Add helper to generate padded pair ec signature

Source/AwsCommonRuntimeKit/crt/EC.swift

@@ -83,6 +83,44 @@ public class ECKeyPair {
     }
   }
 
+  /// Decode der ec signature into padded R || S
+  /// both R and S are first padded and then concatenated
+  static public func decodeDerEcSignatureToPaddedPair(signature: Data, padTo: Int) throws -> Data {
+
+    return try signature.withUnsafeBytes { signaturePointer -> Data in
+      let signatureCur = aws_byte_cursor_from_array(
+        signaturePointer.baseAddress,
+        signature.count
+      )
+
+      if padTo > 256 {
+        throw CommonRunTimeError.crtError(CRTError(code: AWS_ERROR_INVALID_ARGUMENT.rawValue))
+      }
+
+      let bufferSize = padTo * 2
+      var outData = Data(count: bufferSize)
+      var newBufferSize = 0
+
+      try outData.withUnsafeMutableBytes { outPointer in
+        var outBuf = aws_byte_buf_from_empty_array(outPointer.baseAddress, bufferSize)
+        guard
+          aws_ecc_decode_signature_der_to_raw_padded(
+            allocator.rawValue,
+            signatureCur,
+            &outBuf,
+            padTo
+          ) == AWS_OP_SUCCESS
+        else {
+          throw CommonRunTimeError.crtError(.makeFromLastError())
+        }
+        newBufferSize = outBuf.len
+      }
+
+      outData.count = newBufferSize
+      return outData
+    }
+  }
+
   /// Encode raw ec signature into der format
   static public func encodeRawECSignature(signature: ECKeyPair.ECRawSignature) throws -> Data {
     return try signature.r.withUnsafeBytes { rPointer in

Test/AwsCommonRuntimeKitTests/crt/ECTests.swift

@@ -63,4 +63,30 @@ class ECTests: XCBaseTestCase {
     XCTAssertTrue(ecKey.verify(digest: sha256, signature: signature))
   }
 
+  func testDecodePadded() throws {
+
+    let signature: [UInt8] = [
+      0x30, 0x42, 0x02, 0x1f, 0x2d, 0x2a, 0xad, 0xce, 0xbc, 0x1b, 0x3f, 0x78,
+      0xec, 0xd1, 0x12, 0x53, 0x9e, 0xc0, 0xe3, 0x44, 0x7b, 0x37, 0x5f, 0x6a,
+      0x99, 0xca, 0x0b, 0x27, 0xb5, 0x4c, 0x31, 0xda, 0x0e, 0x6c, 0x5e, 0x02,
+      0x1f, 0x47, 0xfb, 0x3d, 0xbd, 0xff, 0xb8, 0x58, 0xf4, 0xba, 0x8a, 0x03,
+      0xe7, 0xb4, 0x83, 0xe6, 0xb8, 0xc9, 0x46, 0xa8, 0x0a, 0xd8, 0x46, 0xfa,
+      0x80, 0x0a, 0xd8, 0xca, 0xc5, 0x3f, 0x8e, 0xbd,
+    ]
+    let signatureData = Data(_: signature)
+
+    let expected: [UInt8] = [
+      0x00, 0x2d, 0x2a, 0xad, 0xce, 0xbc, 0x1b, 0x3f, 0x78, 0xec, 0xd1, 0x12, 0x53,
+      0x9e, 0xc0, 0xe3, 0x44, 0x7b, 0x37, 0x5f, 0x6a, 0x99, 0xca, 0x0b, 0x27, 0xb5,
+      0x4c, 0x31, 0xda, 0x0e, 0x6c, 0x5e, 0x00, 0x47, 0xfb, 0x3d, 0xbd, 0xff, 0xb8,
+      0x58, 0xf4, 0xba, 0x8a, 0x03, 0xe7, 0xb4, 0x83, 0xe6, 0xb8, 0xc9, 0x46, 0xa8,
+      0x0a, 0xd8, 0x46, 0xfa, 0x80, 0x0a, 0xd8, 0xca, 0xc5, 0x3f, 0x8e, 0xbd,
+    ]
+    let expectedData = Data(_: expected)
+
+    let raw = try ECKeyPair.decodeDerEcSignatureToPaddedPair(signature: signatureData, padTo: 32)
+
+    XCTAssertEqual(expectedData, raw)
+  }
+
 }