Add format conversion function for convenience (#707)

Author: zeugmaster

Sources/zkp/Asymmetric.swift

@@ -151,6 +151,24 @@ public extension secp256k1 {
                     return try Self(dataRepresentation: negatedKey.dataRepresentation, format: negatedKey.format)
                 }
             }
+            
+            /// Returns a public key in uncompressed 65 byte form
+            public var uncompressedRepresentation: Data {
+                let context = secp256k1.Context.rawRepresentation
+                var pubKey = rawRepresentation
+                var pubKeyLen = ByteLength.uncompressedPublicKey
+                var pubKeyBytes = [UInt8](repeating: 0, count: pubKeyLen)
+
+                _ = secp256k1_ec_pubkey_serialize(
+                    context,
+                    &pubKeyBytes,
+                    &pubKeyLen,
+                    &pubKey,
+                    UInt32(SECP256K1_EC_UNCOMPRESSED)
+                )
+
+                return Data(pubKeyBytes)
+            }
 
             /// Generates a secp256k1 public key.
             ///

Sources/zkp/secp256k1.swift

@@ -64,6 +64,9 @@ extension secp256k1 {
         /// Number of bytes in a secp256k1 signature.
         @inlinable
         static var partialSignature: Int { 36 }
+        
+        @inlinable
+        static var uncompressedPublicKey: Int { 65 }
     }
 }
 

Tests/zkpTests/secp256k1Tests.swift

@@ -648,6 +648,12 @@ final class secp256k1Tests: XCTestCase, @unchecked Sendable {
 
         XCTAssertEqual(combinedKeyBytes, expectedCombinedKey)
     }
+    
+    func testKeyFormatConversion() throws {
+        let pubkey = try secp256k1.Signing.PublicKey(dataRepresentation: "02a9acc1e48c25eeeb9289b5031cc57da9fe72f3fe2861d264bdc074209b107ba2".bytes, format: .compressed)
+        let uncompressed = pubkey.uncompressedRepresentation
+        XCTAssertEqual(String(bytes: uncompressed.bytes), "04a9acc1e48c25eeeb9289b5031cc57da9fe72f3fe2861d264bdc074209b107ba270b2031fef3acf8e13ea7a395e375491bdc37be1cd79e073d82bfd5ba8d35d68")
+    }
 
     func testPrivateKeyPEM() {
         let privateKeyString = """