Bugs in our Pockets: The Risks of Client-Side Scanning
arXiv:2110.07450 [cs.CR], October 14, 2021.
Our increasing reliance on digital technology for personal, economic, and government affairs has made it essential to secure the communications and devices of private citizens, businesses, and governments. This has led to pervasive use of cryptography across society. Despite its evident advantages, law enforcement and national security agencies have argued that the spread of cryptography has hindered access to evidence and intelligence. Some in industry and government now advocate a new technology to access targeted data: client-side scanning (CSS). Instead of weakening encryption or providing law enforcement with backdoor keys to decrypt communications, CSS would enable on-device analysis of data in the clear. If targeted information were detected, its existence and, potentially, its source, would be revealed to the agencies; otherwise, little or no information would leave the client device. Its proponents claim that CSS is a solution to the encryption versus public safety debate: it offers privacy—in the sense of unimpeded end-to-end encryption—and the ability to successfully investigate serious crime.
In this report, we argue that CSS neither guarantees efficacious crime prevention nor prevents surveillance. Indeed, the effect is the opposite. CSS by its nature creates serious security and privacy risks for all society while the assistance it can provide for law enforcement is at best problematic. There are multiple ways in which client-side scanning can fail, can be evaded, and can be abused.
Its proponents want CSS to be installed on all devices, rather than installed covertly on the devices of suspects, or by court order on those of ex-offenders. But universal deployment threatens the security of law-abiding citizens as well as lawbreakers.
Technically, CSS allows end-to-end encryption, but this is moot if the message has already been scanned for targeted content. In reality, CSS is bulk intercept, albeit automated and distributed. As CSS gives government agencies access to private content, it must be treated like wiretapping. In jurisdictions where bulk intercept is prohibited, bulk CSS must be prohibited as well.
Although CSS is represented as protecting the security of communications, the technology can be repurposed as a general mass-surveillance tool. The fact that CSS is at least partly done on the client device is not, as its proponents claim, a security feature. Rather, it is a source of weakness. As most user devices have vulnerabilities, the surveillance and control capabilities provided by CSS can potentially be abused by many adversaries, from hostile state actors through criminals to users’ intimate partners. Moreover, the opacity of mobile operating systems makes it difficult to verify that CSS policies target only material whose illegality is uncontested.
The introduction of CSS would be much more privacy invasive than previous proposals to weaken encryption. Rather than reading the content of encrypted communications, CSS gives law enforcement the ability to remotely search not just communications, but information stored on user devices.
Introducing this powerful scanning technology on all user devices without fully understanding its vulnerabilities and thinking through the technical and policy consequences would be an extremely dangerous societal experiment. Given recent experience in multiple countries of hostile-state interference in elections and referenda, it should be a national-security priority to resist attempts to spy on and influence law-abiding citizens. CSS makes law-abiding citizens more vulnerable with their personal devices searchable on an industrial scale. Plainly put, it is a dangerous technology.
Even if deployed initially to scan for child sex-abuse material, content that is clearly illegal, there would be enormous pressure to expand its scope. We would then be hard-pressed to find any way to resist its expansion or to control abuse of the system.
The ability of citizens to freely use digital devices, to create and store content, and to communicate with others depends strongly on our ability to feel safe in doing so. The introduction of scanning on our personal devices—devices that keep information from to-do notes to texts and photos from loved ones—tears at the heart of privacy of individual citizens. Such bulk surveillance can result in a significant chilling effect on freedom of speech and, indeed, on democracy itself.
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