ACA Cipher Types

History

The American Cryptogram Association’s members set and solve ciphers for one another in its journal, The Cryptogram, each of a standard type. A sheet for each type gives its rule, the length a puzzle should have and a worked example. BION (William Mason) published nine statistics for telling the types apart, with their values for each type, in The Cryptogram of May–June 2005; SHMOO’s Automated Cryptogram Solving (2025) sets out how a computer can solve each.

How it works

Each type enciphers and deciphers with the fields its key takes. Where the encipherer chooses among several signs, the tool takes them in turn, so its ciphertext may differ from the sheet’s and still decipher alike. Without the key, a small key space is tried whole — every rail fence, route, Morbit order, Bazeries number below a million, Periodic Gromark keyword in its primer’s order — and a large one is climbed or annealed, scored by quadgrams; an annealing search runs from a different start on every core. Annealing a whole alphabet can make random letters read as English on a short text, so a reading counts only when it reads clearly better than random letters of its length do under the same search; a Numbered Key’s reading is shown, never taken as read.

How it was checked

All 60 worked examples (two sheets print none) decipher to the sheet’s plain text in the tests, and encipher to its print where the type leaves no choice. Two misprints are corrected from the sheets themselves: the Phillips-RC’s final ciphertext against its own table, the Interrupted Key’s grouping. Eighteen searches are tested on passages of the ACA’s lengths: thirteen read them letter for letter, the Checkerboard and Monome-Dinome all but a letter met once. Nicodemus also reads 28 of 30 passages 16 rows deep; the Autokey, on eight cores, 16 of 16 of 42 letters.

Limits

Seventeen types have no search, each with the reason. A Cadenus, Swagman or Grille may come out begun at another place, since every turn reads alike. Morse is International Morse; the ACA’s own table is not public. The Sequence Transposition’s check digit has no rule on its sheet; it is read as the running sequence’s last digit, as the Gromark’s sheet has it. A Twin Bifid or Trifid is read one message at a time. A Numbered Key of the ACA’s length was read in places, never wholly, in the runs measured.