Add ECDH support, tested with Wycheproof test vectors

Author: Sajjon

.gitignore

@@ -0,0 +1,7 @@
+.DS_Store
+/.build
+/Packages
+/*.xcodeproj
+xcuserdata/
+DerivedData/
+.swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata

.swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata

@@ -23,7 +23,8 @@ let package = Package(
 
     dependencies: [
         .package(url: "https://github.com/apple/swift-crypto.git", "2.0.0" ..< "3.0.0"),
-        .package(url: "https://github.com/attaswift/BigInt.git", from: "5.3.0")
+        .package(url: "https://github.com/attaswift/BigInt.git", from: "5.3.0"),
+        .package(url: "https://github.com/filom/ASN1Decoder", .branch("master"))
     ],
 
     targets: [
@@ -51,6 +52,7 @@ let package = Package(
                 "libsecp256k1/autogen.sh",
                 "libsecp256k1/libsecp256k1.pc.in",
                 "libsecp256k1/doc",
+                "libsecp256k1/contrib",
                 "libsecp256k1/ci",
                 "libsecp256k1/sage",
                 "libsecp256k1/build-aux",
@@ -59,19 +61,21 @@ let package = Package(
                 "libsecp256k1/COPYING",
                 "libsecp256k1/SECURITY.md"
             ],
+            sources: ["src", "libsecp256k1/src"],
             cSettings: [
-                .headerSearchPath("secp256k1"),
+                .headerSearchPath("secp256k1/include/"),
                 // Basic config values that are universal and require no dependencies.
                 // https://github.com/bitcoin-core/secp256k1/blob/master/src/basic-config.h#L12-L13
                 .define("ECMULT_WINDOW_SIZE", to: "15"),
                 .define("ECMULT_GEN_PREC_BITS", to: "4"),
-                // Enabling additional secp256k1 modules.
-                .define("SECP256K1_ECDH_H"),
-                .define("SECP256K1_MODULE_ECDH_MAIN_H"),
-                .define("SECP256K1_EXTRAKEYS_H"),
-                .define("SECP256K1_MODULE_EXTRAKEYS_MAIN_H"),
-                .define("SECP256K1_SCHNORRSIG_H"),
-                .define("SECP256K1_MODULE_SCHNORRSIG_MAIN_H"),
+
+                // Enable modules in secp256k1.
+                // See bottom of: Sources/secp256k1/libsecp256k1/src/secp256k1.c
+                // For list
+                .define("ENABLE_MODULE_ECDH"),
+                .define("ENABLE_MODULE_RECOVERY"),
+                .define("ENABLE_MODULE_SCHNORRSIG"),
+                .define("ENABLE_MODULE_EXTRAKEYS"),
             ]
         ),
 
@@ -87,7 +91,9 @@ let package = Package(
         .testTarget(
             name: "K1Tests",
             dependencies: [
-                "K1"
+                "K1",
+//                "ASN1Swift"
+                "ASN1Decoder"
             ],
             resources: [
                 .process("Resources/"),

.swiftpm/xcode/package.xcworkspace/xcuserdata/sajjon.xcuserdatad/UserInterfaceState.xcuserstate

@@ -29,6 +29,7 @@ public extension K1 {
         case failedToSerializeCompactSignature
         case failedToSerializeDERSignature
         case failedToSignDigest
+        case failedToPerformDiffieHellmanKeyExchange
     }
 
 }

.swiftpm/xcode/xcuserdata/sajjon.xcuserdatad/xcschemes/xcschememanagement.plist

@@ -34,6 +34,45 @@ extension Bridge {
             count: MemoryLayout.size(ofValue: signatureBridgedToC.data)
         )
     }
+    
+    static func ecdh(
+        publicKey publicKeyBytes: [UInt8],
+        privateKey: SecureBytes
+    ) throws -> Data {
+
+        var publicKeyBridgedToC = secp256k1_pubkey()
+
+        try Self.call(ifFailThrow: .failedToSerializePublicKeyIntoBytes) { context in
+            /* "Serialize a pubkey object into a serialized byte sequence." */
+            secp256k1_ec_pubkey_parse(
+                context,
+                &publicKeyBridgedToC,
+                publicKeyBytes,
+                publicKeyBytes.count
+            )
+        }
+
+        var sharedPublicPointBytes = [UInt8](
+            repeating: 0,
+            count: K1.Format.uncompressed.length
+        )
+        
+        try Self.call(
+            ifFailThrow: .failedToPerformDiffieHellmanKeyExchange
+        ) { context in
+            /** Compute an EC Diffie-Hellman secret in constant time
+             */
+            secp256k1_ecdh(
+                context,
+                &sharedPublicPointBytes, // output
+                &publicKeyBridgedToC, // pubkey
+                privateKey.backing.bytes, // seckey
+                ecdh_skip_hash_extract_x_and_y, // hashfp
+                nil // arbitrary data pointer that is passed through to hashfp
+            )
+        }
+        return Data(sharedPublicPointBytes)
+    }
 }
 
 public extension K1.PrivateKey {
@@ -47,4 +86,17 @@ public extension K1.PrivateKey {
             signatureData
         )
     }
+    
+    /// Performs a key agreement with provided public key share.
+    ///
+    /// - Parameter publicKeyShare: The public key to perform the ECDH with.
+    /// - Returns: Returns the public point obtain by performing EC mult between
+    ///  this `privateKey` and `publicKeyShare`
+    /// - Throws: An error occurred while computing the shared secret
+    func sharedSecret(with publicKeyShare: K1.PublicKey) throws -> Data {
+        let sharedSecretData = try withSecureBytes { secureBytes in
+            try Bridge.ecdh(publicKey: publicKeyShare.rawRepresentation, privateKey: secureBytes)
+        }
+        return sharedSecretData
+    }
 }

Package.resolved

@@ -67,7 +67,7 @@ internal extension K1.PrivateKey.Wrapped {
 
         var attempt = 0
 
-        while attempt < 10_000 {
+        while attempt < 100 {
             defer { attempt += 1 }
             do {
                 let secureBytes = SecureBytes(count: Self.byteCount)
@@ -81,7 +81,7 @@ internal extension K1.PrivateKey.Wrapped {
 
         // Probability of this happening is:
         // n = 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141
-        // (n / 2^256) ^ 10_000 = lim 0
+        // (n / 2^256) ^ 100 = lim 0
         // I.e. will not happen.
         fatalError("""
             Failed to generate private key after #\(attempt) attempts.