Add Ed25519.pointsMulVarTime

Crypto/ECC/Ed25519.hs

@@ -26,6 +26,7 @@ module Crypto.ECC.Ed25519
     , pointAdd
     , pointDouble
     , pointMul
+    , pointsMulVarTime
     ) where
 
 import           Data.Bits
@@ -204,6 +205,21 @@ pointMul (Scalar scalar) (Point base) =
         withByteArray base   $ \pbase   ->
              ed25519_point_scalarmul out pbase pscalar
 
+-- | Multiply the point @p@ with @s2@ and add a lifted to curve value @s1@.
+--
+-- @
+-- pointsMulVarTime s1 s2 p = 'pointAdd' ('toPoint' s1) ('pointMul' s2 p)
+-- @
+--
+-- /WARNING:/ variable time
+pointsMulVarTime :: Scalar -> Scalar -> Point -> Point
+pointsMulVarTime (Scalar s1) (Scalar s2) (Point p) =
+    Point $ B.allocAndFreeze pointArraySize $ \out ->
+        withByteArray s1 $ \ps1 ->
+        withByteArray s2 $ \ps2 ->
+        withByteArray p  $ \pp  ->
+             ed25519_base_double_scalarmul_vartime out ps1 pp ps2
+
 foreign import ccall "cryptonite_ed25519_scalar_eq"
     ed25519_scalar_eq :: Ptr Scalar
                       -> Ptr Scalar
@@ -273,3 +289,10 @@ foreign import ccall "cryptonite_ed25519_point_scalarmul"
                             -> Ptr Point  -- base
                             -> Ptr Scalar -- scalar
                             -> IO ()
+
+foreign import ccall "cryptonite_ed25519_base_double_scalarmul_vartime"
+    ed25519_base_double_scalarmul_vartime :: Ptr Point  -- combo
+                                          -> Ptr Scalar -- scalar1
+                                          -> Ptr Point  -- base2
+                                          -> Ptr Scalar -- scalar2
+                                          -> IO ()

cbits/ed25519/ed25519-cryptonite-exts.h

@@ -138,3 +138,9 @@ ED25519_FN(ed25519_point_scalarmul) (ge25519 *r, const ge25519 *p, const bignum2
         }
     }
 }
+
+void
+ED25519_FN(ed25519_base_double_scalarmul_vartime) (ge25519 *r, const bignum256modm s1, const ge25519 *p2, const bignum256modm s2) {
+    // computes [s1]basepoint + [s2]p2
+    ge25519_double_scalarmult_vartime(r, p2, s2, s1);
+}

tests/ECC/Ed25519.hs

@@ -87,6 +87,9 @@ tests = testGroup "ECC.Ed25519"
         , testProperty "scalarmult distributive" $ \x y p ->
             let pR = Ed25519.pointMul x p `Ed25519.pointAdd` Ed25519.pointMul y p
              in pR `propertyEq` Ed25519.pointMul (x `Ed25519.scalarAdd` y) p
+        , testProperty "double scalarmult" $ \n1 n2 p ->
+            let pR = Ed25519.pointAdd (Ed25519.toPoint n1) (Ed25519.pointMul n2 p)
+             in pR `propertyEq` Ed25519.pointsMulVarTime n1 n2 p
         ]
     ]
   where