Pseudonyms Generated and Managed by Authority, Communicated to Users

[nadimkobeissi]

The ROBERT summary document contains the following diagram, showing authorities generating pseudonyms and transmitting them directly to users:

However, Section 1.3 of version 1.0 of the ROBERT specification states that, as a security and privacy requirement, ROBERT mandates the following:

Anonymity of users from a central authority. The central authority should not be able to learn information about the identities or locations of the participating users, whether diagnosed as COVID-positive or not.

And yet, this assumption is only meant to hold under an honest authority:

The authority running the system, in turn, is ”honest-but-curious”. Specifically, it will not deploy spying devices or will not modify the protocols and the messages. However, it might use collected information for other purposes such as to re-identify users or to infer their contact graphs. We assume the back-end system is secure, and regularly audited and controlled by external trusted and neutral authorities (such as Data Protection Authorities and National Cybersecurity Agencies).

Furthermore, Section 2.2 states the following:

When a user wants to use the service, she installs the application, App, from an official App store (Apple or Google). App then registers to the server that generates a permanent identifier (ID) and several Ephemeral Bluetooth Identifiers (EBIDs). The back-end maintains a table, IDTable, that keeps an entry for each registered ID. The stored information are “anonymous” and, by no mean, associated to a particular user (no personal information is stored in IDTable).

In short, all of ROBERT is built on trust from central authorities and the assumption that they will behave honestly and be impervious to third-party compromise. I am unable to determine how this is a strong, or even serious and realistic approach to real user privacy. Could you please justify how this protocol achieves any privacy from authorities, and how the current model of assuming that all authorities are:

• Completely honest,
• Impervious to server/back-end compromise,
• Impervious to any transport-layer compromise or impersonation,

...is in any way realistic or something that can be taken seriously as a privacy-preserving protocol? Given the level of trust assurances that you are attributing to authorities, and given that authorities are responsible for generating, storing and communicating all pseudonyms directly to users to their devices, what security property is actually achieved in ROBERT in terms of pseudonymity between authorities and users?

Furthermore, it appears that the trust model for ROBERT is such that the server allocates pseudonyms and is thereafter trusted to never examine the social graph or any network relationship graph for users, ever. How could this possibly be a reasonable assumption for a privacy-preserving protocol?

[dbeniamine]

Why is there a need for a central authority to manage peusodnyms anyway ?

In a similar protocol DP3T the devices generates it's own randomized pseudonyms.

The central authority is only contacted to get pseudonyms of infected people thus get way less information about users.

[bortzmeyer]

Why is there a need for a central authority to manage peusodnyms anyway ?

I don't want you to believe I support the design of the ROBERT protocol (I don't) but your specific point is addressed in appendix A. (Summary: it is to avoid repetition of the "one contact" attack.)

[ThomasFournaise]

@bortzmeyer the "one contact" attack is dismissed by sending random "false positive" @dbeniamine with a central verification you don't know when you may have been in contact with covid (except one contact) you only receive an information. With a local verification, you receive a list of covid ID, if you logged when you receive these IDs (by creating your ownapp for example) you will know that you have been contaminated this day at this time and then at this place. If you write all your agenda you may be able to find which people contaminated you....

[PRIVATICS-Inria]

Thanks @kaepora for raising this issue.

In ROBERT scheme v1.0, the server is indeed responsible for generating, storing and communicating pseudonyms. This is done for two main reasons:

• A user can only perform an Exposure Status Request phase corresponding to its pseudonyms. Indeed, the pseudonyms are linked to a secret key K_A on the server, and the server verifies that the request originates from the owner of the secret key K_A before answering. Therefore, an attacker cannot query the server for the exposure status of another user.

• The pseudonyms of infected users are not exposed to other users. Since the computation of exposure status is done on the server, there is no need to publish pseudonyms of infected users as it may be the case with other solutions.

About the pseudonymity of users with regard to the server. The server does not store any other identifiers than the one included in the IDTable database (see section 3.2. Application Registration (Server Side)). During the Application Registration and the Exposure Status Request, the app may expose network identifiers that could compromise her real identity. To mitigate this risk, solutions like Mixnet or proxies could be used (as suggested in Section 6, footnote 11).

More generally, concerning the “honest but curious” assumption: this is a key assumption for the ROBERT v1.0 design as you noticed. It is not our responsibility, as privacy researchers, to judge whether or not this assumption is valid.

This topic could be discussed for hours, clearly. However, when looking at the “avis CNIL sur le projet d’application mobile StopCovid”, we have the feeling this is a reasonable assumption.

[nadimkobeissi]

@PRIVATICS-Inria

More generally, concerning the “honest but curious” assumption: this is a key assumption for the ROBERT v1.0 design as you noticed. It is not our responsibility, as privacy researchers, to judge whether or not this assumption is valid.

Could you please care to justify this statement, which appears to be laughably absurd if taken at face value?

This topic could be discussed for hours, clearly. However, when looking at the “avis CNIL sur le projet d’application mobile StopCovid”, we have the feeling this is a reasonable assumption.

Incredibly disappointing, but somehow not surprising, to see an appeal to authority used in a scientific discussion by the INRIA Privatics team.

[everdha]

The statement of the CNIL (n° 2020-056 of May 25th 2020) relies precisely on the assumption that the server cannot know the contact list of an infected user (see point 41). You are responding to our concern that this point has not been adressed by the protocol by saying that CNIL states it's ok? Aren't we in an infinite non-sense loop here? How are you precisely and technically adressing this issue please, if you are?

And yes indeed, we do not have hours and days to discuss this, since the app was voted in France and will be deployed within hours, that is why we would like a more scientific and clear answer.

[nadimkobeissi]

This topic could be discussed for hours, clearly. However, when looking at the “avis CNIL sur le projet d’application mobile StopCovid”, we have the feeling this is a reasonable assumption.

Hey @PRIVATICS-Inria, still have the feeling this is a reasonable assumption?