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# Digital contact/presence tracing in FR/Europe: lessons learned after two years

Vincent Roca

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## To cite this version:

Vincent Roca. Digital contact/presence tracing in FR/Europe: lessons learned after two years. 2022. [hal-03693797](https://hal HAL Id: hal-03693797)

https://inria.hal.science/hal-03693797

Preprint submitted on 13 Jun 2022

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Distributed under a Creative Commons Attribution 4.0 International License
# Digital contact/presence tracing in FR/Europe: lessons learned after two years

V. Roca for the PRIVATICS team

PRIVASKI seminar, March 8th, 2022

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Inria
# Some background
France: TAC

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#TOUS
AntiCovid

Consortium led by Inria

ROBERT & CLÉA protocols designed by PRIVATICS

Rest of the world (almost): GAEN

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Google

Exposure Notifications: Help slow the spread of
COVID-19, with one step on your phone

The easy, on-the-shelf solution, supposedly at no
risk...
EU Member States status

* GAEN: AT, IT, LV, DE, PL, DK, IE, HR, ES, NL, FI, EE, PT, MT, BE, CZ, SI, LT, NO, CY (22, of which 19 are interconnected through EFGS)

* Centralized: FR, HU
(Warning: map not accurate)

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# Contact... or presence tracing? (2)

**Step 1: remember...**

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Alice
CT: keep a 14-days history of **contacts**
{contact pseudo, time, distance}

PT: keep a 14-days history of **locations**
{QRcode (includes location pseudo), time}

**Step 2: check risk...**

**decentralized:** **locally** (CLÉA for PT and GAEN for CT)

**centralized:** by polling a **server** that computes risks
(ROBERT for CT)
# Contact... or presence tracing? (3)

Step 3: share if COVID+...

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if Alice agreed, upload to server :

GAEN for CT: Alice pseudos (only)

CLÉA for PT: scanned QR codes (only)

ROBERT for CT: contact list (only)
# Contact... or presence tracing? (4)

* different **epidemiologic** assumptions

* direct virus transmission (close to a COVID+ person), or

* virus remains in a location in the air for some time...

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Q: where's the main risk?

* different **technological** requirements

* QRcode scanning is less demanding than BLE
# Forward tracing or backward tracing?

Assume Alice is tested COVID+...

Contact Tracing

* **Forward:** who might have been contaminated by Alice when contagious?

→ inform her contacts

Presence Tracing
(does both 😊)

* **Backward:** on what occasion/where could Alice be contaminated?

→ inform people who were there

→ (potentially) identify who is key index, inform him (was he asymptomatic?) and his contacts
Let's talk about threats and privacy
Decentralized **risk analysis requires sharing data 😞**

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**Step 1:** server collects (GAEN) or computes (CLÉA) a **black list**

**Step 2:** distribute **publicly** the black list through a CDN (w/o access control)

**Step 3:** download the black list and compare with local history

Key question: nature of information in the black list?
# Decentralized CT **threat and privacy**

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**Decentralized** risk analysis (e.g., GAEN) requires a smartphone to know pseudonyms of all COVID+ users

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The GAEN black list of “diagnosed pseudos” is **public**
(see https://hal.inria.fr/hal-02899412).

No effective and scalable way to add access control

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Not GDPR friendly... and creates discrimination risks
(see https://coronadetective.eu)
# There are bad centralized CT designs 😞

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→ side effect: server knows the social graph

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COVID+

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uploads her pseudonyms

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here are my contacts, is one of them at risk?

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* Problem: Bob exposes his full contact list to the server 😞
# But there are good centralized CT designs 😊

(e.g., ROBERT)

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→ knows only a list of pseudos at risk

* Server only knows an unordered list of pseudonyms at risk, nothing else

* Bob does not reveal anything to the server (idem GAEN 😊)
# Centralized CT **threat and privacy**

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list of contacts with
COVID+ users
« 0x5B08 », 10mn
« 0xAE20 », 5mn
...

**Centralized** risk analysis (e.g., **ROBERT**) requires
the server to know the contacts of COVID+ users

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No public black list (neither “diagnosed pseudos” nor “at risk” pseudos)

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GDPR compliant 😊

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Yet there are **major** assumptions regarding the threat model!

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# About CT threat and privacy modes!

if ((you can **trust** the institutions of your country)
&& (the use of the app is **optional**)
&& (a trusted **3rd party** audits the whole system)) {

privacy
model

/* health authority has more control, no sensitive
 * data shared publicly, GDPR compliant, safe */
use a **centralized CT**;

} else {

/* risk of abuse cannot be mitigated,
 * GDPR non-compliance is perhaps secondary */
use a **decentralized CT**;

}
# Decentralized Presence Tracing **threat and privacy**

**Decentralized** risk analysis (e.g., **CLÉA**) requires a smartphone to know locations at risk (pseudo)

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The CLÉA black list of “location pseudos” is **public**

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It’s not sensitive health data 😊 and all locations Bob visited remain local (unless COVID+)

black list of location pseudos

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## Centralized PT **threat and privacy models (2)**

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**Centralized** risk analysis **LUCA** collects name/phone number of participants to an event for the HA to contact them by phone if needed

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Potentially helpful from an **epidemiologic** viewpoint...

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...but risky from a **privacy** viewpoint.

(see: [abuse of LUCA by police to find witnesses 2022-01](https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/cloth-face-cover-guidance.html#about-the-covid-19-virus) 2022-01)
# On-the-shelf Google's GAEN must be audited too!

