Talking to Vula is the story of the secret communications network of Operation Vula, a project of the African National Congress that facilitated clandestine messaging between South African exiles in London and leaders of the resistance in Zambia and in South Africa. The account (which was pointed out to me by Finn Brunton) talks of an inventive use of encrypted text transmitted via analog modem tones recorded to cassette tape and then played back via untraceable pay phones. It's a remarkable tale of late 80s underground crypto, leading up to the release of Nelson Mandela from prison.
From the text, to give you a flavor of it:
To test this hypothesis I wrote a little program to send some computer output to the modem. Sure enough the sounds came out of the modem`s speaker. These I recorded and played back into the microphone end of the modem while running a communications program on the computer. Eureka! The characters appeared on the screen. I had done with a modem what we were attempting to do with our tone machine.
This seemed to be the real breakthrough. I adapted our encryption program to work with the acoustic modem and recorded the output on a tape recorder. This I took to a public telephone booth and played back to my answering machine. Then I played the answering machine
messageback into the modem and the computer deciphered it successfully. As the plaintext message appeared on the screen I realised that we had finally discovered an absolutely safe method of communicating with the underground using computers.
The work originally appeared in a series of six articles in the ANC`s monthly journal Mayibuye from May 1995 to October 1995.
More: Operation Vula: ICT versus Apartheid (2008).
More: Garrett and Edwards (2007): “[Revolutionary Secrets]: Technology’s Role in the South African Anti-Apartheid Movement,” Social Science Computer Review, 24(4). Preprint at U of Michigan.