Pseudorandom generation algorithm

As explained in Hacking Cryptography

RC4's pseudorandom generation algorithm updates the cipher state and produces successive keystream bytes.

The RC4 pseudorandom generation algorithm (PRGA) turns the shuffled S-box into a keystream. After key scheduling initializes the permutation, PRGA repeatedly updates the cipher state and emits one byte at a time. section 4.3.1

Each PRGA iteration performs four state and output operations:

• Increment i modulo 256. section 4.3.1
• Update j by adding the current S[i], modulo 256. section 4.3.1
• Swap S[i] and S[j], preserving the S-box as a permutation. section 4.3.1
• Output S[(S[i] + S[j]) mod 256] as the next keystream byte. section 4.3.1

Go implementation

A length-limited implementation repeats the PRGA loop, stores each selected state byte in keyStream, and returns the resulting sequence. RC4 then XORs that sequence with plaintext to produce ciphertext. In WEP, the first keystream byte can be recovered from a known SNAP header and ciphertext byte, enabling statistical key recovery under weak IV conditions. section 4.3.1 section 4.3.2

Figure 4.10 RC4 internal state: a 256-byte S-box and two pointers, i and j

Shows the mutable 256-byte S-box and the i and j pointers that drive each PRGA iteration. section 4.3.1

How Hacking Cryptography covers it

How deep it goes

Traces pointer updates, swapping, indexed output, and a Go implementation, then applies the first output byte to FMS key recovery.

Examples

Uses Go code, a 256-byte S-box, pointer updates, known SNAP-header bytes, and weak-IV WEP packets.

What it assumes

Assumes familiarity with RC4 key scheduling, modular arithmetic, byte arrays, swapping, and XOR-based stream encryption.

What you can do after

Build RC4 keystream generation and reproduce its XOR-based encryption routine in Go.

How it connects

Connects RC4, key-scheduling algorithm, S-box, keystream, stream cipher, WEP, and the Fluhrer-Mantin-Shamir attack.

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Related concepts

• keystreamproduces · A keystream is a sequence of cryptographic bits or symbols generated for combination with plaintext or ciphertext in a stream cipher.
• RC4part of · RC4 is a stream-cipher algorithm that generates a pseudorandom keystream from a variable-length secret key using a permutation-based internal state.
• algorithmkind of · An algorithm is a finite, precisely specified procedure for performing a computation, transforming inputs into outputs, or solving a problem.

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.