Editing Stream cipher (section)

===Synchronous stream ciphers===
[[File:Lorenz Cipher Machine.jpg|thumb|[[Lorenz cipher|Lorenz SZ cipher]] machine as used by the German military during World War II]]
In a '''synchronous stream cipher''' a stream of pseudorandom digits is generated independently of the plaintext and ciphertext messages, and then combined with the plaintext (to encrypt) or the ciphertext (to decrypt). In the most common form, binary digits are used ([[bit]]s), and the keystream is combined with the plaintext using the [[exclusive or]] operation (XOR). This is termed a '''binary additive stream cipher'''.

In a synchronous stream cipher, the sender and receiver must be exactly in step for decryption to be successful. If digits are added or removed from the message during transmission, synchronisation is lost. To restore synchronisation, various offsets can be tried systematically to obtain the correct decryption. Another approach is to tag the ciphertext with markers at regular points in the output.

If, however, a digit is corrupted in transmission, rather than added or lost, only a single digit in the plaintext is affected and the error does not propagate to other parts of the message. This property is useful when the transmission error rate is high; however, it makes it less likely the error would be detected without further mechanisms. Moreover, because of this property, synchronous stream ciphers are very susceptible to [[Attack (computing)#Phenomenology|active attack]]s: if an attacker can change a digit in the ciphertext, they might be able to make predictable changes to the corresponding plaintext bit; for example, flipping a bit in the ciphertext causes the same bit to be flipped in the plaintext.