International Conference on the Theory and Applications of Cryptographic Techniques

EUROCRYPT 2003: Advances in Cryptology — EUROCRYPT 2003 pp 294-311

CAPTCHA: Using Hard AI Problems for Security

  • Luis von Ahn
  • Manuel Blum
  • Nicholas J. Hopper
  • John Langford
Conference paper

DOI: 10.1007/3-540-39200-9_18

Volume 2656 of the book series Lecture Notes in Computer Science (LNCS)
Cite this paper as:
von Ahn L., Blum M., Hopper N.J., Langford J. (2003) CAPTCHA: Using Hard AI Problems for Security. In: Biham E. (eds) Advances in Cryptology — EUROCRYPT 2003. EUROCRYPT 2003. Lecture Notes in Computer Science, vol 2656. Springer, Berlin, Heidelberg

Abstract

We introduce captcha, an automated test that humans can pass, but current computer programs can’t pass: any program that has high success over a captcha can be used to solve an unsolved Artificial Intelligence (AI) problem. We provide several novel constructions of captchas. Since captchas have many applications in practical security, our approach introduces a new class of hard problems that can be exploited for security purposes. Much like research in cryptography has had a positive impact on algorithms for factoring and discrete log, we hope that the use of hard AI problems for security purposes allows us to advance the field of Artificial Intelligence. We introduce two families of AI problems that can be used to construct captchas and we show that solutions to such problems can be used for steganographic communication. CAPTCHAs based on these AI problem families, then, imply a win-win situation: either the problems remain unsolved and there is a way to differentiate humans from computers, or the problems are solved and there is a way to communicate covertly on some channels.

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Copyright information

© International Association for Cryptologic Research 2003

Authors and Affiliations

  • Luis von Ahn
    • 1
  • Manuel Blum
    • 1
  • Nicholas J. Hopper
    • 1
  • John Langford
    • 2
  1. 1.Computer Science Dept.Carnegie Mellon UniversityPittsburghUSA
  2. 2.IBM T.J. Watson Research CenterYorktown HeightsUSA