Chris Spackman – Cryptography

Traditional Cryptography

Before computers, people used pen-and-paper ciphers to keep messages secret. These pages walk through seven classic ciphers, from the simplest to the theoretically unbreakable. Each one is interactive — type in a message and see how it gets encrypted and decrypted, step by step.

The Story

These ciphers tell a story of escalating sophistication. The Caesar cipher has only 26 possible keys and can be broken in seconds. The substitution cipher has an astronomically large key space but is still breakable with frequency analysis. The Vigenère cipher defeated cryptanalysts for 300 years. The rail fence cipher takes a completely different approach. And the one-time pad is provably unbreakable — but has a fatal practical problem that leads directly into the world of modern cryptography.

The Ciphers

1. Caesar Cipher — shift each letter by a fixed amount. Trivially breakable: only 26 possible keys.
2. Substitution Cipher — scramble the entire alphabet. Huge key space (26! ≈ 4 × 10²⁶) but still breakable with frequency analysis.
3. Playfair Cipher — encrypt letter pairs using a 5×5 grid built from a keyword. Harder to break than single-letter substitution.
4. Vigenère Cipher — use a repeating keyword to apply different shifts. Considered "unbreakable" for 300 years.
5. Rail Fence Cipher — rearrange letters in a zigzag pattern instead of replacing them. A completely different approach (transposition instead of substitution).
6. Columnar Transposition Cipher — write the message into a grid and read columns in keyword-determined order. A more structured transposition cipher.
7. One-Time Pad — a truly random key as long as the message, used once. Provably unbreakable — but how do you share the key?

What Comes Next?

None of these ciphers stayed secret for good. Simple substitution fell to frequency analysis, first described by the Arab scholar al-Kindi in the 9th century. The Vigenère cipher held out until Charles Babbage (around 1854, unpublished) and Friedrich Kasiski (1863) showed how to find the keyword length. In the twentieth century William Friedman and others turned such one-off methods into general statistical tools — the early modern cryptanalysis covered in the next section. And the one-time pad's fatal weakness, the key distribution problem, eventually led to modern cryptography.

• Cryptanalysis: Breaking Ciphers — the other side of the coin. Hands-on tools for breaking substitution, Vigenère, and transposition ciphers using frequency analysis, the Kasiski examination, and column anagramming.
• Early Modern Cryptanalysis — the Zimmermann Telegram, Yardley's Black Chamber, and Friedman's statistical methods that turned codebreaking into a science.
• Diffie-Hellman Key Exchange — how two people create a shared secret over an insecure channel.
• RSA Key Generation — how public and private key pairs are created for encryption and digital signatures.