1. On Parallel and Concurrent Security of HB and HB+
Jonathan Katz and Ji-Sun Shin
University of Maryland

2. Motivation
Juels and Weis analyze and propose the HB and HB+ protocols (tomorrow)
Building on prior work of Hopper-Blum
HB: secure against passive adversary HB+: secure against active adversary
They are extremely efficient
Suitable for low-power devices

3. But… Juels-Weis analyze “basic”, two-round protocols with soundness ½
Then claim security under sequential repetition
Yields protocols with O(k) rounds!
Also, no security under concurrent executions…

4. Our Results
Using recent results of Regev (STOC ’05), we prove security of HB/HB+ under concurrent executions
HB/HB+ can be “collapsed” to only 2-3 rounds!
We believe our approach also greatly simplifies the proofs…