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[[Image:A5-1 GSM cipher.svg|280px|thumbnail| The operation of the keystream generator in A5/1, an LFSR-based stream cipher used to encrypt mobile phone conversations.]]
A stream cipher is a symmetric key cipher where plaintext digits are combined with a pseudorandom cipher digit stream (keystream). In a stream cipher, each plaintext numerical digit is encrypted one at a time with the corresponding digit of the keystream, to give a digit of the ciphertext stream. Since encryption of each digit is dependent on the current state of the cipher, it is also known as state cipher. In practice, a digit is typically a bit and the combining operation is an exclusive-or (XOR).
The pseudorandom keystream is typically generated serially from a random seed value using digital . The seed value serves as the cryptographic key for decrypting the ciphertext stream. Stream ciphers represent a different approach to symmetric encryption from . Block ciphers operate on large blocks of digits with a fixed, unvarying transformation. This distinction is not always clear-cut: in some modes of operation, a block cipher primitive is used in such a way that it acts effectively as a stream cipher. Stream ciphers typically execute at a higher speed than block ciphers and have lower hardware complexity. However, stream ciphers can be susceptible to security breaches (see stream cipher attacks); for example, when the same starting state (seed) is used twice.
A stream cipher makes use of a much smaller and more convenient key such as 128 bits. Based on this key, it generates a pseudorandom keystream which can be combined with the plaintext digits in a similar fashion to the one-time pad. However, this comes at a cost. The keystream is now pseudorandom and so is not truly random. The proof of security associated with the one-time pad no longer holds. It is quite possible for a stream cipher to be completely insecure.
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 active attacks: 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.
An example of a self-synchronising stream cipher is a block cipher in cipher feedback (CFB) mode.
An alternating step generator comprises three LFSRs, which we will call LFSR0, LFSR1 and LFSR2 for convenience. The output of one of the registers decides which of the other two is to be used; for instance, if LFSR2 outputs a 0, LFSR0 is clocked, and if it outputs a 1, LFSR1 is clocked instead. The output is the exclusive OR of the last bit produced by LFSR0 and LFSR1. The initial state of the three LFSRs is the key.
The stop-and-go generator (Beth and Piper, 1984) consists of two LFSRs. One LFSR is clocked if the output of a second is a 1, otherwise it repeats its previous output. This output is then (in some versions) combined with the output of a third LFSR clocked at a regular rate.
The shrinking generator takes a different approach. Two LFSRs are used, both clocked regularly. If the output of the first LFSR is 1, the output of the second LFSR becomes the output of the generator. If the first LFSR outputs 0, however, the output of the second is discarded, and no bit is output by the generator. This mechanism suffers from timing attacks on the second generator, since the speed of the output is variable in a manner that depends on the second generator's state. This can be alleviated by buffering the output.
As with other attacks in cryptography, stream cipher attacks can be certificational so they are not necessarily practical ways to break the cipher but indicate that the cipher might have other weaknesses.
Securely using a secure synchronous stream cipher requires that one never reuse the same keystream twice. That generally means a different nonce or key must be supplied to each invocation of the cipher. Application designers must also recognize that most stream ciphers provide not authenticity but privacy: encrypted messages may still have been modified in transit.
Short periods for stream ciphers have been a practical concern. For example, 64-bit block ciphers like DES can be used to generate a keystream in output feedback (OFB) mode. However, when not using full feedback, the resulting stream has a period of around 232 blocks on average; for many applications, the period is far too low. For example, if encryption is being performed at a rate of 8 per second, a stream of period 232 blocks will repeat after about an hour.
Some applications using the stream cipher RC4 are attackable because of weaknesses in RC4's key setup routine; new applications should either avoid RC4 or make sure all keys are unique and ideally related key (such as generated by a well-seeded CSPRNG or a cryptographic hash function) and that the first bytes of the keystream are discarded.
The elements of stream ciphers are often much simpler to understand than block ciphers and are thus less likely to hide any accidental or malicious weaknesses.
Another advantage of stream ciphers in military cryptography is that the cipher stream can be generated in a separate box that is subject to strict security measures and fed to other devices such as a radio set, which will perform the XOR operation as part of their function. The latter device can then be designed and used in less stringent environments.
ChaCha is becoming the most widely used stream cipher in software; others include: RC4, A5/1, A5/2, Chameleon, FISH, Helix, ISAAC, MUGI, Panama, Phelix, Pike, Salsa20, SEAL, SOBER, SOBER-128, and WAKE.
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