misc: edits to StoreDataCommand and ChannelEncryption

ChannelEncyrption: use Span<byte> instead of byte[]
StoreDataCommand: change order of fields

Yubico.YubiKey/src/Yubico/YubiKey/Scp/Commands/StoreDataCommand.cs

@@ -28,11 +28,10 @@ namespace Yubico.YubiKey.Scp.Commands
     /// </remarks>
     internal class StoreDataCommand : IYubiKeyCommand<StoreDataCommandResponse>
     {
+        public YubiKeyApplication Application => YubiKeyApplication.SecurityDomain;
         private const byte GpStoreDataIns = 0xE2;
         private readonly ReadOnlyMemory<byte> _data;
 
-        public YubiKeyApplication Application => YubiKeyApplication.SecurityDomain;
-
         /// <summary>
         /// Initializes a new instance of the <see cref="StoreDataCommand"/> class, with the given data to be stored.
         /// </summary>

Yubico.YubiKey/src/Yubico/YubiKey/Scp/Helpers/ChannelEncryption.cs

@@ -36,14 +36,14 @@ public static ReadOnlyMemory<byte> EncryptData(
             ReadOnlySpan<byte> encryptionKey,
             int encryptionCounter)
         {
-            // NB: Could skip this if the payload is empty (rather than sending a 16-byte encrypted '0x800000...' payload
-            byte[] countBytes = new byte[sizeof(int)];
+            Span<byte> countBytes = stackalloc byte[sizeof(int)];
             BinaryPrimitives.WriteInt32BigEndian(countBytes, encryptionCounter);
 
-            byte[] ivInput = new byte[16];
-            countBytes.CopyTo(ivInput, 16 - countBytes.Length); // copy to rightmost part of block
-            var iv = AesUtilities.BlockCipher(encryptionKey, ivInput);
+            Span<byte> ivInput = stackalloc byte[16];
+            int offset = 16 - countBytes.Length;
+            countBytes.CopyTo(ivInput[offset..]);
 
+            var iv = AesUtilities.BlockCipher(encryptionKey, ivInput);
             var paddedPayload = Padding.PadToBlockSize(dataToEncrypt);
             var encryptedData = AesUtilities.AesCbcEncrypt(encryptionKey, iv.Span, paddedPayload.Span);