[Cryptography] Points of compromise

Jerry Leichter leichter at lrw.com
Mon Sep 9 06:44:42 EDT 2013

On Sep 8, 2013, at 1:53 PM, Phillip Hallam-Baker wrote:

> I was asked to provide a list of potential points of compromise by a concerned party. I list the following so far as possible/likely:
It's not clear to me what kinds of compromises you're considering.  You've produced a list of a number of possibilities, but not even mentioned whole classes of them - e.g., back doors in ECC.

I've expanded, however, on one element of your list.
> 2) Covert channel in Cryptographic accelerator hardware.
> It is possible that cryptographic accelerators have covert channels leaking the private key through TLS (packet alignment, field ordering, timing, etc.) or in key generation (kleptography of the RSA modulus a la Motti Young). 
There are two sides to a compromise in accelerator hardware:  Grabbing the information, and exfiltrating it.  The examples you give - and much discussion, because its fun to consider such stuff - look at clever ways to exfiltrate stolen information along with the data it refers to.

However, to a patient attacker with large resources, a different approach is easier:  Have the planted hardware gather up keys and exfiltrate them when it can.  The attacker builds up a large database of possible keys - many millions, even billions, of keys - but still even an exhaustive search against that database is many orders of magnitude easier than an exhaustive search on an entire keyspace, and quite plausible - consider Venona.  In addition, the database can be searched intelligently based on spatial/temporal/organizational "closeness" to the message being attacked.

An attack of this sort means you need local memory in the device - pretty cheap these days, though of course it depends on the device - and some way of exfiltrating that data later.  There are many ways one might do that, from the high tech (when asked to encrypt a message with a particular key, or bound to a particular target, instead encrypt - with some other key - and send - to some other target - the data to be exfiltrated) to low (pay someone with physical access to plug a USB stick into the device periodically).

                                                        -- Jerry

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