Kryha Pocket cipher machine · 1924
Sold as the unbreakable cipher of the future. Friedman broke a 1135-letter test message in 2 hours 41 minutes.
Why This Matters
Alexander von Kryha was a Ukrainian-born engineer who emigrated to Berlin after the Russian Revolution and patented his “Kryha Chiffriermaschine” in 1924. It was the first widely-marketed civilian cipher machine: a brass case the size of a pocket watch, sold to German banks, embassies, and businesses with extravagant claims of unbreakability.
Sales pitches included challenge messages with cash prizes. Kryha’s machine was used commercially through the late 1920s and was even adopted by the German Foreign Office for some diplomatic traffic — a decision that intelligence historians regard as one of the more avoidable cryptographic mistakes of the inter-war period.
Two concentric wheels: a fixed outer ring of plaintext letters and an inner wheel carrying a mixed alphabet. The operator types a letter on the outer ring, reads off the cipher letter from the inner wheel, then triggers a spring-driven escapement that advances the inner wheel by an irregular number of positions — between 1 and roughly 26 — according to a stepping pattern fixed by the wheel design.
The advertising claimed astronomical key spaces by counting wheel orientations and stepping pawls. The reality is that the inner wheel is a single mixed alphabet rotated by a finite, repeating sequence of step sizes — a polyalphabetic cipher with a long but eventually periodic key.
William Friedman and Solomon Kullback at the US Army Signal Intelligence Service were given a 1135-letter test message generated by a Kryha. Recognising the periodic structure of the stepping, they reduced the cipher to a small number of correlated substitutions, applied frequency analysis to each, and recovered the plaintext in 2 hours 41 minutes. Their internal report became a standard demonstration of how marketing claims fail under analysis.
Because the stepping repeats, the Kryha is asymptotically a Vigenère-like cipher with a long, mixed-alphabet key. The same techniques that work on Vigenère — Kasiski examination to find the period, then frequency analysis on each column — work on Kryha as soon as you have a few hundred characters of ciphertext.
| Kryha lesson | Modern echo |
|---|---|
| Counting wheel orientations ≠ key strength | Key length ≠ entropy in modern key analysis |
| Vendor unbreakability claims with no public review | “Trust us” crypto vs. open peer-reviewed standards |
| Fixed stepping pattern = finite period | Why modern stream ciphers need state at least as large as the security level |
| 2 hours 41 minutes is a long time to admire a cipher | Time-to-first-break is the only honest security claim |
| Origin | Alexander von Kryha, Berlin |
| Year | Patented 1924; sold widely 1924–1930s |
| Mechanism | Single mixed alphabet wheel, clockwork-driven |
| Marketed as | Unbreakable; recommended by German banks and diplomats |
| Broken by | William Friedman & Solomon Kullback, US Army SIS, 1933 |