How XChaCha20 Works

A Stream of Pseudorandomness

XChaCha20 works by taking your secret key and a 192-bit random nonce, and feeding them through the ChaCha core cryptographic function to generate a massive, infinite stream of pseudorandom data.

This random stream is then XOR'd (a fast mathematical operation) against your plaintext files. To decrypt the file, the receiver simply generates the exact same random stream using the same key and nonce, and XORs the ciphertext back to its original state.

The XOR Operation

Stream ciphers work on incredibly simple fundamental binary mathematics. The cipher generates a random stream of 1s and 0s (the keystream). It then applies the XOR (exclusive OR) logic gate against the plaintext.

Everyday Example

Imagine a machine that prints an endless ribbon of perfectly random static noise. You lay your secret letter directly on top of this ribbon. You cut out shapes where the letter overlaps the noise. To read the letter, your friend must spin up the exact same ribbon and align the cutouts perfectly!

The Deep Mathematics

XChaCha20 operates purely via Addition, Rotation, and XOR (ARX). The initial 4x4 coordinate matrix is populated by constants, the key, a counter, and the nonce. To generate a 64-byte keystream block, the algorithm executes 20 'rounds' (10 column rounds, 10 diagonal rounds) of ARX mixing, finalizing by adding the resulting mixed matrix to the original state to eliminate invertibility.