Paper 2016/763

Human Public-Key Encryption

Houda Ferradi, Rémi Géraud, and David Naccache


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.

Available format(s)
Publication info
Preprint. MINOR revision.
Contact author(s)
remi geraud @ ens fr
2016-08-10: received
Short URL
Creative Commons Attribution


      author = {Houda Ferradi and Rémi Géraud and David Naccache},
      title = {Human Public-Key Encryption},
      howpublished = {Cryptology ePrint Archive, Paper 2016/763},
      year = {2016},
      note = {\url{}},
      url = {}
Note: In order to protect the privacy of readers, does not use cookies or embedded third party content.