An English Polyalphabetic Cipher from the Seventeenth Century

A Polyalphabetic Cipher Table

Add MS 11252 (BL) contains a polyalphabetic cipher table (f.7).

(I made one correction in the above transcription.)

Each column represents a substitution cipher table. Taking column A as an example, "a" is enciphered as 4, "b" as 11, and so on. By switching columns to be used (e.g., according to a keyword), ciphers can be changed. This polyalphabetic cipher is better than the well-known Vigenere cipher because the cipher symbols in each column are in random order (mixed alphabet).

The deciphering table is on the verso.

Although it is not dated, it probably belongs to the mid-17th century (f.5 is from 1648, f.8 is from 1681). Typical English ciphers at the time were those for substituting Arabic numerals for letters of the alphabet. The following is one of the simplest (Ormonde-Clanricarde Cipher (1643)).

Most ciphers were better than this in providing more homophones or employing random assignment (see examples in another article). The polyalphabetic table above is an alternative way to attain greater strength, but actual use of this cipher is not known to me. (It is not granted that different columns are used for different letters. There is a possibility that one column was used for an entire word or an entire line, as described in John Falconer, Cryptomenysis Patefacta (1685).)

A Short Code List

The same volume, Add MS 11252, contains "Cipher key, reign of James II" (f.14), which is transcribed below.

M The King
N The Queene
A Lord Treasurer
F. Hallifax
G Prince of Orange
Q Parliament
X Ireland
Y Scotland
W.W. Dutch
66 the Lord Lt of Ireland
W. Test
K Papists
92 Councell
py Army
f fleet

The catalogue dating to James II's reign is possible, but the entry "Test" may also suggest the period of the Exclusion Crisis (1679-1681) or legislation of the Test Act in 1673).

©2023 S.Tomokiyo
First posted on 11 August 2023. Last modified on 11 August 2023.
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