AMSCO Transposition

AMSCO belongs to the American Cryptogram Association’s repertoire of hand-solvable transposition puzzles. This exhibit uses the ACA specification and complete worked example, printed p. 32 of its Cipher Exchange Guidelines. The sheet permits either a single letter or a digraph in the first cell.

The specification is not a dated military operating manual. It does not establish a particular invention date or operational deployment, so this page does not invent one. Its educational value lies in a small change to columnar transposition that makes decryption geometry less obvious.

1. Choose a permutation of the column ranks. This demo uses 41325: physical columns are read in order 2, 4, 3, 1, 5 (ranks 1–5).
2. Choose the starting cell length: two letters by default here, as in the worked example. Fill left to right, alternating digraph and single-letter cells. Alternate the starting length on each new row as well. Thus the first column also alternates.
3. If text ends inside a digraph cell, leave that final cell with one letter. Do not append null letters or fill later cells.
4. Read each column from top to bottom, in ascending rank order.
5. To decrypt, use the ciphertext length, column count and first-cell convention to reconstruct the cell geometry and actual final-cell length. Calculate each column’s capacity, fill columns in rank order, then read cells by rows.

For an even number of columns, alternating the first cell on the next row matters: a continuous alternation through a flattened list of cells implements a different cipher.

Key 41325, digraph first. The first row is IN | C | OM | P | LE; the second is T | EC | O | LU | M. The final row is incomplete: A | PH | S. The prepared message has 57 letters.

Plaintext:
INCOMPLETECOLUMNARWITHALTERNATINGSINGLELETTERSANDDIGRAPHS

Ciphertext:
CECRTEGLENPHPLUTNANTEIOMOWIRSITDDSINTNALINESAALEMHATGLRGR

This complete answer is transcribed from the ACA sheet. An additional hand-derived even-width check uses key 12, digraph first: ABCDEF makes rows AB | C and D | EF, yielding ABDCEF.

AMSCO preserves letters and their frequencies. A solver has to discover the width, column order and initial cell length, then judge reconstructed neighbouring letters. Probable words, anagramming and scored permutation search are applicable transposition attacks. There is no claim here of a named wartime break: the cited ACA sheet specifies a puzzle, not an operational cryptanalytic history.

For a fixed width n, the rank order has n! possibilities and there are two starting-cell conventions. This is a settings count, not a security estimate; language structure provides information that exhaustive counting ignores.

The engine implements the ACA’s cell-length convention and exact incomplete-row reconstruction. Both first-cell conventions are selectable. It needs a genuine permutation, with no repeated or missing ranks: use 2–25 columns in this interface. Ranks greater than nine must be space-separated.

Case folding, accent stripping and nonletter removal are stated input conveniences. No escape sequence or padding is introduced, and decryption returns the normalized letters rather than the original punctuation. Animated cell boundaries show why the receiver must reconstruct geometry before inserting ciphertext.