private byte[] EncryptAesCbcPkcs7(Span <byte> key, byte[] plaintext)
{
var iv = new Span <byte>(new byte[IvLength]);
_randomNumberGenerator.Fill(iv);
using var encryptor = _aesCryptoProvider.CreateEncryptor(key.ToArray(), iv.ToArray());
using var ms = new MemoryStream();
using var cs = new CryptoStream(ms, encryptor, CryptoStreamMode.Write);
using var sw = new BinaryWriter(cs);
sw.Write(plaintext);
cs.FlushFinalBlock();
return(Concat(iv.ToArray(), ms.ToArray()));
}