Tyler Stowell

Work / Software & Algorithms

Keyless Caesar Cipher Decryption

Used a character-frequency dataset to decrypt a Caesar-shifted plaintext without the key. A fun little project that seemed like magic to me at the time.

When
2018
Status
Completed
  • Java
  • Information Theory
Source
Bar chart of English letter frequencies, with E highest at 12.0%, then T at 9.1% and O at 8.1%
The decoder looks for how often each letter appears in English, from the reference table in FrequencyTest.java.

Overview

I wrote two Java programs: one that uses a key to encrypt a message, and one that tries to decrypt it without the key. I used the most insecure encryption method ever invented, the Caesar shift, so it’s more a proof of concept and something that I enjoyed as opposed to anything novel.

Three panels: plaintext from A Tale of Two Cities, the scrambled ciphertext with key 42, and the recovered plaintext The full round trip: the opening of A Tale of Two Cities is encrypted with key 42, then recovered with no key at all. Click to enlarge.

How it works

Line plot of the discrepancy score for each of the 96 candidate shifts, with a sharp minimum of 0.53 at shift 42
For each candidate key, the program checks how well the resulting message lines up with English. At a shift of 42, the message has the same letter distribution as common English.

This Caesar shift was done along the ASCII alphabet, so admittedly most of the characters had a probability near 0 and it was far too easy for the key to be decrypted. However, when I made this, I thought it was magic, and I’m including it here because not every project I worked on was significant. Some of them were just significant to me.