I recently read two articles which made me think that we still do not understand well enough what “information” is. Both articles consider ways of managing information by “side channels” or through “covert channels”. In other words, whatever we do, produces much more information than what we believe.

The first article is “Attack of the week: searchable encryption and the ever-expanding leakage function” by cryptographer Matthew Green in which he explains the results of this scientific article by P. Grubbs et al. The scenario is an encrypted database, that is a database where column data in a table is encrypted so that whoever accesses the DB has no direct access to the data (this is not the case where the database files are encrypted on the filesystem). The encryption algorithm is such that a remote client, who knows the encryption key, can make some simple kind of encrypted searches (queries) on the (encrypted) data, extracting the (encrypted) results. Only on the remote client data can be decrypted. Now an attacker (even a DB admin), under some mild assumptions, with some generic knowledge of the type of data in the DB and able to monitor which encrypted rows are the result of each query (of which she cannot read the parameters), applying some advanced statistical mathematics in learning theory, is anyway able to reconstruct with good precision the contents of the table. A simple example of this is a table containing the two columns employee_name and salary, both of them with encrypted values. In practice this means that this type of encryption leaks much more information than what we believed.

The second article is “ExSpectre: Hiding Malware in Speculative Execution” by J.Wampler et al. and, as the title suggests, is an extension of the Spectre CPU vulnerability. Also the Spectre and Meltdown attacks have to do with information management, but in these cases the information is managed internally in the CPU and it was supposed not to be accessible from outside it. In this particular article the idea is actually to hide information: the authors have devised a way of splitting a malware in two components, a “trigger” and a “payload”, such that both components appear to be benign to standard anti-virus and reverse engineering techniques. So the malware is hidden from view. When both components are executed on the same CPU, the trigger alters the internal state of the branch prediction of the CPU in such a way to make the payload execute malign code as a Spectre speculative execution. This does not alter the correct execution by the CPU of the payload program, but through Spectre, extra speculative instructions are executed and these, for example, can implement a reverse shell to give external access to the system to an attacker. Since the extra instructions are retired by the CPU at the end of the speculative execution, it appears as if they have never been executed and thus they seem to be untraceable. Currently this attack is mostly theoretical, difficult to implement and very slow. Still it is based on managing information in covert channels as both Spectre and Meltdown are CPU vulnerabilities which also exploit cache information side-channel attacks.