  Logic Encryption is one of the most popular hardware security techniques
which can prevent IP piracy and illegal IC overproduction. It introduces
obfuscation by inserting some extra hardware into a design to hide its
functionality from unauthorized users. Correct functionality of an encrypted
design depends upon the application of correct keys, shared only with the
authorized users. In the recent past, extensive efforts have been devoted in
extracting the secret key of an encrypted design. At the same time, several
countermeasures have also been proposed by the research community to thwart
different state-of-the-art attacks on logic encryption. However, most of the
proposed countermeasures fail to prevent the powerful SAT attack. Although a
few researchers have proposed different solutions to withstand SAT attack,
those solutions suffer from several drawbacks such as high design overheads,
low output corruptibility, and vulnerability against removal attack. Almost all
the known logic encryption strategies are vulnerable to scan based attack. In
this paper, we propose a novel encryption technique called Encrypt Flip-Flop,
which encrypts the outputs of selected flip-flops by inserting multiplexers
(MUX). The proposed strategy can thwart all the known attacks including SAT and
scan based attacks. The scheme has low design overhead and implementation
complexity. Experimental results on several ISCAS'89 and ITC'99 benchmarks show
that our proposed method can produce reasonable output corruption for wrong
keys.
