Cipher methods Symbols
Friderici (window-shutter alphabet)
The Friderici window-shutter alphabet is one of the ciphers published by the German theologian and cryptologist Johann Balthasar Friderici in his treatise Cryptographia (Hamburg, 1685). The book compiles about twenty secret alphabets then in circulation in the chanceries and religious orders of Central Europe — Friderici contributes several of his own, including this “windows” family where each letter is drawn as a shutter open in a specific direction.
The system follows the medieval 24-letter Latin convention: J is folded onto I and V onto U, a distinction that only stabilised in the late 17th century. On CipherChronicle, encryption automatically applies the J→I and V→U folds before rendering, as for the other medieval alphabets in the catalogue (Templars, Lingua Ignota, Copiale). 24 effective glyphs for 26 modern Latin letters.
How does the alphabet work?
The cipher relies on a monoalphabetic substitution: each cleartext letter is replaced by a shutter-shaped glyph. The technique goes back to Antiquity (Caesar cipher, ~50 BC) — Friderici contributes a visual variant where the “key” is the image rather than a shift.
The table holds 24 glyphs for 24 medieval letters (J is rendered via I, V via U). The rendering matches the engraved plate from Cryptographia (1685), as reproduced by dCode in the Symbol Ciphers section.
Cryptographic strength: low. Like any monoalphabetic substitution, frequency analysis breaks it in a few dozen words. For 17th-century diplomacy it was however enough: the rarity of the treatise and the confidentiality of the correspondence sufficed for short messages. Today the interest is purely historical and decorative.
Historical and modern usage
- 17th-c. diplomacy — Holy Roman Empire, Lutheran chanceries.
- Pedagogy — example of a 24-letter medieval alphabet with J/V folds.
- Historical escape rooms — “parchment” look with shutters.
- History of cryptography — pre-Vigenère, post-Trithemius.
Related variants
- Templars — see our entry, another 24-letter medieval alphabet.
- Lingua Ignota (Hildegard of Bingen) — see our entry, medieval J→I, V→U.
- Copiale — see our entry, 24-letter masonic code.
What are the weaknesses?
- Monoalphabetic substitution — frequency analysis is immediate.
- J→I and V→U folds — these letters cannot be written distinctly.
- Documented table — available in Cryptographia (1685) and on dCode.
The 24 glyphs
