Hacking the GCHQ Backdoor
Last week, I evaluated the security of a recent GCHQ backdoor proposal for communications systems. Furthering the debate, Nate Cardozo and Seth Schoen of EFF explain how this sort of backdoor can be detected:
In fact, we think when the ghost feature is active—silently inserting a secret eavesdropping member into an otherwise end-to-end encrypted conversation in the manner described by the GCHQ authors—it could be detected (by the target as well as certain third parties) with at least four different techniques: binary reverse engineering, cryptographic side channels, network-traffic analysis, and crash log analysis. Further, crash log analysis could lead unrelated third parties to find evidence of the ghost in use, and it’s even possible that binary reverse engineering could lead researchers to find ways to disable the ghost capability on the client side. It should be obvious that none of these possibilities are desirable for law enforcement or society as a whole. And while we’ve theorized some types of mitigations that might make the ghost less detectable by particular techniques, they could also impose considerable costs to the network when deployed at the necessary scale, as well as creating new potential security risks or detection methods.
EDITED TO ADD (1/26): Good commentary on how to defeat the backdoor detection.
EDITED TO ADD (3/1): Another good essay on the security risks of this back door.