified Crypto.PubKey.Ed25519 as Ed25519
+import Data.ByteArray as BA (pack,unpack,Bytes)
+import Data.Modular
+import Data.Word
+
+-- 2^255 - 19
+type P25519 = 57896044618658097711785492504343953926634992332820282019728792003956564819949
+
+newtype FieldElement = FE (ℤ / P25519)
+
+
+fe_frombytes :: X25519.PublicKey -> FieldElement
+fe_frombytes pub = FE $ toMod $ decodeLittleEndian $ BA.unpack pub
+
+fe_tobytes :: FieldElement -> Ed25519.PublicKey
+fe_tobytes (FE x) = throwCryptoError $ Ed25519.publicKey (b :: Bytes)
+ where
+    b = BA.pack $ take 32 $ (encodeLittleEndian $ unMod x) ++ repeat 0
+
+fe_1 :: FieldElement
+fe_1 = FE $ toMod 1
+
+fe_sub :: FieldElement -> FieldElement -> FieldElement
+fe_sub (FE x) (FE y) = FE $ x - y
+
+fe_add :: FieldElement -> FieldElement -> FieldElement
+fe_add (FE x) (FE y) = FE $ x + y
+
+fe_invert :: FieldElement -> FieldElement
+fe_invert (FE x) = FE $ inv x
+
+fe_mul :: FieldElement -> FieldElement -> FieldElement
+fe_mul (FE x) (FE y) = FE (x * y)
+
+decodeLittleEndian :: [Word8] -> Integer
+decodeLittleEndian [] = 0
+decodeLittleEndian (x:xs) = fromIntegral x + 256 * decodeLittleEndian xs
+
+encodeLittleEndian :: Integer -> [Word8]
+encodeLittleEndian 0 = []
+encodeLittleEndian x = let (bs,b) = divMod x 256
+                        in fromIntegral b : encodeLittleEndian bs
+