* **None** of the EU Members States checked GAEN security sufficiently

* **data breach** during > 1 year

* 400 pre-installed apps had access to everything

* **no fix** during 2 months after AppCensus responsible discl. whereas the fix is trivial

https://themarkup.org/privacy/2021/04/27/google-promised-its-contact-tracing-app-was-completely-private-but-it-wasnt

https://blog.appcensus.io/2021/04/27/why.google-should-stop-logging-contact-tracing-data/

## The Markup

Big Tech Is Watching You. We're Watching Big Tech.

### Privacy

## Google Promised Its Contact Tracing App Was Completely Private—But It Wasn't

Researchers say hundreds of preinstalled apps can access a log found on Android devices where sensitive contact tracing information is stored

By Alfred Ng

April 27, 2021 08:00 ET

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# Sovereignty
* What place for the national Health Authority?

* at the center vs. periphery

* Who should decide what features are authorized?

* should G&A decide in place of democratic gov?

* Don't you think the OS be neutral?

* iOS restrictions on use of BLE in background make CT very hard. Apple always refused to white list the FR app. It had epidemiologic consequences. Normal?

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Tech

NHS Covid-19 app update blocked for breaking Apple and Google's rules

By Leo Kellon

Technology desk editor

6 hours ago

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Coronavirus pandemic

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Makers of the NHS Covid-19 app had planned to ask infected users to share their check-in histories

An update to England and Wales's contact tracing app has been blocked for breaking the terms of an agreement made with Apple and Google.
« if we're faced with any kind of trade-off one of the guiding insights that we've used through this process is that **social graph is more sensitive and more privacy risky than infection status data.**»

G. Hogben (Dir. privacy Android) Oct. 2020: https://www.youtube.com/watch?v=0gqZJXOO9Ko [offset 3:35]

* Who should decide that GDPR compliance is of lesser importance? G&A?

* CNIL preferred ROBERT to GAEN

* what would have happened if this key question had been discussed in April 2020?

* why does the "**joint statement on CT**" (April 19th, 2020) totally omit this trade-off?

* No longer valid in case of higher death rates

* GAEN prohibited
Did it work?

Epidemiologic and technical efficiency
# Epidemiologic: Yes, CT worked in UK

* In March 2020 there were suspicions it could work (simulations)

* One year later [1] proved CT **saved 4200 - 8700 lives** in UK in Sept.-Dec. 2020

* to compare to the 32 500 deaths during period

* but ~28% of regular users within population

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[1] "The epidemiological impact of the NHS COVID-19 app", Nature, Vol 594, 17 June 2021. https://www.nature.com/articles/s41586-021-03606-z
Epidemiologic: In FR it's hard to conclude

* Missing a sound epidemiologic study!

○ Usage analytics since June 2021 only (required a decree, published Feb. 2021)

○ The existing efficiency evaluation document is limited in its methodology 😞

○ Raises the question of evaluation-by-design...

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#Tous
AntiCovid
# Epidemiologic: no clue for Presence Tracing

* No evaluation as far as I know

* In FR, after one month, use of PT compromised by generalization of sanitary pass in bars/restaurants/sport centers

* Important to know since CT and PT rely on different virus spreading models...
Which one is valid?

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wearing masks or not?
**Technical: Centralized/ROBERT more efficient**

* **Scalability** in peak contamination phases

* one of the 3 goals of Digital CT

* ROBERT scales better (computation on server + no need to download a black list)

**download traffic** over days TAC vs. GAEN (CWA DE app)

NB: collected Feb. 17-22nd, 2022 during Omicron wave, keeping only traffic from TAC (resp. CWA) servers

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Technical: Centralized/ROBERT more efficient (2)

* Being centralized enables **symmetric cryptography**... prohibited with GAEN

* keep a shared key on the server per registered app

* enables efficient use of 16 bytes of payload for BLE messages

→ ROBERT BLE messages have (limited) **anti-replay** + encrypted **country** code

→ ROBERT avoids the “single world” syndrome of the pan-European interconnection of GAEN apps (EFGS service): each app downloads all diagnosed keys of all EU countries
Takeout
* There's **no single universal solution...** Depends on:

* the threat model WRT institutions versus Google/Apple

* the threat model WRT data that decentralized systems share

* GAEN acceptable for COVID... But not with a virus with **higher death rate**

* GAEN not being GDPR compliant is an issue... see: www.coronadetective.eu

* Result is anyway a trade-off as different stakeholders have different expectations

* Digital CT proved to be **efficient** (UK / GAEN / 28% of usage)

* essentially a matter of user acceptance + a few tricks (easy upload, etc.)

* however efficiency is complex question... What about **evaluation-by-design**?

* Centralized scales better and enables technical optimizations

* Acceptance raises different questions... ask **sociologists**, not only privacy experts
# DÉSIRÉ 3rd way CT: “identify encounters, not devices”

A paradigm shift: from “public device pseudonyms” (GAEN, ROBERT) to “private encounter pseudonyms”:

* that **change** across time and devices

* are **private** to users who met (eavesdroppers powerless)

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compute encounter
pseudo: "0x20E17"

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compute encounter
pseudo: "0x20E17"

And major benefits:

* better **privacy** protection

* highly **flexible**: centralized or decentralized risk evaluation, at a country own discretion

* full **interoperability** across different deployments (≠ GAEN or ROBERT)

“DESIRE: Leveraging the best of centralized and decentralized contact tracing systems”, ACM DTRAP, 2021.

https://hal.inria.fr/hal-03476799/en
Thank you...

vincent.roca@inria.fr

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#Tous
AntiCovid

L'imposer, on ne peut pas,
le recommander on doit