Rabin signature

docs/index.rst

@@ -37,6 +37,7 @@ Script is designed to facilitate writing smart contract running on chain.
    p2pkh
    rpuzzle
    ackermann
+   rabin
 
 ..
     contributing: PR for example contracts

docs/rabin.rst

@@ -0,0 +1,41 @@
+===============
+Rabin Signature
+===============
+
+`Rabin signature <https://nchain.com/app/uploads/2018/09/Rabin-Signatures-in-Bitcoin-Cash.pdf>`_ is an alternative form of digital signature to ECDSA used in Bitcoin.
+
+.. code-block:: solidity
+
+    contract RabinSignature {
+        Ripemd160 nHash;
+
+        constructor() {
+            nHash = Ripemd160(0x88d9931ea73d60eaf7e5671efc0552b912911f2a);
+        }
+
+        function hash3072(bytes x) returns (bytes) {
+            Sha256 y = sha256(x);
+
+            int i = 0;
+            // start with empty
+            bytes ret = 0x;
+            // divide into 3072 / 256 = 12 slices
+            loop (12) {
+                // 256 bits is 32 bytes
+                bytes slice = y[32 * i : 32 * (i+1)];
+                // concatenate
+                ret = ret . sha256(slice);
+                i = i + 1;
+            }
+
+            // 3072 bit hash
+            return ret;
+        }
+
+        public function verifySig(int S, bytes U, bytes m, int lambda, bytes n) {
+            require(hash160(n) == nHash);
+
+            int h = bin2num(hash3072(m . U));
+            require(S * S == h + lambda * bin2num(n));
+        }
+    }