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 Detective Allan Pinkerton, working under his wartime pseudonym Major E. J. Allen, and Pinkerton’s agents tried for months to decipher Rose Greenhow’s secret dispatches, many of which they seized when she was arrested in her home on August 23, 1861. They pored over singed and torn scraps of paper that they recovered from her stove and compared copies of letters she kept with others they intercepted when she thought she had charmed her guards into getting messages out for her. Pinkerton, who had been a railway detective not a cryptographer, undoubtedly would have benefitted from Mrs. Greenhow’s cipher. It was given to her at the start of the war by Thomas Jordan, a U.S. army officer who resigned his commission to join the Confederate forces in his native Virginia. Jordan taught her what he described as a rudimentary cipher, and although she continued to get encrypted messages out after her arrest, he lost confidence in the security of the system and told his superiors some months later to ignore reports written in Mrs. Greenhow’s cipher because he feared it had been compromised. It is possible he was able to provide her a replacement during her imprisonment, but that is not clear.
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