Rasterschlüssel 44 (RS44)

The Rasterschlüssel 44 was a German paper-and-pencil field cipher intended to replace Double Playfair in 1944. This exhibit follows Schlüsselanleitung zum Rasterschlüssel 44, Ausgabe 27. März 1944, with the second addendum’s clarification of the beginning boundary and column counting. It does not implement the October 1944 change to a directly selected readout column.

A daily key supplied the black-and-white stencil, permuted column ranks, coordinate labels, name alphabets and indicator substitution. Operators wrote on thin grid paper over the stencil. A planned 1 August introduction appears in contemporary Allied reports; actual adoption varied across services and units.

1. Select an indicated beginning row and physical column. If their intersection is black, advance in row order to the next white cell. Include the beginning itself when it is white.
2. Enter prepared plaintext left to right through white cells only. Continue through successive rows, wrapping from the bottom to the top if necessary. The stencil has 24 × 25 cells, with exactly ten writing cells in each row: 240 available cells.
3. Add the digits of the tactical-time minute and message length. From the indicated beginning column, move that many physical columns to the right, wrapping at column 25. The first step excludes the beginning column; returning to it after a full circuit counts normally.
4. Read the occupied cells top to bottom in the selected column, then by ascending numerical column rank, cycling from 25 to 1. Skip every unoccupied cell, including an unused gap when the message wraps.
5. For decryption, mark the same occupied cells from the length and starting position. Fill them in that column order, then read in the original writing order.

The transmitted body was 60–200 letters, leaving at least 40 white cells unused. Shorter messages required operator-chosen Wahlwörter, with doubled boundary letters. Longer messages were divided into separately keyed parts of differing lengths. The teaching mode permits shorter bodies without silently adding fillers.

Printed pp. 12–13 and Fig. 2 on p. 18 give a complete 77-letter example. Beginning: row ae (5), column bb (14); tactical time 1203. The digit sum is 0 + 3 + 7 + 7 = 17, so readout begins in physical column ee (6), rank 19.

Prepared plaintext:
FEINDGREIFTSEITELFXVIERFUENFUHRBEIAAIGQALFISIGQALFISEEMITAQTPANZNAQSUEDWESTAN

Ciphertext body:
DIANMRQTVFNNRISIFFGPUEFZGNAEEHAEUTAIIEADAGQQLWIBSFFXUTITEEAAENIQSSIRLIEFESELT

The test suite also reproduces the 64-letter wrap example on printed pp. 14–15 and Fig. 4, p. 20. These expected ciphertexts were transcribed from the manual, not generated by the new engine.

Captain Walter J. Fried’s reports document Allied work during 1944. Report F-79 (22 August) describes fitting messages to a captured stencil. F-93 (18 September) reports recovering stencil rows from retransmissions and messages sent twice from the same starting point, then exploiting stereotyped traffic and cribs. It also describes constructing 36 sliding masks to test probable plaintext against known column order.

These are conditional attacks, not a claim that every isolated message was solved. Captured material, repeated arrangements and operator habits mattered. The daily stencil and 25-column permutation create many possible settings, but counting them as “bits of security” ignores their structure and the traffic available to an attacker. Letter frequencies are preserved.

The default stencil and ranks are factual transcriptions of the manual’s worked key. You can vary the beginning and minute; the engine also accepts a validated custom stencil and rank permutation. This page fixes the daily stencil so the published answer stays reproducible.

The demo accepts prepared A–Z text. It does not reproduce German preprocessing (CH/CK → Q, ß → SZ, umlauts written out, number words), place-name substitution, encoded message headers, key distribution, coordinate-use marks, emergency procedures or operator-chosen filler words. Accent stripping is a museum input convenience and is not historical German message preparation. No padding or escape scheme is added.