Anxiety over the public snooping of police radios using smartphones is persuading a growing number of US police forces to take the controversial step of moving their communications to fully-encrypted operation.
The Washington D.C. police department has become the latest to adopt radio encryption after mounting evidence that criminals were listening in to police conversations using cheap applications running on mass-market phones, the Associated Press has reported.
The same adoption is happening in Orange County Florida, Santa Monica California and even small out-of-the-way towns in Kansas, the agency discovered.
Although scanning open analogue and digital radio services has been possible for decades using fixed radios, doing so reliably from any location or while moving is extremely difficult - the frequencies vary widely for different services across county and state boundaries.
Smartphone apps take the legwork out of eavesdropping by streaming the content for a wide range of emergency services from the Internet in almost real time. One prominent iPhone app, Scanner 911, costs only $1.99 and supports hundreds of North American services.
"It concerns the officers when you see the same vehicle keep showing up at your scenes. What is their intent when they keep showing up?," Garden City police department officer Sgt Michael Reagle is quoted as saying.
In addition to criminal monitoring, journalists are also enthusiastic eavesdroppers, keen to use radio traffic between emergency and police teams as a way of getting ahead on news stories.
The system used for encryption on US radio communications at this level is ACPO P25, which adds complexity (such as key management and distribution) and expense, which has until now slowed its take-up. A recent technical paper also found fault with P25, noting its vulnerability to denial of service disruption using low-cost equipment.
Others worry about the implications of making police communications closed to public and journalistic oversight, which is encouraging some police forces to adopt a mixed approach, with some communications secured and others open.
In the UK and many other countries the dominant digital radio standard for police communication is terrestrial trunked radio (TETRA), which comes with a built-in encryption interface.