A cellphone was thrown at President Trump as he addressed the NRA convention on Friday. The individual responsible was detained and Trump was not hit. Still, the incident does raise an interesting point.

Namely, that the U.S. Secret Service does not like its protectees and cellphones close together. There are two elements at play here: physical protection and intelligence targeting vulnerability.

The first concern centers on assailants hiding explosives inside a fake cellphone. This matters because explosives screening is focused prior to an event rather than during an event per se, and because it is somewhat difficult to detect a relatively small amount of explosives in a cellphone. In 2019, it is also impractical for the Secret Service to ask thousands of attendees to surrender their cellphone as a condition of entry.

The Secret Service thus fears an undetected assailant asking a protectee to "speak to my friend/loved one" and then detonating the hidden explosives as the protectee raises the phone to their ear. This is why Secret Service agents dislike individuals taking selfie photos with protectees or asking a protectee to speak to someone on their phone. Fortunately, some of the risk here has been diminished by the demise of al Qaeda's innovative bombmaker, Ibrahim al-Asiri.

The intelligence monitoring issue is more obvious. The most capable intelligence services can use one end of a line to gather a range of intelligence on the front end. Consider, for example, that if an intelligence service can effectively target a cellphone, it can also geo-locate that cellphone and thus the person using it. The Secret Service, White House Communications Agency, and NSA do implement protective safeguards here, but they are not absolute.

Cellphones are also vulnerable to hacking. Contrary to the assumption of those who post their PGP and encrypted numbers on Twitter, this vulnerability extends to encrypted apps. Beyond the U.S. and British services, the Chinese and Israeli intelligence services are particularly talented at using cellphone apps for intelligence access and acquisition.

In short, whether it's Trump, Obama, or whoever comes next, the Secret Service will keep trying to keep their protectees away from that which most of us cannot go without.