Sunday, March 21, 2010

In Search of Encrypted Search

Last December Moxie Marlinspike announced a $34 cloud service for auditing the strength of WPA passwords. The service tries to reconstruct the password from the user-supplied WPA session material and a hand-crafted dictionary of 135 million passwords. Marlinspike's cloud service searches through the optimised dictionary in just 20 minutes, a computation that would otherwise take 5 days or so on a standard desktop. Notice then that Moxie learns the password if it is found in his dictionary. This might be an example of where customers would prefer Moxie to perform an encrypted search on your behalf – the parameters are encrypted and Moxie just learns a simple yes or no as to whether the corresponding password was found.

This is an encrypted search problem that could be solved in principle by the “fully homomorphic” encryption system devised by IBM researcher Craig Gentry in June of last year. There was much fanfare around the breakthrough and googling “ibm homomorphic encryption” returns over 21,000 results today. There is a long and informative article in Forbes under the headline IBM's Blindfolded Calculator, and you can also see a lecture by Gentry on his result here, given at the Fields Institute in Toronto last December.

Homomorphisms in RSA

But what is homomorphic encryption? Well RSA gives us a nice example, and recall that the “textbook” RSA encryption of a message M is given as

E(M) = M^e mod N

It is not hard to verify that

E(M1) * E(M2) = E(M1 * M2)

which in words states that the product of two encryptions is equal to the encryption of their product. And this is the defining property of a homomorphism – given a function (here encryption) and two inputs (here messages) with a binary operation defined on those inputs (here multiplication), we can interchange the order of applying the function and the operation to get the same result. So we say that the RSA function is a homomorphism with respect to multiplication, or more simply, just a multiplicative homomorphism.

So for an unknown message M2, if we are given E(M2) we can multiply E(M2) by E(M1) to produce E(M1*M2) without needing to decrypt M2. In the early days of RSA this homomorphic property was deemed to be undesirable, due to posited scenarios like the following one (which in fact seems more likely today than they did 20 years ago). Let’s say Alice is going to transfer $M to the account of Bob and sends an encrypted message E(M) instructing her bank to perform the transfer. But Bob intercepts the message and multiples it by E(2), the encryption of 2. The new message becomes E(2)*E(M) = E(2*M) and Bob doubles his money without having to decrypt E(M). The PKCS 1 format has removed the threat of manipulating messages in this way by adding additional padding and fixed fields to messages which can detect such (multiplicative) manipulation.

While RSA is multiplicatively homomorphic it is not additively homomorphic, since in general

E(M1) + E(M2) != E(M1 + M2).

Getting back to the encrypted search problem, does RSA being a multiplicative homomorphism help us? Well not really. Say we are searching for a string S in some data D, where we are only given E(D). We could encrypt S to get get E(S), and then compute

E(S) * E(D)  =  E(S*D)

but E(S*D) won’t tell us that S is somewhere in D because treating S and D as numbers then multiplying them is not related to searching. So we need to find an encryption scheme that has more homomorphic properties than just multiplication if we are to perform encrypted search. And this is what Mr. Gentry did.

Fully Homomorphic Encryption

This elusive property is what Gentry has created in his “fully homomorphic” encryption system, a system which can support the secure homomorphic evaluation of any function given an inout, such as search over data, and not just simple arithmetic functions like multiplication. The underlying computational problem that furnished this property was neither RSA nor discrete logarithm systems but rather a geometric construction referred to as an ideal lattice. Lattices are special vector spaces that furnish computationally difficult problems that have been used in several other cryptosystems such as NTRU, and provide an alternative to the “standard” hard problems used from number theory.

Using the ideal lattice Gentry was able to show how to compute a “circuit” for problems such as encrypted search that can be evaluated without decrypting the data. Both the encrypted term and data are used as inputs to the circuit, and the trick is to devise a method for securely and correctly evaluating the gates of the circuit. Your computer uses built-in gates and circuits to evaluate computations, such as performing the multiplications and divisions to evaluate an RSA encryption or decryption during an SSL session for example. And as such the computations are small and fast. However for encrypted search the corresponding circuit needs to be built and represented in software, which leads to large and inefficient computations. So this is why the result is a breakthrough but not a practical one at the moment. Gentry has estimated that building a circuit to perform an encrypted Google search with encrypted keywords would multiply the current computing time by around 1 trillion.

The Outlook for Encrypted Search

It remains to be seen whether Gentry’s theoretical breakthrough can be converted into a practical service, in the cloud or otherwise. Gentry has worked with some other researchers from IBM and MIT to remove the requirement of using lattices and to just treat the fully homomorphic encryption as operating over the integers, which provides a simpler conceptual framework. Making the central ideas more accessible will increase the chances of finding more practical solutions. There is a recent review of homomorphic encryption by Craig Stuntz, giving more details and promising code in an upcoming post. Stuntz also points to an article by Gentry in the respected Communications of the ACM which begins

Suppose that you want to delegate the ability to process your data, without giving away access to it. We show that this separation is possible: we describe a "fully homomorphic" encryption scheme that keeps data private, but that allows a worker that does not have the secret decryption key to compute any (still encrypted) result of the data, even when the function of the data is very complex. In short, a third party can perform complicated processing of data without being able to see it. Among other things, this helps make cloud computing compatible with privacy.

Reading further requires a subscription but it is clear that the work of Gentry and his co-authors is being positioned as a core service for cloud platforms, and I am suspecting IBM’s platform in particular. However, just at the moment, you will have to trust Moxie with your WPA password.


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