The Playfair Cipher
Learn how Playfair encrypts pairs of letters using a 5x5 key square — a leap beyond simple substitution.
Beyond Single-Letter Substitution
Simple substitution ciphers like Caesar encrypt one letter at a time, making them vulnerable to frequency analysis. English letters have predictable frequencies: E is most common, followed by T, A, O, and so on.
The Playfair cipher encrypts pairs of letters (digraphs) instead of single letters. This dramatically increases the number of possible bigrams and flattens frequency analysis.
The 5x5 Key Square
The Playfair cipher uses a 5x5 grid of letters called the key square. It contains all 26 letters of the alphabet, with I and J combined into one cell (since there are only 25 cells).
To create the key square, write the keyword first (removing duplicates), then fill in the remaining alphabet letters in order. The keyword determines the entire arrangement.