V2b: Understanding Stream Ciphers with ChaCha20 🔐

Welcome to "V2b: Stream Cipher: ChaCha20," a lecture in Alfred Menezes's "Crypto 101: Building Blocks" course. This video explains the inner workings of the ChaCha20 stream cipher, a modern, high-performance encryption algorithm designed by Dan Bernstein. ChaCha20 is widely deployed in real-world applications such as VPNs, and secure messaging, providing secure encryption with a focus on simplicity and speed.

In this lecture, you’ll learn how ChaCha20 works as a stream cipher, which generates a keystream to encrypt plaintext one byte at a time. We discuss how the algorithm operates by using a 256-bit key and a 96-bit nonce, and how it produces a pseudorandom keystream using a series of simple operations. We’ll also cover the importance of keystream uniqueness and the dangers of reusing a keystream, which can lead to catastrophic security vulnerabilities, such as revealing plaintext.

ChaCha20 is considered more secure and efficient than other stream ciphers, making it a popular choice in modern cryptographic systems. By the end of this video, you’ll have a clear understanding of ChaCha20’s structure, its advantages over traditional stream ciphers like RC4, and its role in ensuring secure communications in a variety of contexts.

Lecture playlist:
https://www.youtube.com/playlist?list=PLA1qgQLL41SRsHMtsb4up5fTytDeHV_yJ

Course web page: https://cryptography101.ca/crypto101-building-blocks/
The slides are available on the course web page.

Other cryptography courses: https://cryptography101.ca

Slides
00:00 Introduction
00:42 Slide 46: The one-time pad
03:11 Slide 47: Binary messages
04:32 Slide 48: Example: reusing a key in the one-time pad
05:52 Slide 49: Security of the one-time pad
07:05 Slide 50: Stream ciphers
10:22 Slide 51: Security requirements for the PRBG
12:25 Slide 52: ChaCha20 stream cipher
14:48 Slide 53: ChaCha20 initial state
16:52 Slide 54: ChaCha20 quarter round function
18:18 Slide 55: ChaCha20 keystream generator
23:08 Coming up

Corrections
20:57 Slide 55: " S \oplus S' " should by "S \boxplus S' ", i.e., replace the xor symbol with the symbol for addition modulo 2^{32}.