Between 1967 and 1970, the breakaway Republic of Biafra fought a civil war against the Nigerian federal government. Biafra maintained a diplomatic mission in Lisbon and passed cable traffic to and from its leadership using tranposition ciphers. Those messages, held privately, had never been deciphered. With George Lasry and Frode Weierud, I broke the principal Biafran cipher systems and recovered the plaintexts of cables that had sat unread for fifty years.
The work combined classical cryptanalysis with modern stochastic search. A detailed account of the attack, the recovered keys and the decrypted traffic appears in the Cryptologia paper below.
Bean, R., Lasry, G. and Weierud, F. (2020). Eavesdropping on the Biafra-Lisbon link: breaking historical ciphers from the Biafran war. Cryptologia. doi:10.1080/01611194.2020.1753265. Local copy: The_Biafran_Ciphers.pdf.
Bean, R., Lasry, G. and Weierud, F. (2020). We decrypted messages from the Biafran war that have remained secret for 50 years. The Conversation.
Conference talk: NSA 2022 Cryptologic History Symposium, Online
See also the full publications list and the presentations archive.