Improve code for .NET 6.

Author: bgrainger

src/MySqlConnector.Authentication.Ed25519/ParsecAuthenticationPlugin.cs

@@ -32,11 +32,16 @@ public byte[] CreateResponse(string password, ReadOnlySpan<byte> authenticationD
 	{
 		// First 32 bytes are server scramble
 		var serverScramble = authenticationData.Slice(0, 32);
-		
+
 		// Generate client scramble
+#if NET6_0_OR_GREATER || NETSTANDARD2_1_OR_GREATER
+		Span<byte> clientScramble = stackalloc byte[32];
+		RandomNumberGenerator.Fill(clientScramble);
+#else
 		var clientScramble = new byte[32];
 		using var randomNumberGenerator = RandomNumberGenerator.Create();
 		randomNumberGenerator.GetBytes(clientScramble);
+#endif
 
 		// Parse extended salt from remaining auth data
 		var extendedSalt = authenticationData.Slice(32);
@@ -49,7 +54,10 @@ public byte[] CreateResponse(string password, ReadOnlySpan<byte> authenticationD
 		var salt = extendedSalt.Slice(2);
 
 		// Derive private key using PBKDF2-SHA512
-		var privateKey = new byte[32];
+		byte[] privateKey;
+#if NET6_0_OR_GREATER
+		privateKey = Rfc2898DeriveBytes.Pbkdf2(Encoding.UTF8.GetBytes(password), salt, iterationCount, HashAlgorithmName.SHA512, 32);
+#else
 		using (var pbkdf2 = new Rfc2898DeriveBytes(
 			Encoding.UTF8.GetBytes(password),
 			salt.ToArray(),
@@ -58,6 +66,7 @@ public byte[] CreateResponse(string password, ReadOnlySpan<byte> authenticationD
 		{
 			privateKey = pbkdf2.GetBytes(32);
 		}
+#endif
 
 		// Generate Ed25519 keypair and sign concatenated scrambles
 		// var keyPair = Chaos.NaCl.Ed25519.GenerateKeyPair(privateKey);
@@ -71,7 +80,7 @@ public byte[] CreateResponse(string password, ReadOnlySpan<byte> authenticationD
 
 		// Return client scramble followed by signature
 		var response = new byte[clientScramble.Length + signature.Length];
-		clientScramble.CopyTo(response);
+		clientScramble.CopyTo(response.AsSpan());
 		signature.CopyTo(response.AsSpan(clientScramble.Length));
 
 		return response;