A backdoor is a vulnerability in a form of encryption that would allow anyone who found it to undermine the protections that the encryption offers. "Backdoor" is a pejorative term used by people who oppose government efforts to require tech companies to design their encryption so that they can break it. These critics argue that the only way to do this is for the companies to deliberately insert vulnerabilities—the "backdoors"—into their encryption. Backdoors can take many forms; one example is a random-number generator algorithm whose number-generation is not truly random, but rather, predictable to anyone who knows how it was designed. By predicting the number generation, it is possible to foil the encryption built on top of it.

Theresa May wants social media companies to create controversial ‘backdoors’ for police
Critics accuse Prime Minister May of political grandstanding.
CIA director clashes with Sen. Ron Wyden over encryption backdoors
A consensus of technologists argue that requiring backdoors would weaken cybersecurity.
FBI director says 12 percent of devices inaccessible because of encryption
James Comey, the FBI director, predicted that the number would only continue to grow.
FTC commissioner: Mandating encryption backdoors ‘is a terrible idea’
Terrell McSweeny said that encryption would soon become even more vital as smart devices proliferated.
Indian leader frames encryption as cybercrime concern in meeting with Apple CEO
Modi apparently urged Cook to accept limits on encryption by saying it would help stop cybercrime.
The pro-encryption bill that Congress is ignoring
Neither chamber of Congress has moved the bill out of the committee stage.
A complete guide to the new ‘Crypto Wars’
Get up to speed on encryption with a thorough timeline of the events that brought us to this point.
U.S. government drops New York iPhone case after third party gives it the passcode
The government won't say who gave it the password.
Police shutter privacy-focused Dutch phone network for allegedly aiding criminals
The company marketed itself toward business professionals.
Congressional working group lays out plan to study encryption
The question remains: Can this working group get anything done?
U.S. tech firms warn of ‘unintended consequences’ from Senate encryption bill
The companies banded together for a strongly worded letter urging the bill's sponsors to back down.
U.K. official confirms surveillance bill would let cops force companies to decrypt data
Comments by a senior U.K. law-enforcement officer confirm what many companies have feared.
Law-enforcement officials condemn unbreakable encryption with baffling claims
Technologists on social media puzzled over many of the officials' comments.
Americans side with tech companies over government on risks and priorities in encryption debate
Far more Americans trust Apple to secure their digital data than trust the FBI.
NYPD joins push for backdoors in commercial encryption
The police department appealed to its audience's emotions in demanding guaranteed access to data.