Cryptography - Sage

6.2 Properties of Stream CiphersSynchronous stream ciphers• Synchronization: Sender and receiver are required to be synchronized in terms ofboth state and key.• Error propagation: None — a bit error in the ciphertext affects precisely one bit inthe deciphered plaintext, provided that synchronization is maintained.• Attacks and features: Property (1) means that an active adversary can use insertion,deletion, or replay of ciphertext; property (2) implies that the affects of these changeseffect direct changes on the deciphered plaintext, which might be exploited.Asynchronous stream ciphers• Synchronization: An insertion, deletion, or change in ciphertext characters resultsin loss of only a fixed number of deciphered plaintext characters, after which thedeciphering self-synchronizes.• Error propagation: A ciphertext error in transmission affects at most t characters ofthe deciphered plaintext.• Attacks and features: The error propagation makes active modification more easilydetected, while self-synchronization makes insertion, deletion, or reply of ciphertextblocks more difficult to detect. Since each plaintext character influences subsequentciphertext, an asynchronous stream cipher is better at masking plaintext structureor redundancies.6.3 Linear Feedback Shift RegistersA linear feedback shift register implements a keystream function, and which can be simplydescribed by a schematic diagram of the following form:⊕ ⊕ ⊕ ⊕c 1⊗c 2⊗c n−1⊗c n⊗ s n+i · · s i+2 · s i+1 · s i . . . s 1 s 0R n−1 R n−2 · · · R 1 R 050 Chapter 6. Stream Ciphers