The public-key encryption method, achieving 128-bit security, typically requires from the sender to solve one CAPTCHA. The receiver does not need to resort to any human aid.
A second symmetric encryption method allows to encrypt messages using very short passwords shared between the sender and the receiver. Here, a simple 5-character alphanumeric password provides sufficient security for all practical purposes.
We conjecture that the automatic construction of Decisional and Existential CAPTCHAs is possible and provide candidate ideas for their implementation.Category / Keywords: Date: received 8 Aug 2016 Contact author: remi geraud at ens fr Available format(s): PDF | BibTeX Citation Version: 20160810:204348 (All versions of this report) Short URL: ia.cr/2016/763 Discussion forum: Show discussion | Start new discussion