Here's a link to one-time password system with lots of explanation and discussion.
https://www.grc.com/ppp.htm
There are several free and open implementations:
"GRC offers a complete and free (though not open source) PPP CryptoSystem implementation for Windows platforms, and other open source solutions are already available for Windows, Mac, Linux, and Java-equipped cell phones."
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Here's a link to one-time password system with lots of explanation and discussion. https://www.grc.com/ppp.htm There are several free and open implementations: "GRC offers a complete and free (though not open source) PPP CryptoSystem implementation for Windows platforms, and other open source solutions are already available for Windows, Mac, Linux, and Java-equipped cell phones."