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Defining a "VPN" will be extremely difficult, but that's not my point.
My point is that it is impossible to block access to VPNs, and equally impossible to ban them.
This is a mathematical certainty. We can't un-learn how to have securely encrypted communications.
@cstross @oschonrock @PeterSommerlad We can happily discuss whether age restrictions on "VPN users" is a Good or Bad idea for a law.
My point is that it's impossible to enforce such a law.
It would be as pointless as the Online Safety Act. Well-intentioned, no doubt, but embarrassing when ignored. The 4chan bulletin board has been fined £20,000 and more for breaching the Online Safety Act. Their response has been "we don't care, we're not complying with a UK law, we're not going to pay any fines". The only thing Ofcom can do is to ask UK ISPs to block access to 4chan. They haven't yet, but if they do it'll be easily bypassed by a VPN or TOR.
@oschonrock @cstross @PeterSommerlad A ban on Twitter in the EU would also be impossible to enforce.
@Nicovel0 @cstross @david_chisnall Yeah, but I'm not going to be carrying my desktop computer on foreign trips.
@oschonrock @PeterSommerlad @cstross Yes. They can legislate as much as they want to age-verify all VPN users. Mathematics and logic makes this impossible to enforce in any meaningful way, though.
@cstross @david_chisnall The likelihood of the police taking my computer for forensic examination is zero.
I have plenty of things that I must keep private. So does everyone.
@oschonrock @PeterSommerlad @cstross I'm presuming they'd want to check your age every time you USE a VPN connection? Otherwise the restriction on underage use would be meaningless.
@cstross @david_chisnall I agree. Allowing for inspection of innocent people's personal computers is both unacceptable and also counter-productive for law enforcement.
The police are not going to be randomly doing "illegal VPN inspections" on everyone. They can only target the few big public VPN services, and persuade them to add age verification.
@cstross @david_chisnall I'm not planning to do anything that would result in the government seizing my computer 
There is no way the government can know whether or not I use a VPN or not, nor whether I use TOR.
Unless the law allows the police to randomly inspect people's computers, and they do this to a significant proportion of the population, I can use any VPN I like without fear. We don't live in a police state yet...
@david_chisnall @cstross The government has to discover that there is an illegal VPN being used in the first place.
It is quite possible for millions of VPNs to be made available to UK children, hosted all over the world. Perhaps hosted by children, sharing the small monthly server costs. Quite secret, extremely difficult to find.
The proposed law could only ever hope to apply to a few big VPN companies. Which just moves the VPN usage by children underground, where other dangers lurk.
@cstross @david_chisnall That assumes that I do something that is bad enough for government to seize my computer.
Unless they do, there is no way (without GCHQ spending a lot of time and effort) that a VPN ban could be enforced.
If I did do something that got the attention of the security services, having a VPN without age restrictions is going to be the least of my problems!
It's the same as the Online Safety Act. It makes a lot of noise, but is almost entirely unenforceable (see: Ofcom's fine for 4chan).
@david_chisnall @cstross But it's almost impossible for a government to detect a VPN service that doesn't have age restrictions. Unless it's one of the big well-known ones.
A foreign entity could set them up, or someone aged less than 16 for themselves (and perhaps also their mates).
You need:
1. A cheap server, anywhere in the world, connected to the internet.
2. VPN server software, available for free from lots of places.
3. Some instructions, easily available.
@etchedpixels @cstross I'm not sure that TOR is a "VPN". It's not an encrypted tunnel, it's a distributed internet packet routing system.
But it rather depends on what any law defines a VPN as. Something that lawmakers will have a lot of fun with!
@cstross It's impossible to restrict access to VPNs.
They could perhaps persuade some of the big providers to add access controls, but that would only result in more people using smaller or even self-hosted VPN services.
You can't un-invent encryption algorithms.