As Angel pointed out, the system as you described is vulnerable to MitM and proxying attacks.
For a secure alibi, we need the process to work like this:
- Every 2 minutes, give or take a few random seconds to avoid predictability, the Cell Tower (CT) sends Alibi Tracker (AT) a "ping" request that contains a unique one-time code. This request is encrypted with AT's public key, so only AT can read it. To get AT's private key, you'd need to dig open your head and perform some serious hackery on the AT.
- AT replies with the code, the GPS location, the GPS timestamp, and the encoded brain wave signature, all encrypted with CT's public key.
CT then knows these things:
- Whether AT has responded to this and/or any previous requests.
- Whether AT has responded with the correct code.
- The brainwaves that AT has previously responded with.
- The triangulated location AT's response came from, accurate to about 1000m.
- The GPS location encoded in AT's response, accurate to about 10m.
- The GPS location of CT, accurate to about 1m.
- The time it should take messages to travel between CT and AT at light speed, accurate to about 0.00001% due to variations in the refractive index of the atmosphere.
- The exact amount of time it takes AT to encrypt and send its response, accurate to maybe a few nanosecs.
- Any internal signal-processing time of the tower, accurate to maybe a few nanosecs.
- The GPS time encoded in AT's response, accurate to about 10 nanosecs.
- The GPS time CT requested a response from AT, accurate to about 10 nanosecs.
- The GPS time CT received the response, accurate to about 10 nanosecs.
The following tricks are therefore avoided:
- 1 detects merely silencing the response with a tin-foil hat. This is probably the only trick that would get cops sent out in an obvious way to investigate.
- 2 detects attempts to replay the response, or to generate the response before the request was sent.
- 3 detects unusual thought patterns. Not something that would get anyone sent out obviously, but literally as an alibi: if someone was murdered where you were, you claim you saw nothing, but your brainwaves were very unusual for that location and time of day, then you have no alibi.
- Comparing 4 to 5 detects GPS spoofing.
- Comparing 4 and 5 to 6 gives the distance between the two, which gives item 7.
- Adding 7+8+9+10 and comparing to 12 detects the delays caused by message proxying as described in Angel's answer.
- Subtracting 9 from 7 and comparing to 11 detects generation or transmission delays between requesting the message and getting a response, such as might be caused when trying to spoof the response.
Light speed is ~300,000 km/sec, so 10km is 10/300,000 seconds, or about 33 microseconds. 66 for a round trip.
The inaccuracy of cell triangulation is quite high: only to the nearest 1000m (about 3 light-milliseconds). So we can only calculate transmission times to the tower with an accuracy of about 3ms.
Still, if a signal is delayed by 33 microseconds each way, that's 10 times the deviation we're willing to accept, and it means there's something up.
With a system like this, proxying as described by Angel could work, but only within at most 1000m of where you're meant to be.
You might be able to get a few more meters out of it, if instead of a single proxy, you put one on each triangulating cell tower, with a directional antenna to speak only to that tower. But it wouldn't help much.
Worse, the 1000m becomes MUCH less in areas where there are a lot of cell towers so triangulation is more accurate. And you can never tell how many towers there are, since they could be hidden. It would be in BB's interest to have lots of them.
Benevolent Brother is not stupid.
In the case of sophisticated tricks, no alarm would sound, no paddy-wagons get sent out. That's for people who just put on tin-foil hats and call it a day, not for sophisticated hackers.
Benevolent Brother knows that everything like this is an arms race: once people know that their clever tricks don't work any more, they go to ground until they can develop cleverer and cleverer tricks. So it's important to let them think they are winning the arms race, so they don't develop better tricks.
So, BB is unlikely to act overtly against someone using Angel's proxying trick, and instead will focus other surveillance methods on them until it's certain that everyone involved in producing the system has been identified, and he knows what they are planning... which he might even allow to continue. A little smuggling of goods is a small price for society to pay, for the ability to closely monitor all those who might be likely to cause problems to that society.
They then become the coalmine canary: if someone else more dangerous needs some trouble, he's bound to contact the local hoods, and so expose himself.
If BB does make any busts, he'll make sure to publicly credit "informants" rather than technology.
Since anyone who made proxying or spoofing equipment would be known, and all uses of the equipment would be of highest interest to BB, anyone getting hold of the equipment for a sophisticated trick would likely already be under very close surveillance before even using it.
So, if you can't fool it, what can you do?
I think the answer is obvious. They're tracking the movements of your head, your phone, your car, your public transport use, your creditcard use. They're using facial recognition on the surveillance cameras on every corner, and every cellphone and car is also a surveillance camera. They will know if you go anywhere you shouldn't be.
So if the smuggler's den is under a big farmer's market, take up an interest in fresh fruit and veg, and then you're just visiting the market. The resistance contact works in an art gallery? Well, it's the weekend, why not visit some museums and galleries? Why it's practically your civic duty to gaze upon those portraits and busts of Benevolent Brother every now and then.
To evade the Alibi Tracker, you don't need fancy technology hacks. You need... an ALIBI.