Add Secp curve library and function to derive address from priv key

.gitmodules

@@ -7,3 +7,6 @@
 [submodule "lib/Solidity-RLP"]
 	path = lib/Solidity-RLP
 	url = https://github.com/hamdiallam/Solidity-RLP
+[submodule "lib/elliptic-curve-solidity"]
+	path = lib/elliptic-curve-solidity
+	url = https://github.com/witnet/elliptic-curve-solidity

README.md

@@ -170,6 +170,22 @@ contract Example {
 }
 ```
 
+## crypto/Secp256k1.sol
+
+Helper library to interact with the `secp256k1` curve.
+
+```solidity
+import "src/crypto/Secp256k1.sol";
+
+contract Example {
+    function example() public {
+        // string memory privateKey = Suave.privateKeyGen(Suave.CryptoSignature.SECP256);
+        string memory privateKey = "b71c71a67e1177ad4e901695e1b4b9ee17ae16c6668d313eac2f96dbcda3f291";
+        address found = Secp256k1.deriveAddress(privateKey);
+    }
+}
+```
+
 ## Forge integration
 
 In order to use `forge`, you need to have a running `Suave` node and the `suave` binary in your path.
@@ -210,13 +226,14 @@ Use the `setConfidentialInputs` function to set the confidential inputs during t
 import "forge-std/Test.sol";
 import "src/Test.sol";
 import "src/suavelib/Suave.sol";
+import "src/Context.sol";
 
 contract TestForge is Test, SuaveEnabled {
     function testConfidentialInputs() public {
         bytes memory input = hex"abcd";
         ctx.setConfidentialInputs(input);
 
-        bytes memory found2 = Suave.confidentialInputs();
+        bytes memory found2 = Context.confidentialInputs();
         assertEq0(input, found2);
     }
 }

src/crypto/Secp256k1.sol

@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: Unlicense
+pragma solidity ^0.8.0;
+
+import "elliptic-curve-solidity/contracts/EllipticCurve.sol";
+import "../utils/HexStrings.sol";
+
+/// @notice Secp256k1 is a library with utilities to work with the secp256k1 curve.
+library Secp256k1 {
+    uint256 internal constant GX = 0x79BE667EF9DCBBAC55A06295CE870B07029BFCDB2DCE28D959F2815B16F81798;
+    uint256 internal constant GY = 0x483ADA7726A3C4655DA4FBFC0E1108A8FD17B448A68554199C47D08FFB10D4B8;
+    uint256 internal constant AA = 0;
+    uint256 internal constant BB = 7;
+    uint256 internal constant PP = 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC2F;
+    uint256 internal constant NN = 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141;
+
+    function derivePubKey(uint256 privKey) internal pure returns (uint256 qx, uint256 qy) {
+        (qx, qy) = EllipticCurve.ecMul(privKey, GX, GY, AA, PP);
+    }
+
+    function deriveAddress(uint256 privKey) internal pure returns (address) {
+        (uint256 qx, uint256 qy) = derivePubKey(privKey);
+        bytes memory ser = bytes.concat(bytes32(qx), bytes32(qy));
+        return address(uint160(uint256(keccak256(ser))));
+    }
+
+    // @notice deriveAddress returns the address corresponding to the private key
+    // @param privKey is the private key
+    // @return address is the address derived from the private key
+    function deriveAddress(string memory privKey) internal pure returns (address) {
+        bytes memory privKeyBytes = HexStrings.fromHexString(privKey);
+        require(privKeyBytes.length == 32, "Invalid private key length");
+
+        return deriveAddress(uint256(bytes32(privKeyBytes)));
+    }
+}

test/crypto/Secp256k1.t.sol

@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: Unlicense
+pragma solidity ^0.8.13;
+
+import "forge-std/Test.sol";
+import "src/Test.sol";
+import "src/suavelib/Suave.sol";
+import "src/Context.sol";
+import "src/crypto/Secp256k1.sol";
+
+contract TestSecp256K1 is Test, SuaveEnabled {
+    function testRecoverAddress() public {
+        string memory signingKey = "b71c71a67e1177ad4e901695e1b4b9ee17ae16c6668d313eac2f96dbcda3f291";
+
+        address expected = 0x71562b71999873DB5b286dF957af199Ec94617F7;
+        address found = Secp256k1.deriveAddress(signingKey);
+        assert(expected == found);
+    }
+}

tools/stdchecker/remappings.txt

@@ -3,3 +3,4 @@ Solidity-RLP/=../../lib/Solidity-RLP/contracts/
 ds-test/=../../lib/forge-std/lib/ds-test/src/
 forge-std/=../../lib/forge-std/src/
 solady/=../../lib/solady/
+elliptic-curve-solidity/=../../lib/elliptic-curve-solidity