Stream cipher

In cryptography, a stream cipher is a symmetric cipher in which the plaintext digits are encrypted one at a time, and in which the transformation of successive digits varies during the encryption. An alternative name is a state cipher, as the encryption of each digit is dependent on the current state. In practice, the digits are typically single bits or bytes.

Stream ciphers represent a different approach to symmetric encryption from block ciphers. Block ciphers operate on large blocks of digits with a fixed, unvarying transformation. This distinction is not always clear-cut: some modes of operation use a block cipher primitive in such a way that it then 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 serious security problems if used incorrectly: see stream cipher attacks — in particular, the same starting state must never be used twice.

National Security Agency documents sometimes use the term combiner-type algorithms, referring to algorithms that use some function to combine a pseudorandom number generator (PRNG) with a plaintext stream.

Loose inspiration from the one-time pad

Stream ciphers can be viewed as approximating the action of a theoretically unbreakable cipher, the one-time pad (OTP), sometimes known as the Vernam cipher. A one-time pad uses a key stream of completely random digits. The key stream is combined with the plaintext digits one at a time to form the ciphertext. This system was proved to be theoretically secure by Shannon in 1949. However, the key stream must be (at least) the same length as the plaintext, and generated completely at random. This makes the system very cumbersome to implement in practice, and as a result the one-time pad has not been widely used, except for the most critical applications.

A stream cipher makes use of a much smaller and more convenient key — 128 bits, for example. Based on this key, it generates a pseudorandom key stream which can be combined with the plaintext digits in a similar fashion to the one-time pad. However, this comes at a cost: because the keystream is now pseudorandom, and 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.

Types of stream cipher

A stream cipher generates successive elements of the keystream based on an internal state. This state is updated in essentially two ways: if the state changes independently of the plaintext or ciphertext messages, the cipher is classified as a synchronous stream cipher. By contrast, self-synchronising stream ciphers update their state based on previous ciphertext digits.

Synchronous stream ciphers

In a synchronous stream cipher a stream of pseudo-random 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 (bits), 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 active attacks — if an attacker can change a digit in the ciphertext, he 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.

Self-synchronizing stream ciphers

Another approach uses several of the previous N ciphertext digits to compute the keystream. Such schemes are known as self-synchronizing stream ciphers, asynchronous stream ciphers or ciphertext autokey (CTAK). The idea of self-synchronization was patented in 1946 (), and has the advantage that the receiver will automatically synchronise with the keystream generator after receiving N ciphertext digits, making it easier to recover if digits are dropped or added to the message stream. Single-digit errors are limited in their effect, affecting only up to N plaintext digits. It is somewhat more difficult to perform active attacks on self-synchronising stream ciphers by comparison with their synchronous counterparts.

An example of a self-synchronising stream cipher is a block cipher in cipher-feedback mode (CFB).

LFSR-based stream ciphers

Binary stream ciphers are often constructed using linear feedback shift registers (LFSRs) because they can be easily implemented in hardware and can be readily analysed mathematically. The use of LFSRs on their own, however, is insufficient to provide good security. Various schemes have been proposed to increase the security of LFSRs.

Non-linear combining functions

Because LFSRs are inherently linear, one technique for removing the linearity is to feed the outputs of several parallel LFSRs into a non-linear Boolean function to form a combination generator. Various properties of such a combining function are critical for ensuring the security of the resultant scheme, for example, in order to avoid correlation attacks.

Clock-controlled generators

Normally LFSRs are stepped regularly. One approach to introducting non-linearity is to have the LFSR clocked irregularly, controlled by the output of a second LFSR. Such generators include the stop-and-go generator, the alternating step generator and the shrinking generator.

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.

Filter generator

Another approach to improving the security of an LFSR is to pass the entire state of a single LFSR into a non-linear filtering function.

Other designs

Instead of a linear driving device, one may use a nonlinear update function. For example, Klimov and Shamir proposed triangular functions (T-Functions) with a single cycle on n bit words.

Security

Main article: Stream cipher attack

To be secure, the period of the keystream, that is, the number of digits output before the stream repeats itself, needs to be sufficiently large. If the sequence repeats, then the overlapping ciphertexts can be aligned against each other "in depth", and there are techniques which could allow the plaintext to be extracted. This can be a practical concern: for example, the DES block cipher was initially allowed to be used in a certain mode (OFB) with a varying parameter. However, for most choices of this parameter, the resulting stream had a period of only 2³² — for many applications, this period is far too low. For example, if encryption is being performed at a rate of 1 megabyte per second, a stream of period 2³² will repeat after around 8.5 minutes.

Usage

Stream ciphers are often used in applications where plaintext comes in quantities of unknowable length—for example, a secure wireless connection. If a block cipher were to be used in this type of application, the designer would need to choose either transmission efficiency or implementation complexity, since block ciphers cannot directly work on blocks shorter than their block size. For example, if a 128-bit block cipher received separate 32-bit bursts of plaintext, three quarters of the data transmitted would be padding. Block ciphers must be used in ciphertext stealing or residual block termination mode to avoid padding, while stream ciphers eliminate this issue by naturally operating on the smallest unit that can be transmitted (usually bytes).

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, e.g. 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.

RC4 is the most widely-used stream cipher in software; others include: A5/1, A5/2, Chameleon, FISH, Helix, ISAAC, MUGI, Panama, Phelix, Pike, SEAL, SOBER, SOBER-128 and WAKE.

References

Matt J. B. Robshaw, Stream Ciphers Technical Report TR-701, version 2.0, RSA Laboratories, 1995 (PDF).
Thomas Beth and Fred Piper, The Stop-and-Go Generator. EUROCRYPT 1984, pp88-92.

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