Cryptology Academy · Lesson

Stream Cipher Concepts & PRNG

Learn how stream ciphers generate a keystream from a seed.

Lesson 1 of 413 steps

Stream Cipher Concepts & PRNG is a free Cryptology Academy lesson on CoddyKit. This is lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Cryptology Academy learning path, and your progress syncs across the web and the CoddyKit app. The Cryptology Academy course includes 4 lessons in total.

Welcome

Stream ciphers generate a pseudorandom keystream from a key and encrypt byte by byte. In this lesson we understand how keystreams work and what makes a stream cipher secure.

Stream Cipher Model

KeyGen(K, Nonce) → keystream K1, K2, K3, ... Ci = Pi XOR Ki Pi = Ci XOR Ki Encryption and decryption are identical: XOR with the keystream. The keystream must be pseudorandom and never reused.

PRNG vs CSPRNG

PRNG (Pseudorandom Number Generator): deterministic output that looks random. NOT cryptographically secure — statistical tests pass but output is predictable with enough samples. CSPRNG: cryptographically secure — output is computationally indistinguishable from random even with unlimited compute.

Security Requirements

A secure stream cipher keystream must be: 1. Computationally indistinguishable from random 2. Different for each (key, nonce) pair 3. Impossible to predict from partial output 4. Not invertible — cannot derive key from keystream

Linear Congruential Generators (Insecure)

LCG: s(n+1) = (a × s(n) + c) mod m. Used in many rand() implementations. Completely insecure for crypto — knowing a few outputs reveals all future and past outputs mathematically.

Seed Size and Security

A stream cipher keystream is only as secure as its seed (key). A 128-bit key provides 128-bit security regardless of how much keystream is generated. The seed is the entropy source.

Synchronous vs Self-Synchronizing

Synchronous: keystream is independent of plaintext/ciphertext (RC4, ChaCha20). Self-synchronizing: keystream depends on previous ciphertext (CFB mode). Lost bits desynchronize synchronous ciphers.

Key Stream Generation Architecture

Most modern stream ciphers use: 1. Key + Nonce → initial state 2. Permutation function applied repeatedly to state 3. Some state bits output as keystream 4. Counter incremented for next block AES-CTR follows this pattern.

eSTREAM Portfolio

The eSTREAM project (2004-2008) standardized stream ciphers. Software finalists: HC-128, Rabbit, Salsa20/12, SOSEMANUK. Salsa20 led to ChaCha20, now the dominant stream cipher.

Keystream Reuse: Fatal Flaw

The cardinal sin of stream ciphers: never reuse (key, nonce). C1 XOR C2 = P1 XOR P2. Multiple WEP packets with the same IV (equivalent to key+nonce) allowed complete decryption.

Stream Cipher vs Block Cipher in CTR Mode

AES-CTR is a block cipher used as a stream cipher. Dedicated stream ciphers (ChaCha20) are often faster in software. Security properties are similar when properly keyed and nonced.

Quick Check

What is the fundamental vulnerability when a stream cipher's (key, nonce) pair is reused?

Answer it in the interactive lesson

Recap

Stream cipher fundamentals are clear. Next we study RC4 in depth — how it works and the devastating weaknesses that retired it.

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Frequently Asked Questions

Is the “Stream Cipher Concepts & PRNG” lesson free?

Yes — the full text of “Stream Cipher Concepts & PRNG” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Cryptology Academy course, upgrade to CoddyKit PRO. The Cryptology Academy course includes 4 lessons in total.

What will I learn in “Stream Cipher Concepts & PRNG”?

Learn how stream ciphers generate a keystream from a seed. You practise Cryptology Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Cryptology Academy?

No prior experience is required. Cryptology Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 1 of 4.

How long does the “Stream Cipher Concepts & PRNG” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Cryptology Academy lesson?

Yes. Every Cryptology Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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