Cryptology ePrint Archive: Report 2016/763

Human Public-Key Encryption

Houda Ferradi and Rémi Géraud and David Naccache

Abstract: This paper proposes a public-key cryptosystem and a short password encryption mode, where traditional hardness assumptions are replaced by specific refinements of the CAPTCHA concept called Decisional and Existential CAPTCHAs.

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.

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Date: received 8 Aug 2016

Contact author: remi geraud at ens fr

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Version: 20160810:204348 (All versions of this report)

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