Add tweak negation (#28)

src/Crypto/Secp256k1.hs

@@ -57,6 +57,7 @@ module Crypto.Secp256k1
     , tweakAddPubKey
     , tweakMulPubKey
     , combinePubKeys
+    , tweakNegate
 
 #ifdef ECDH
     -- * Diffie Hellman
@@ -480,6 +481,18 @@ recover (RecSig frg) (Msg fm) = withContext $ \ctx ->
         ret <- withForeignPtr fp $ \pp -> ecdsaRecover ctx pp prg pm
         if isSuccess ret then return $ Just $ PubKey fp else return Nothing
 
+tweakNegate :: Tweak -> Maybe Tweak
+tweakNegate (Tweak fk) = withContext $ \ctx -> do
+    fnew <- mallocForeignPtr
+    peeked <- withForeignPtr fk peek
+    ret <- withForeignPtr fnew $ \n -> do
+        poke n peeked
+        ecTweakNegate ctx n
+    return $
+        if isSuccess ret
+            then Just (Tweak fnew)
+            else Nothing
+
 #ifdef ECDH
 -- | Compute Diffie-Hellman secret.
 ecdh :: PubKey -> SecKey -> ByteString

src/Crypto/Secp256k1/Internal.hs

@@ -391,6 +391,13 @@ foreign import ccall
     -> Ptr Tweak32
     -> IO Ret
 
+foreign import ccall
+    "secp256k1.h secp256k1_ec_privkey_negate"
+    ecTweakNegate
+    :: Ptr Ctx
+    -> Ptr Tweak32
+    -> IO Ret
+
 foreign import ccall
     "secp256k1.h secp256k1_ec_pubkey_tweak_add"
     ecPubKeyTweakAdd

test/Crypto/Secp256k1Spec.hs

@@ -52,6 +52,7 @@ spec = do
         it "add public key" $ property $ tweakAddPubKeyTest
         it "multiply public key" $ property $ tweakMulPubKeyTest
         it "combine public keys" $ property $ combinePubKeyTest
+        it "negates tweak" $ property $ negateTweakTest
 #ifdef ECDH
     describe "ecdh" $ do
         it "computes dh secret" $ property $ computeDhSecret
@@ -239,6 +240,18 @@ combinePubKeyTest =
     expected = importPubKey $ fst $ B16.decode $ B8.pack
         "043d9a7ec70011efc23c33a7e62d2ea73cca87797e3b659d93bea6aa871aebde56c3bc6134ca82e324b0ab9c0e601a6d2933afe7fb5d9f3aae900f5c5dc6e362c8"
 
+negateTweakTest :: Assertion
+negateTweakTest =
+    assertEqual "can recover secret key 1 after adding tweak 1" oneKey subtracted
+  where
+    Just oneKey = secKey $ fst $ B16.decode $ B8.pack
+        "0000000000000000000000000000000000000000000000000000000000000001"
+    Just oneTwk = tweak $ fst $ B16.decode $ B8.pack
+        "0000000000000000000000000000000000000000000000000000000000000001"
+    Just minusOneTwk = tweakNegate oneTwk
+    Just twoKey = tweakAddSecKey oneKey oneTwk
+    Just subtracted = tweakAddSecKey twoKey minusOneTwk
+
 #ifdef ECDH
 computeDhSecret :: Assertion
 computeDhSecret =