Trithemius Progressive Cipher Steganographia · 1518
The first published polyalphabetic cipher: a tabula recta with every letter advanced by one more shift than the last.
Why This Matters
Johannes Trithemius (1462–1516) was a Benedictine abbot, occult scholar, and one of the most consequential cryptographers of the German Renaissance. His Steganographia (written ~1499, published 1606) disguised a cipher manual as a book of angel-summoning magic; his Polygraphia (1518) was the first printed book on cryptography in the West. The progressive cipher described here appears in Polygraphia's opening tables.
Trithemius's contribution was structural, not numerical. By writing 26 successively-shifted alphabets in a square — the tabula recta — he made the polyalphabetic principle visible: each letter could be enciphered by a different alphabet. Belaso (1553) and Vigenère (1586) added the keyword on top of Trithemius's table.
Encrypt the first letter with shift 0, the second with shift 1, the third with shift 2, and so on. The keystream is just the position counter:
Plain: S T E G A N O G R A P H I A Shift: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Cipher: S U G J E S U N Z J Z S U N
The optional starting offset simply moves the counter's origin. There is no keyword. The system is polyalphabetic in form — every letter uses a different alphabet — but the alphabet sequence is public knowledge, so it offers no real secrecy.
Because the keystream is fixed and public, the only secret is the starting position. Twenty-six guesses recover the plaintext — Caesar-grade work.
After 26 letters the shifts wrap around and a 26-period structure emerges. Splitting the ciphertext into 26 columns and frequency-analysing each one yields the offset.
Trithemius is the proof-of-concept that a single ciphertext letter need not always come from the same alphabet. That single idea unlocks Vigenère, Beaufort, Porta, the Jefferson disk, the Bazeries cylinder, the M-94, the Hagelin lug-and-pin family, and ultimately the rotor machines (Enigma, SIGABA, Typex). Every twentieth-century mechanical cipher is a descendant of Trithemius's table.
The flaw — a public, predictable keystream — is also the lesson. Modern stream ciphers (RC4, ChaCha20) are polyalphabetic in exactly Trithemius's sense, but their keystreams are pseudorandom outputs of a keyed PRF. Same architecture, immensely larger key.
| Origin | Johannes Trithemius, abbot of Sponheim |
| Published | Polygraphia, 1518 (posthumous) |
| Key Type | Optional starting offset only |
| Significance | Introduced the tabula recta to Europe |
| Modern Lesson | A predictable keystream is barely a keystream |