Caesar Cipher

Shift each letter by a fixed amount — the simplest substitution cipher.

Cracking Caesar

Brute-force all 26 keys and let English letter frequencies pick the winner.

Vigenère Cipher

A repeating keyword turns Caesar into a polyalphabetic cipher — once unbreakable.

Cracking Vigenère

Kasiski examination finds the period; brute-forcing each Caesar slice finds the key.

Chi-squared Test

The English-likeness score that powers every frequency-based attack on this site.

Binary Numbers

Click bits to toggle them and explore positional notation, decimal, and hex.

AND

Both bits must be 1. The fundamental bitmask operation.

OR

At least one bit must be 1. Used to set bits without disturbing others.

NOT

Flips every bit. The one's complement — foundation of two's complement negation.

XOR

Bits differ → 1. Self-inverse, so the same operation encrypts and decrypts.

Vernam Cipher

The one-time pad: XOR with a truly random key. Provably unbreakable when used correctly.

Crib Dragging

Reusing an OTP key lets you XOR two ciphertexts and drag known words to read both messages.

WEP IV Collision

WEP's 24-bit IV space causes keystream reuse after ~4 900 packets. Birthday paradox in action.

Weak PRNG

Netscape SSL seeded its PRNG with time ^ PID — guessable values that collapse 128-bit keys.

S-DES

Simplified DES: an 8-bit block cipher with a 10-bit key — follow every bit through the full pipeline.