The Lorenz Wheel Patterns
1941 · Cam Settings · Work 464 of 1,871
A plain ruled sheet of paper lying flat on a workbench, its whole face taken up by the rows of marks, twelve long rows of tiny crosses and dots pencilled in columns, the marks reading as a fine grey texture, a few small pencil ticks in the margin, a pair of fine tweezers and a single small removed brass cam lying on the paper, a magnifying glass at one corner, the sheet under a single work lamp from above at an angle, the paper bright against the dark wood of the bench, the workshop around it in darkness.
- What it was used for
- The wheel patterns were the long-term key of the Lorenz machine: the setting of every single cam on the twelve wheels, raised or not — 501 cams, the sum of the wheel lengths Tiltman lists. They applied to all stations on a link and were distributed on paper; the wheels' start positions were the message key and changed from message to message. Copeland gives the change intervals: before the summer of 1944 the chi patterns were changed monthly, the psi patterns at first quarterly, then monthly from October 1942. The Newmanry report records, for its own period, that the motor wheel patterns changed every day. After 1 August 1944 the patterns changed daily.
- What was new
- What matters here is no mechanism but a key regime: the separation of the rarely changed, physically distributed long-term key from the message key, which until March 1942 was sent in the open. That layering only holds as long as the long-term key is changed often enough — and that is exactly where the Germans economised. Copeland writes that in January 1942 Tutte single-handedly deduced the fundamental structure of the machine from some 4,000 characters of key recovered by Tiltman, without ever having seen it. By the time the patterns began changing daily on 1 August 1944, Bletchley already had the machinery to keep up with that rhythm.
Connections
Every connection names its kind. “Forerunner” on its own would be a claim.
- Works withLorenz SZ401940The wheel pattern is the setting of the cams on the twelve wheels of the cipher attachment: 23 to 61 cams per wheel, each made operative or inoperative. From them the machine builds its key stream; Tiltman calls it a prearranged pattern of crosses and dots. According to Tiltman the chi and psi patterns were changed monthly in 1941; from 1 August 1944 they changed daily.Source: John Tiltman, The 'Tunny' Machine and Its Solution (1961) · B. Jack Copeland, Colossus: Breaking the German 'Tunny' Code at Bletchley Park, The Rutherford Journal