On July 7, 2005, four suicide bombers attacked London's transport system, killing 52 people. Within days, investigators had traced their movements using footage from closed circuit television cameras. In 2018 a man suspected of a string of murders and rapes in California in the 1970s and 1980s, known as the Golden State Killer, was identified when investigators uploaded crime scene DNA to a genealogy website and found his distant relatives. Surveillance solves real crimes and can save lives. Any honest account of privacy has to begin there, and then ask what the price is and who pays it.

The case for more surveillance

Supporters argue that the first duty of government is to protect life, and that information is the main tool for doing it. Terrorists, child abusers and organized criminals use the same phones and apps as everyone else. Law enforcement agencies warn of "going dark": as more communication is protected by end-to-end encryption, in which only the sender and recipient can read a message, not the company carrying it, investigators lose evidence even when they have a warrant. Many citizens, polls suggest, accept some surveillance as the price of safety.

The case for limits

Critics question the idea that privacy and security are simply traded against each other. They raise four main points.

First, the arithmetic of rare events. Suppose a system scans 100 million people for 1,000 real threats and is wrong only 1 percent of the time. It will catch nearly all 1,000, but it will also flag about a million innocent people. When the target is rare, even an accurate system flags far more innocent people than guilty ones, and investigators drown in false leads.

Second, function creep: data collected for one purpose gets used for others. During the COVID pandemic, police in Western Australia accessed data from a contact tracing check-in app to investigate crimes, despite assurances it would be used only for health. The state then passed a law to forbid it.

Third, abuse. The history of COINTELPRO and the Stasi shows that powers given for security can be turned against critics. Fourth, security can be weakened by surveillance itself. In 2016 the FBI asked a court to order Apple to write software to unlock an iPhone used by one of the San Bernardino shooters. Apple refused, arguing that a tool to break into one phone would endanger every phone if it leaked. The FBI eventually paid an outside firm to open the device. Many security experts agree that a "back door" for the police can be found by criminals and hostile governments too.

Safeguards in the middle

Most democracies try to keep useful surveillance while limiting it. Common tools include warrants approved by independent judges, rules on how long data may be kept, independent oversight boards, published transparency reports, and sunset clauses, which make a law expire unless lawmakers renew it, forcing a fresh debate. European courts ask whether a measure is necessary and proportionate: whether it truly serves its aim and intrudes no more than needed.

What citizens can do

Individuals cannot fix a system alone, but they can reduce their exposure. Security experts recommend starting with threat modeling: asking what you want to protect, from whom, and how bad it would be to lose it. A journalist protecting sources faces different risks from a teenager worried about a stalker.

A few habits help almost everyone: unique passwords kept in a password manager; two-factor authentication, preferably with an app or security key rather than a text message, since phone numbers can be hijacked; prompt software updates; and reviewing which apps can see your location, microphone and contacts. Encrypted messaging, privacy minded browsers and opt-out requests to data brokers reduce what others can gather.

Privacy is also collective. Your contacts list exposes your friends; your DNA test exposes your relatives, as the Golden State Killer case showed. That is why individual choices are not enough, and why voting, commenting on proposed laws and supporting local rules that require public approval of police surveillance tools all matter.

Worked example: leaving an abusive partner

Ana is leaving a controlling partner who once knew all her passwords. Her threat model is specific: the adversary is one person who knows her habits and may have had access to her phone.

So her steps are specific too. She checks her phone for stalkerware and unknown apps, or gets help from a domestic violence advocate to do so safely. She changes passwords from a device her partner never touched, turns on two-factor authentication, and removes his access to shared accounts and family location sharing. She asks people search sites to remove her new address and, where available, joins a state program that keeps survivors' addresses confidential.

None of this protects her from a government agency, and it does not need to. Threat modeling matches defenses to real risks. Her case also shows why privacy is a matter of human dignity: for Ana, control over who knows where she lives is the difference between fear and a life of her own.