# The 1944 Bletchley Park Cryptographic Dictionary

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Tony Sale's
Codes and Ciphers

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A series of figures or letters (or a group or single unit of such a series) which is added figure by figure or letter by letter, usually non-carrying, especially to the figures or letters of code-groups in the process of reciphering, and subtracted from cipher groups in the process of stripping a reciphered code message; a reciphering key or subtractor.

A daily-changing number which is added to a basic number in the process of determining the page of the call-sign book, known as the Bird book, to be used on a particular day.

A reciphering table applied by addition.

A process (used in enciphering, reciphering, deciphering, etc.) whereby two figures, two letters, or a letter and a figure are combined and the result denoted by a figure, a letter, and a letter respectively, on a conventional basis.

1. Addition of figures is usually non-carrying and in the scale of 10, the series 0 1 2 3 4 5 6 7 8 9 being treated as cyclic; so that 1 + 2 = 3, 5 + 9 = 4; but ordinary "carrying" addition is occasionally used, and non-carrying addition in other scales, e.g. 11, when 5 + 9 = 3.

2. In the addition of letters the letters A - Z may be treated as having the values 0 - 25 and as forming a cyclic series; addition is always non-carrying in the scale of 26, so that A + O = O, I + O = W, M + S = E. Or A - Z may be treated as having the values 1 - 26, i.e. 1 - 0, so that A + O = P I + O = X, M + S = F.

3. Addition of a letter and a figure is similarly non-carrying in the scale of 26, the figure indicating the number of places to move to the right in a cyclic alphabet, so that A + 4 = E Y + 5 = D, cf. Gronsfeld.

(used especially of recipher keys). Applied by addition.