TAPCOD was the first encryption/transmission program used by Operation Vula (1988-1992) in South Africa. It was used from 1988 until 1990, when it was superseded by TECOD.

TAPCOD.BAS was written in PowerBASIC, an advanced form of the Basic programming language that was popular in the 1980s. The PowerBASIC compiler created proper EXE files to run on Microsoft MS-DOS computers.

TAPCOD used a very simple "one-time pad" encryption and decryption algorithm but it had two modes: a "quick" mode that created its random data (key) on the fly and a "data disk" mode that read its random data (key) from a separate data disk. The former was used for messages of less importance that could be disguised with code words in the text; the latter for high security messages where 100% security was essential. Often the two were used together: an important message of high secrecy was first encihpered with the "quick" mode and then re-enciphered with the "disk" mode.

When communicating, TAPCOD had to be used in simplex mode. That is, it was either sending digital data in the form of recorded acoustic modem sounds to a an answering machine, or receiving the same from another answering machine. As the communication was to and from dumb devices, there could be no error checking. To cope with errors, TAPCOD sent and received its data in blocks of 10 characters. Each block was terminated with a block marker that counted the number of blocks. If there were less than 10 characters in a received block, the program inserted a ? for each missing character. If a block marker was missing, the program would insert as many ??s as were required to reach the next block marker in the sequence.

dscho/TAPCOD

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