Use AsnWriter in EcDsaSigFromSig

Author: iamcarbon

Test/Fido2Tests.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Buffers.Binary;
 using System.Collections.Generic;
+using System.Formats.Asn1;
 using System.IO;
 using System.Linq;
 using System.Security.Cryptography;
@@ -718,16 +719,23 @@ public void TestAuthenticatorData()
             Assert.True(ad.Extensions.GetBytes().SequenceEqual(extBytes));
         }
 
-        internal static byte[] EcDsaSigFromSig(byte[] sig, int keySize)
+        internal static byte[] EcDsaSigFromSig(ReadOnlySpan<byte> sig, int keySize)
         {
             var coefficientSize = (int)Math.Ceiling((decimal)keySize / 8);
-            var R = sig.Take(coefficientSize);
-            var S = sig.TakeLast(coefficientSize);
+            var r = sig.Slice(0, coefficientSize);
+            var s = sig.Slice(sig.Length - coefficientSize);
 
-            var intR = AsnElt.MakeInteger(R.ToArray());
-            var intS = AsnElt.MakeInteger(S.ToArray());
-            var ecdsasig = AsnElt.Make(AsnElt.SEQUENCE, new AsnElt[] { intR, intS });
-            return ecdsasig.Encode();
+            var writer = new AsnWriter(AsnEncodingRules.BER);
+
+            ReadOnlySpan<byte> zero = new byte[1] { 0 };
+
+            using (writer.PushSequence())
+            {
+                writer.WriteIntegerUnsigned(r.TrimStart(zero));
+                writer.WriteIntegerUnsigned(s.TrimStart(zero));
+            }
+
+            return writer.Encode();
         }
 
         [Fact]