IRA Cipher Traffic
The Moss Twomey papers preserve interwar IRA cipher traffic. Jim Gillogly recovered many messages; Richard Bean solved the remaining published challenge in 2019.
Jim Gillogly; Richard Bean solved the final published challenge in 2019
Why This Matters
This exhibit is about a body of real ciphertext and the man who read it. In 2001 the historian Tom Mahon found roughly three hundred documents containing encrypted messages in a Dublin archive — the papers of Moss Twomey, IRA chief of staff from 1926 to 1936. Between them they held about 1,300 individual cryptograms, mostly dispatches between headquarters and organisers in Britain and the United States, dating from the 1920s. Mahon brought in the American cryptanalyst Jim Gillogly, and together they published Decoding the IRA in 2008. Their book left one challenge unresolved; Richard Bean subsequently solved it in 2019.
The collection documents IRA correspondence during the 1920s. Dunin and Schmeh describe both columnar transpositions and a smaller set of Vigenère-encrypted passages embedded in otherwise readable letters, including an example dated 4 May 1923. The collection should not be described as using only one cipher.
For the Vigenère passages, Gillogly combined short samples to search for a reused key. The recovered keyword, GVZKLG, is TEAPOT transformed by a reversed alphabet. Ampersands represented Z in the ciphertext. This case shows why reused keys and operator conventions matter alongside the underlying cipher.
Most of the cryptograms used columnar transposition, the mechanism demonstrated here. The message is written into a grid of fixed line length; the columns are then read out in an order set by a keyword. No letter is ever replaced — every letter of the plaintext survives into the ciphertext, only its position changes. Single-letter frequencies cannot recover the column order, but they can help suggest a transposition and a likely language. Letter-group statistics help score candidate orders.
Operators also padded. In the message Gillogly works through as an example, the encipherer had inserted two columns of meaningless vowels to disguise the shape of the grid.
The demo above uses the museum's columnar engine. It is the right mechanism, but it is a clean-room model: it does not reproduce the padding conventions, the five-letter grouping, or the handwriting of the originals.
Gillogly wrote hill-climbing software aimed at columnar transposition. In the worked example described by Dunin and Schmeh, he tried line lengths in the range of about eight to fifteen characters, ran the search around a hundred times from different random starting keys, and watched for the point at which recognisable English words began to fall out of a candidate solution. The approach is the same family this museum covers under hill climbing, but its success depends on message length, language scoring, transcription and the particular cipher conventions.
This surviving 51-letter cryptogram was the unresolved challenge in Decoding the IRA (2008):
GTHOO RCSNM EOTDE TAEDI NRAHE EBFNS INSGD AILLA YTTSE AOITDE
Bean’s reconstruction concerns a report about a raid and gelignite in Scotland. Codebreaking: A Practical Guide, by Elonka Dunin and Klaus Schmeh (2020), explains a missing L within the ciphertext and an omitted four-letter column, ETDS. A reconstructed 56-letter message, retaining the source’s THLS spelling, can be transposed with the equivalent key BCAFIEHGKDLJ; removing those five letters reproduces the displayed 51 letters. This is a reproducible reconstruction, not recovery of the original keyword.
Elonka Dunin and Klaus Schmeh, Codebreaking: A Practical Guide (Robinson, 2020), sections “Jim Gillogly’s solution of IRA Vigenère cryptograms”, “Jim Gillogly’s solution of IRA transposition cryptograms” and “Richard Bean’s solution of the last unsolved IRA cryptogram”. Tom Mahon and James J. Gillogly, Decoding the IRA (2008), is the earlier publication discussed there. The University of Queensland’s 23 August 2019 announcement independently records Bean’s result and the 51-character ciphertext.
| Source | Moss Twomey papers, UCD Archives, Dublin |
| Era | 1920s; includes a 1923 Vigenère example |
| Families | Mainly columnar transposition; also Vigenère |
| Corpus | ~300 documents, ~1,300 cryptograms |
| Discovered by | Tom Mahon (historian) |
| Broken by | Jim Gillogly; Richard Bean (2019 final challenge) |
| Published | Decoding the IRA (Mercier Press, 2008) |
| Final published challenge | 51 surviving letters; solved by Richard Bean in 2019 |
| Modern Lesson | Transposition preserves letters; key reuse helps analysts combine evidence |