Trithemius / progressive key — The shift advances by a fixed step at every letter: a progressive key rather than a repeating one.
Shift increases by a constant step per letter: key[i] = (start + i*step) mod 26.
A worked example§
Encrypting a sample with the cipher itself, at build time:
MEET ME BY THE OLD CLOCK TOWER AT DAWNKey: start=0, step=1
produces the ciphertext
MFGW QJ HF BQO ZXQ QAETC MIRAO YS DBYQThis example is generated by running the cipher when the page is built, and the round trip is checked, so it always matches what the solver does.
How it is broken§
Only the starting point and the step are unknown, so 676 possibilities cover every variant.
History and context§
Johannes Trithemius, abbot and occultist, whose Polygraphia (1518) was the first printed book on cryptography — and whose Steganographia looked so much like sorcery that it spent two centuries on the Index.
See also§
Vigenère · Beaufort · Variant Beaufort · Gronsfeld · Porta · Quagmire III (keyed alphabet)
Browse every cipher in the Polyalphabetic ciphers family, read the history of codebreaking, or return to the wiki main page.
Frequently asked questions§
How is the Trithemius / progressive key broken?
Only the starting point and the step are unknown, so 676 possibilities cover every variant.
What key does the Trithemius / progressive key use?
(start, step). There are 676 possible keys.
How much ciphertext do I need?
At least 16 characters for this solver to attempt it; short messages can be readable and still not be proof.
Can I break a Trithemius / progressive key on this page?
Yes — the browser solver on this page handles it directly. Paste the ciphertext into the solver and press the button.