| Cryptography is an active field of theoretical research. It is also a place where many unproven, but time-tested practical ideas exist. This work exposits a few strands that connect these two sides of the Cryptographic coin.;The two main results presented are, 1. The first traitor tracing scheme based on prime order bilinear groups. While prime order bilinear groups are practical, existing schemes for traitor tracing were based on the more structurally rich, but much less practical composite order bilinear groups. Our work brings the rich structure of composite order bilinear groups and the efficiency of prime order groups together. 2. A formal model for CAPTCHAs which captures the intuition that any automaton must request human help to solve CAPTCHA problems. We use this model to obtain positive results in the field of concurrent security. This is the first result that brings CAPTCHA, a widely adopted practical security tool, into main stream cryptography to achieve tasks that are known to be theoretically impossible in the plain model. |
