POSH : a generalized CAPTCHA with security applications

A puzzle only solvable by humans, or POSH, is a prompt or question with three important properties: it can be generated by a computer, it can be answered consistently by a human, and a human answer cannot be efficiently predicted by a computer. In fact, a POSH does not necessarily have to be verifiable by a computer at all. One application of POSHes is a scheme proposed by Canetti et al. that limits on-line dictionary attacks against password-protected local storage, without the use of any secure hardware or secret storage. We explore the area of POSHes, implement several candidate POSHes and have users solve them, to evaluate their effectiveness. Given these data, we then implement the above scheme as an extension to the Mozilla Firefox web browser, where it is used to protect user certificates and saved passwords. In the course of doing so, we also define certain aspects of the threat model for our implementation (and the scheme) more precisely.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Electrical Engineering and Computer Science.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2008

Author dc:creator

• Daher, Waseem S. (Waseem Sebastian)

Advisor dc:contributor.advisor

• Ran Canetti and Ronald L. Rivest.

• Electrical Engineering and Computer Science.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/45628

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/45628