Signal insists that plans to compel tech companies to scan devices for nude images of children announced by UK Prime Minister Keir Starmer on Monday at London Tech Week "will not keep children safe."

"It endangers us all," the encrypted messaging platform said, adding that the mechanism required to implement it would be "dangerous." And it wouldn't be a pro-privacy statement without calling it "dystopian."

Signal argues that the proposed technology could at some point be repurposed to enable state-sponsored surveillance of all citizens' comms, or used as a mass censorship tool.

"Forcing all UK residents to prove their age and/or have all their content scanned, simply to exercise their fundamental right to communicate, is a perilous proposition," Signal stated. 

"We know that mass surveillance and censorship capabilities, however sincere-sounding the promises of those who initiate them are, never remain narrowly scoped. Once created, they will be expanded, forming a dangerous tool that will be wielded both in the UK and abroad to censor and surveil whatever they might consider 'threats' or 'harmful content.'"

Similar accusations have been leveled against the UK government in response to its various attempts to improve online safety via legislation.

For example, the government has long presented the Investigatory Powers Act as a way to enshrine in law necessary powers available to law enforcement and UK intelligence to intercept communications for the sake of preventing terrorist attacks.

More recently, the Online Safety Act was introduced to impose new obligations on digital platforms to prevent children from accessing online harms.

However, privacy proponents have shunned both.

Rather than simply providing powers to prevent terror attacks, critics say the IPA enables public bodies to spy on people's calls or texts. It's colloquially known as "The Snooper's Charter."

Digital rights organizations have also claimed the OSA is more about online censorship than it is about restricting the types of content children are allowed to view on the web.

The PM's proposals are not law yet. Instead, Starmer's speech amounted to a three-month ultimatum to tech companies: make the changes the UK wants to see or the government will legislate.

Essentially, whichever way the likes of Apple, Google, Microsoft, and others want to play it, some form of device-level scanning appears likely to be pushed onto UK devices soon.

"When it comes to the safety of our children, standing by is not an option. Nobody gets a free pass. That is why I'm making sure Britain is the first country in the world to make it impossible for children to take, share or view nude images," Starmer said.

"And I expect tech firms to make that happen. This is not an impossible challenge – these are some of the most innovative companies in the world. But if they choose not to, then we will act and change the law."

The government's announcement was backed by a slew of campaigners and charities that argued child protection has not been as big a part of tech innovation as it should have been in recent years.

Roxy Longworth, author and founder of Behind Our Screens, said: "I told myself, back in 2021, that if I went public with what happened to me and it stopped one life from being ruined, then it was worth it, but the more I campaigned the angrier I became. 

"Every child needs to be protected from platforms who for far too long have been allowed to turn a blind eye to the damage being done to them. This announcement makes me hopeful that there won't be kids sat in their room feeling the same pressure and shame that consumed my teenage years."

Likewise, Chris Sherwood, chief exec at the NSPCC, said: "Every day these protections are not in place, more children will continue to face devastating harm in the online world. That's why we strongly support the government's decision to make it mandatory for these companies to block inappropriate material at device level. This marks a major step forward in our fight against online child sexual abuse."

The UK government singled out Apple and Google, saying that it demands both block nudity by default across their devices. That includes cameras, third-party apps, and messaging services, which would prevent children from taking, viewing, or sending nude images.

It proposed that the nude-block-by-default approach would keep children safe, while still allowing adults to remove the block by verifying their ages.

Client-side scanning remains a highly controversial technology, but supporters present it as striking a balance between privacy and safety.

Advocates argue it should appeal to the pro-privacy crowd by keeping all data on the device, rather than blurring nude images in transit, for example, which would involve sending that data to an intermediary.

However, in the case of Signal, an encrypted messenger, it breaks the private comms trust model, even if the message content is not sent to a third party.

Client-side scanning can involve checking content against a database of known objectionable material. In the context of child exploitation, image hashes would be checked against a database of other hashes associated with abuse material. If the hashes match, then the image would be blocked.

Some implementations scan using AI, rather than against a database.

So while the image in this scenario is not sent to a third party, it does mean that Signal could no longer say that message content stays between sender and receiver only.

Further, because the databases of objectionable material would need to be updated, this introduces additional problems. 

Updated databases or models would need to be pushed to devices, creating another trust and security dependency. The attack surface also widens, as it is conceivable that attackers could try to manipulate them.

As Signal points out, it would be technically possible for the same scanning mechanisms to be updated to block other things, like messages criticizing the government, to take one hypothetical example.

Authorities could also feasibly implement ways of seeing which device contains images or other content that has registered matches with its objectionable material database, potentially opening the door to surveillance.

The company's statement [PDF] called for public funds to be funneled into other areas to improve child safety, including education, social services, and guardrails on AI technologies and platforms, instead of drafting legislation to block children's nudes by default on devices.

"What the UK government wants instead is invisible surveillance infrastructure, switched on by default and potentially rushed into law under cynical pretexts," it said. "All of this with scant care for the actual needs of the children they claim to be protecting or the horrifying and far-ranging consequences that will ensue in practice."

Signal has not threatened to pull out of the UK, however, despite the government's promises to enact the plans, via legislation or the threat of it.

The company has previously mulled exiting Sweden over proposed encryption-busting laws, and more recently Canada, as it debates a bill that would compel platforms like Signal to gather its users' metadata, which could include their locations and who they are talking to. ®