* Use System.Security.Cryptography in DesCipher and TripleDesCipher; Fall back to use BouncyCastle if BCL doesn't support
* Drop DesCipher; Replace PKCS7Padding with BouncyCastle's implementation.
* Restore `CbcCipherMode`
* Restore AesCipherMode; Use BlockImpl instead of BouncyCastleImpl for 3DES-CFB on lower targets.
* Restore the xml doc comment
