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March 24, 2016 · Cryptography

5-Min Tutorial: Caesar Cipher in Scala

The Caesar cipher is the "hello world" of cryptography: shift every letter forward by a fixed number of places, wrapping around at the end of the alphabet. It is hopelessly insecure and completely charming, which makes it the perfect five-minute tutorial. Here it is in Scala.

The idea

Each letter is a number: A = 0, B = 1, … Z = 25. To encrypt with a shift of k, add k, then take the result modulo 26. To decrypt, shift by 26 - k (or just by -k and let the modulo sort it out).

The code

def caesar(text: String, shift: Int): String =
  text.map {
    case c if c.isUpper => (((c - 'A' + shift) % 26 + 26) % 26 + 'A').toChar
    case c if c.isLower => (((c - 'a' + shift) % 26 + 26) % 26 + 'a').toChar
    case c              => c
  }

val secret = caesar("Et tu, Brute?", 3)   // "Hw wx, Euxwh?"
val plain  = caesar(secret, -3)            // "Et tu, Brute?"

The double-modulo (x % 26 + 26) % 26 is the only trick worth remembering: it keeps the result non-negative even when shift is negative, so the same function both encrypts and decrypts. Non-letters fall through untouched.

Where to go next

Once this clicks, the natural next step is a Vigenère cipher — a Caesar cipher whose shift changes per letter according to a keyword — and from there the whole history of classical cryptanalysis opens up. But that's another five minutes.