Going dark is military lingo for the sudden termination of communication. The term used to describe a scenario in which communication appears to have ceased, but in reality has just moved from a public communication channel, where it could be monitored, to a private communication channel that prevents eavesdropping.
The term has been adopted by law enforcement to describe digital communication that cannot be monitored because of strong encryption. Mobile apps that use end-to-end encryption (E2EE) are designed to protect data at rest and in transit and keep the end user's text messages, emails and video chats private and secure. The same encryption technologies that protect end users from intruders, however, can prevent law enforcement and government agencies with the legal right to monitor transmissions from being able to do so.
In the United States, the question of how much help law enforcement and national security agencies should expect from vendors to make decryption upon demand available is under debate. The National Security Agency (NSA) has proposed vendors use split-key encryption to solve the problem of law-breakers and terrorists going dark.
In a split key encryption approach, also known as secret sharing, the technology vendor or service provider retains half the master key and law enforcement retains the other half. This approach places responsibility for deploying encryption in a way that supports lawful access on the vendor or service provider; it also provides a level of transparency, requiring the participation of both parties in order for lawful access to occur.
Opponents of this approach maintain that it would be prohibitively complex to implement and the complexity would provide points of entry that would ultimately endanger user data security. Another approach, which has been used in the past, is called lawful device hacking. In this scenario, the responsibility for decrypting dark communication is placed on law enforcement and government agencies, who must use exploits and/or external hardware/software to access the content of locked devices.