In response to the horrible terrorist attacks in Paris and San Bernardino, Calif., law enforcement and some ill-informed politicians are trotting out a demand that was soundly rejected more than 20 years ago: government “backdoors” or “keys” to encrypted software and devices that will not prevent terrorists or any other malicious actors from using encryption but instead will make all Americans less secure.

James Comey, director of the Federal Bureau of Investigation, increasingly warns of the “public safety and national security...