I learned at Cipherbrain that telegrams from John A. Macdonald, the first prime minister of Canada, were solved by Matthew Brown. He knew that the Canadian government used a codebook by Slater, but unlike other telegraph codebooks, decoding is not a straightforward table lookup with Slater's code. Specifically, Slater's code involves translating a word into a number and, after some manipulation of the number, translating it back into a different word. Brown succeeded in identifying the key for the manipulation (addition) by running a computer program to check every possibility.
I added a section of this achievement in "How to Break a Code (Not a Cipher)". I also made related changes in "Nonsecret Code: An Overview of Early Telegraph Codes" and "Unsolved Historical Ciphers".
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