What technical methods would platforms use to scan previously stored messages under EU chat control?
Executive summary
The EU “Chat Control” debate centers on mandating providers to detect CSAM by scanning private communications; the technical route most cited is client‑side scanning — analysis on user devices before encryption — which critics say undermines end‑to‑end encryption and raises false‑positive and surveillance risks [1] [2]. Council and Parliament texts and advocacy groups report moves from mandatory to voluntary or targeted scanning, and talk of “vetted technologies” to scan images, videos and links (less consistently texts) on devices or servers [3] [4] [5].
1. What “scanning previously stored messages” means in practice
Scanning previously stored messages refers to analysing content that already exists in a provider’s servers or a user’s device — for example cloud backups, archived chats, or device files — rather than only intercepting messages in transit. EU documents and commentary make a distinction between server‑side scanning (providers inspect stored content they control) and client‑side scanning (software inspects files on a user’s device before encryption or upload) [1] [4]. Several sources say the proposal would reach both stored and in‑flight content where providers operate storage or implement client‑side tools [4] [3].
2. Client‑side scanning: the headline technical method
The most discussed technical method is client‑side scanning (CSS): software on a handset or PC compares local content against databases or detection models before that content is encrypted and sent or stored. Civil‑liberties groups and technical commentators describe CSS as the way to make E2EE services detectable for CSAM, because it acts prior to encryption [2] [1]. EU reporting and industry summaries use the same term when describing the Commission’s or Council’s envisioned solutions and note vendors would be required to deploy “vetted technologies” for this purpose [4] [2].
3. Server‑side and metadata scanning remain parts of the toolbox
Where services do not provide end‑to‑end encryption or hold backups, scanning can and already does happen on the server: providers can index and compare stored images, links and videos against known CSAM hashes or AI classifiers [1]. The EU drafts and press note scenarios where high‑risk services are ordered to scan links, images and videos — though some drafts said texts may be excluded while others explicitly extend scope to text [5] [3] [6].
4. Detection technologies mentioned: hashes, ML models, and vetted tools
Sources cite concrete technical approaches: perceptual hashing or known‑content matching for previously identified CSAM; machine‑learning classifiers for image/video/URL detection; and vendor‑approved (“vetted”) tools that would be mandated to run either on clients or servers [4] [7]. The European Parliament’s impact assessments and independent analysts warn about high false positives and limits to reliably detecting “new” CSAM or nuanced grooming behaviour [7] [1].
5. Technical limits and security trade‑offs reported by experts
Multiple sources stress that CSS cannot be implemented without altering the security properties of end‑to‑end encryption: inserting detection on the client creates new attack surfaces, potential backdoors, or supply‑chain risks that critics say weaken overall security [1] [8]. The European Digital SME Alliance and technologists argue that mandating client‑side tools undermines encryption and digital sovereignty [8]. Independent analyses cited by EU bodies flagged false positives and misidentification as practical barriers [7].
6. Political context shapes what technical methods become law
The proposal’s text and Council negotiations shifted repeatedly: mandatory universal scanning was contested, some presidencies pushed for voluntary or targeted scanning, and drafts differ on whether text is in scope [3] [5] [9]. That political tug‑of‑war will determine whether scanning of stored messages is required server‑side, client‑side, or limited to non‑E2EE contexts [3] [7].
7. Competing viewpoints and implicit agendas
Proponents frame scanning as necessary to detect and report CSAM at scale; drafts reference boosting CSAM reports and existing provider duties [10]. Critics — digital rights groups, technologists, some member states and MEP questions — argue the methods (especially client‑side scanning) create permanent surveillance capabilities, weaken encryption, and may normalize record‑keeping and broad monitoring beyond child protection [2] [8] [11]. Some industry and national positions emphasize “vetted” tools to limit risks, which can be read as an attempt to reconcile enforcement aims with security concerns [4].
8. What the reporting does not say (limits of available sources)
Available sources do not mention detailed, vendor‑level blueprints for how archived chats would be re‑indexed at scale under the final law, nor do they provide performance figures from large‑scale CSS pilots in the EU context; technical efficacy data beyond general warnings about false positives is limited in these reports [7] [1].
9. Bottom line for readers
If the law mandates scanning of stored messages, the primary technical paths are server‑side scanning for provider‑held content and client‑side scanning for end‑to‑end encrypted channels; both approaches carry trade‑offs between investigatory reach and the security/privacy guarantees of encryption, a point stressed across EU questions, NGO analyses and technical commentary [1] [4] [2]. The final mix of methods will depend on ongoing Council and Parliament decisions that have repeatedly reshaped scope and obligations [3] [7].