Stingray countermeasures

A Stingray is a cell tower lookalike device.  It broadcasts its presence and nearby phones connect to the Stingray thinking it is a legitimate tower.  The Stingray can then log each phone or act as a man in the middle to incercept call metadata, text messages, or even call contents.

There are a number of public databases of legitimate cell towers.  For example,  Some databases are government, for example, the FCC license database, while others are crowdsourced.

It should be possible to modify a phone to only connect to towers which are legitimate by checking the purported tower ID against a cached copy of the database for the local area.  A stingray could, of course, use the id of a real tower, but that would disrupt communications in the whole area. This might not prevent the Stingray from logging the presence of such a phone, since the Stingray could hear the protocol handshake with the legitimate tower.

It should also be possible for a phone to passively listen for tower broadcasts, and to compare the tower ID against the database,  An unknown ID might be a new legitimate tower or it might be a Stingray.

It is likely quite difficult to get at and modify the low level radio software in a commercial smartphone, but there is a complete open source suite of cell infrastructure software at

That code could serve as a starting point for a software defined radio device for detecting and tracking Stingrays.  One could make a box with a red light on top which lights up when there is an unknown tower in the area.

In some areas, use of Stingray devices requires a warrant, but this is not universal.  The courts have also determined that use of location data from legitimate cell towers does not require a warrant



PIN Escrow

The FBI has dropped their request to require Apple to write code to unlock the terrorist iPhone.  Supposedly a third party offered a way in.  Yesterday the FBI said they did get in, so they no longer need Apple’s help.

For those whose first instinct is to distrust the government, this looks like the Justice department realized they were going to lose in court and hastily discovered a way out. “Never mind”.  This preserves their option to try again later when public opinion and perhaps law would be more on their side.  I am a little reluctant to think Justice would outright lie to a federal judge, but it wouldn’t be the first time.

This morning on NPR there was a different sort of heartbreaking story.  A woman and her baby were murdered, and there might be evidence on the woman’s phone, but it can’t be unlocked.  So what to do?

My idea is “PIN Escrow”.  Everyone should have a letter written with a list of their accounts and online passwords, to be opened by someone in the event of death or disappearance.  Everyone should have a medical power of attorney and so forth as well, to give a family member or trusted friend the power to act for you in the event of a sudden disability.  Just add your smartphone PIN to the letter,

In the alternative, one could write an app that encrypts your pin with the public key of an escrow service and sends it off.  This facility could even be built into the OS, with opt-in (or even opt-out, after a sufficient public debate), so it would automatically track changes.  The government could operate such a service, or it could be private.  There could be many such services.  Some could be offshore.  Some could use key-sharing for the private key, so PIN recovery could not be done in secret.

Let’s leave it up to individuals whether they want someone to have the power to unlock their phone in the event of an emergency.

From a security perspective, a PIN escrow service would be a dangerous and attractive target, so such a thing would have to be well designed in order to be trustworthy.  It should be kept offline, with no network connection.  The private key should be in a hardware key module.  Several people would have to collude in order to unlock a key, and there ought to be hardware safeguards to prevent bulk PIN recovery.

This is not a general back door for government surveillance, it wouldn’t grant remote access to a phone.  It wouldn’t be useful for hacking into criminal’s or terrorist’s phones (if they are smart), but it might help in cases where the phone owner is the victim of tragedy or accident.

And if you change your mind about having your PIN escrowed?  Just change your PIN.


Apple v FBI

I’m beginning to build up a full head of steam.  The first step seems straightforward.  I’m going to write my congressman.  It may not have much effect, but if enough of us write, it might.

Here’s my letter to Massachusetts Senator Elizabeth Warren.  I’ll be sending similar letters to Sen. Ed. Markey and Rep. Katherine Clark.

2016, March 16

The Honorable Elizabeth Warren
317 Hart Senate Office Building
Washington, DC 20510

Dear Senator Warren:

I write about the Apple FBI affair.  Please oppose any attempt by government to weaken the security and privacy of all Americans by demanding security “backdoors” in our technology or to require the conscription of Americans or American companies to weaken their own security.

First, regarding backdoors. I hold a PhD in Electrical Engineering and have worked with computer systems and computer security for over 40 years.  I am coauthor of the well-regarded book on E-commerce systems “Designing Systems for Internet Commerce.”  In other words, I know quite a lot about this area.  There is simply no way to create a backdoor that does not also reduce the security of the system for everyone.

Second, speaking as an ordinary citizen, I do not know how the courts will rule on the government’s request to use the All Writs Act to compel Apple to write software to unlock the San Bernadino iPhone, but my own view is that the constitution does not and should not allow it.

The government is being deliberately disingenuous when it claims this case is only about one terrorist’s phone. I have no sympathy for the killers, but the privacy and security of everyone is at risk should the government prevail.  Should that happen, I expect you to propose and support legislation that outlaws backdoors and forbids the conscription of individuals or companies into the government’s service.  This has happened before.  In 1980, Congress passed the Privacy Protection Act of 1980 which corrected the overreach of government in Lurcher v. Stanford Daily.

Sincerely yours,

Lawrence C. Stewart