Friedrich Wilhelm Kasiski

(1805–1881)
The man who broke the "unbreakable" Vigenère cipher

Short Biography

Kasiski came from a modest background and had no formal university education in mathematics or linguistics. He was entirely self-taught in cryptology.

Major Contribution to Cryptography (1863)

Title of the book:
Die Geheimschriften und die Dechiffrir-Kunst
(“Secret Writing and the Art of Deciphering”)
Published: 1863, Berlin (self-published, only 84 pages)

Key discovery:
Kasiski was the first person to publish a general and practical method to break polyalphabetic ciphers with repeating keys — especially the famous Vigenère cipher, which had been considered unbreakable for 300 years and was widely called le chiffre indéchiffrable (“the indecipherable cipher”).

The Kasiski Examination / Kasiski Test

(The method that bears his name)

Main idea:
- If the same segment of plaintext is encrypted with the same portion of the key, the resulting ciphertext segments will be identical.
- Therefore, repeated sequences (trigrams, tetragrams, etc.) in the ciphertext are very likely caused by the key repeating.
- The distances between identical repeated sequences must be multiples of the key length.
- Take the greatest common divisor (GCD) of several such distances → probable key length.

Example he used in the book
He demonstrated the attack on real historical ciphertexts and showed that even long keys (10–20 letters) can be found in a few hours.

Before Kasiski (1854), Charles Babbage had already discovered the same principle privately in England while trying to break Vigenère ciphers used during the Crimean War, but Babbage never published it. Therefore, the method is correctly credited to Kasiski.

Other Contributions in the 1863 Book

• Systematic frequency analysis for monoalphabetic ciphers
• Early ideas about probable words
• Analysis of Playfair and two-letter substitution systems
• Discussion of transposition ciphers

Recognition

• During his lifetime: almost none. The book sold very few copies and was forgotten.
• 1880s–1910s: cryptologists slowly rediscovered the method.
• 1920s onward: William F. Friedman and others popularized it and called it the “Kasiski method” or “Kasiski examination”.
• Today: every cryptography textbook teaches the Kasiski test as the classic way to find the key length of a Vigenère or any repeating-key polyalphabetic cipher.

Famous Quote about Kasiski

“Major Kasiski, an officer who never attended university, destroyed with a short pamphlet of 84 pages the illusion that the polyalphabetic cipher was unbreakable.”
— David Kahn, The Codebreakers (1967)

Timeline Summary

Friedrich Kasiski is a perfect example of an amateur who, through careful observation and self-study, made a fundamental and lasting contribution to the science of cryptanalysis.