Connecting Theory and Practice in Modern Cryptography

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 connectthese 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 primeorder bilinear groups are practical, existing schemes for traitor tracing were basedon 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 efficiencyof prime order groups together.2. A formal model for CAPTCHAs which captures the intuition that any automatonmust request human help to solve CAPTCHA problems. We use this model to obtainpositive results in the eld of concurrent security. This is the first result that bringsCAPTCHA, a widely adopted practical security tool, into main stream cryptographyto achieve tasks that are known to be theoretically impossible in the plain model