avoid allocation of key in trithemius cipher

Author: Peter-Juhasz

src/Science.Cryptography.Ciphers/Ciphers/TrithemiusCipher.cs

@@ -13,17 +13,19 @@ public TrithemiusCipher(Alphabet alphabet)
 	{
 		Alphabet = alphabet;
 		_inner = new(alphabet);
+		_innerKey = alphabet.ToCharArray();
 	}
 	public TrithemiusCipher()
 		: this(WellKnownAlphabets.English)
 	{ }
 
 	private readonly VigenèreCipher _inner;
+	private readonly char[] _innerKey;
 
 	public Alphabet Alphabet { get; }
 
 
-	public void Encrypt(ReadOnlySpan<char> plaintext, Span<char> ciphertext, out int written) => _inner.Encrypt(plaintext, ciphertext, Alphabet.ToCharArray(), out written);
+	public void Encrypt(ReadOnlySpan<char> plaintext, Span<char> ciphertext, out int written) => _inner.Encrypt(plaintext, ciphertext, _innerKey, out written);
 
-	public void Decrypt(ReadOnlySpan<char> ciphertext, Span<char> plaintext, out int written) => _inner.Decrypt(ciphertext, plaintext, Alphabet.ToCharArray(), out written);
+	public void Decrypt(ReadOnlySpan<char> ciphertext, Span<char> plaintext, out int written) => _inner.Decrypt(ciphertext, plaintext, _innerKey, out written);
 }