Denial-of-service attacks can only work without incoming traffic from different sources.
 Mechanical encryption devices such as Enigma made these attacks much more difficult (although, historically, Polish cryptographers were able to mount a successful ciphertext-only cryptanalysis of the Enigma by exploiting an insecure protocol for indicating the message settings)
  In the history of cryptography, early ciphers, implemented using pen-and-paper, were routinely broken using ciphertexts alone

