Impeding CAPTCHA Breakers with Visual Decryption

Simon Lang, Neville Williams

    Research output: Contribution to conferencePaperpeer-review

    6 Citations (Scopus)


    Abuse of free Internet resources and services from false account creation, to spam, to identity theft, excessive bandwidth usage, or even vote stuffing online polls is a big problem. The Completely Automatic Public Turing Test to tellComputers and Humans Apart (CAPTCHA) controls access to resources but automated systems are increasingly adept at overcoming them. In this paper a method of access control is introduced as an extra layer of security on top of existing CAPTCHA implementations. It uses visual encryption to encrypt images, which are presented to clients like a CAPTCHA. It's purpose is to compress many sub-images into a small image format that humans can decode visually but is hard for automated systems due to decrypting overhead, and having to process more images to nd the hidden image. This paper introducesvisual encryption as a viable method to encrypt CAPTCHAs, and tests a prototype to measure how efficiently users can nd them. It also measures whether this method could impede a real CAPTCHA breaker. Results show humans detect images within 16-33 seconds, and deciphering images is almost 100%. Estimates on CAPTCHA breaking benchmarks show automated systems would be slowed signicantly, even assuming the image is found and decoded. As sub-images increase, humans can process the visually encrypted images faster than automated systems can.

    Original languageEnglish
    Number of pages8
    Publication statusPublished - 1 Dec 2010
    Event8th Australasian Information Security Conference (AISC 2010) -
    Duration: 18 Jan 2010 → …


    Conference8th Australasian Information Security Conference (AISC 2010)
    Period18/01/10 → …


    • Access control
    • Authentication
    • Automated attacks
    • Cryptography
    • Security
    • Spamming
    • Visual decryption
    • Visual processing


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