Keystream

As explained in Hacking Cryptography

A keystream is a sequence of cryptographic bits or symbols generated for combination with plaintext or ciphertext in a stream cipher.

A keystream is the generated sequence of bits or bytes XORed with plaintext to produce ciphertext in a stream cipher. Unlike the fixed-length key or seed, it can grow to match the message: both parties initialize the same generator from the shared seed and independently produce the required keystream length. section 4.1.2

Figure 4.4 The difference between the key and the keystream

Distinguishes the fixed-length seed, which initializes a generator once, from its expandable keystream output used for encryption. section 4.1.2

Encryption and decryption use XOR. For each position, combine plaintext with the corresponding keystream value; applying the same keystream again reverses the operation. A 1 GB message therefore needs a 1 GB keystream, but the communicating parties need share only the generator seed rather than a key as long as the message. section 4.1.2 section 4.3.1

Generating bytes with RC4

RC4 runs key scheduling once to initialize a 256-byte S-box, then its pseudorandom generation algorithm mutates that state and emits one keystream byte per iteration. Encryption generates exactly as many bytes as the plaintext and XORs each pair. This contrasts with an LFSR implementation, which emits individual bits and calls its bit generator eight times per plaintext byte. section 4.3.1 section 4.2.1

Figure 4.12 One iteration of PRGA producing a keystream byte (after i and j are already swapped)

Shows one RC4 PRGA iteration selecting a byte from the evolving S-box as keystream output. section 4.3.1

Seeing keystream values can expose the corresponding plaintext positions: known plaintext XORed with ciphertext yields the keystream. A CSPRNG-based design aims to prevent recovered output from revealing its seed or predicting unseen future output. That protection is not automatic for every generator. A single LFSR is fast and hardware-friendly, but its linear output can let an attacker recover its internal state from observed keystream bits and then predict later bits. section 4.1.2 section 4.2

Known-prefix LFSR attack

If intercepted ciphertext begins with the known prefix “ATTACK AT ”, XORing that prefix with the ciphertext recovers its keystream bits. Those bits can be used to clone an LFSR by trying lengths and solving its linear relationships. The cloned generator predicts the remaining keystream, allowing XOR decryption of the unknown timestamp; successful parsing validates the guess. section 4.2.2

Keystream recovery can have different consequences depending on the cipher. Reversing the LFSR example predicts output without necessarily recovering its original key. In the WEP use of RC4, however, statistical biases tied to weak initialization vectors allow repeated packet observations to turn known keystream bytes into guesses for bytes of the preshared key. More packets improve the frequency-based recovery; too few can produce incorrect key bytes. section 4.3 section 4.3.2

Treat keystream handling as a security boundary:

• Use a generator whose output is intended to resist seed recovery and prediction after some output is known. section 4.1.2
• Do not rely on a lone LFSR when its linear output permits state recovery from observed keystream. section 4.2
• Avoid weak-IV RC4/WEP constructions, where biased keystream observations support preshared-key recovery. section 4.3.2

How Hacking Cryptography covers it

How deep it goes

Works through CSPRNG, LFSR, and RC4 generation internals, then traces known-plaintext and statistical attacks.

Examples

Uses a 1 GB video, an LFSR-encrypted “ATTACK AT” timestamp, RC4 code, and weak-IV WEP packets.

What it assumes

Introduces XOR use and generator roles concretely while using linear algebra and RC4 state transitions for the attacks.

What you can do after

Diagnose/debug with it by recovering known keystream positions and recognizing when generator output enables prediction or key recovery.

How it connects

Connects keystreams to stream ciphers, XOR, CSPRNGs, one-time pads, LFSRs, RC4, WEP, and initialization vectors.

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Also explained in

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  Real-World CryptographyA keystream is the sequence of pseudorandom bytes generated by a stream-encryption construction and XORed with plaintext or ciphertext.

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Related concepts

• pseudorandom generation algorithmproduced by · RC4's pseudorandom generation algorithm updates the cipher state and produces successive keystream bytes.
• linear feedback shift registerproduced by · A linear-feedback shift register is a deterministic shift-register mechanism whose next input bit is computed as a linear function of selected state bits.
• cryptographically secure random number generatorproduced by · A cryptographically secure random number generator produces computationally unpredictable values while resisting prediction and recovery of its internal state by feasible adversaries.
• stream cipherproduced by · A stream cipher is an encryption algorithm that combines plaintext sequentially with a generated keystream, typically using XOR.
• known-plaintext attackproduced by · A known-plaintext attack is a cryptanalytic method that uses plaintext and corresponding ciphertext to infer information about the encryption process or key.
• logical XORuses · Logical XOR is a binary operation that produces true when exactly one operand is true and false when both operands have the same truth value.

This explainer was generated by Manning's knowledge graph from the text of Hacking Cryptography. The section links point to the parts of the book it draws on.