The U.K.’s Conservative government is following through on one of its election manifesto promises by cracking down on kids’ access to online pornography.

Porn providers based in the U.K. already have to have solid age-verification processes, but those serving Brits from outside the country (which happen to be the most-used porn services in the country) are now likely to have to do the same.

The government published a consultation Tuesday about the best way to make sure only adults view online porn.

The key will be making it illegal to not have proper age-verification processes, forcing payment service providers to stop serving sites that lack these controls. This “follow the money” approach is necessary because, given that the sites in question are not based in the U.K., enforcing the law directly on them could prove tricky.

As for the mechanisms sites will have to use, that’s where the picture gets a bit fuzzier. What’s clear is that simple “I’m over 18” checkboxes and date-of-birth fields won’t be sufficient.

Get Data Sheet, Fortune’s technology newsletter.

The existing guidance for porn sites operating in the U.K. is that they should demand confirmation of credit card ownership, use an identity-management service that checks lists such as the electoral roll, or ask to see some other kind of account-ownership proof that shows age.

Aha, you might say, but much of the pornography on the web is free—there doesn’t seem to be any payment provider associated, as advertising is paying for everything.

The consultation documents have an answer to that: The paid-for and free sites are often run by the same companies, which are using the latter as an on-ramp to the former. In other words, forcing their payment providers as leverage still forces the free sites to fall in line.

The government’s big example of another country bringing in similar rules is Germany.

“International commercial pornography providers have age verification controls in place on sites targeted at German audiences since 2003. In practise, this means that domain names which end in ‘.de’ (i.e. those specifically aimed at German users) all have age verification controls in place,” the consultation states.​

For more on adult entertainment, watch:

That’s not necessarily an accurate representation of what happens when you try to visit the “.de” version of several well-known international free porn sites—they simply claim porn is not allowed on a “.de” domain. Visiting their standard “.com” incarnations from Germany results in no age checks at all.

That includes Pornhub, which has said it intends to comply with the upcoming British legislation.

To its credit, the U.K. government seems aware of the limitations and risks of its age-check drive. As the consultation notes, people may not be very keen on giving identifying information to porn websites. Also, “the potential for online fraud could raise significantly, as criminals adapt approaches in order to make use of false [age verification systems] and access user data.”

Still, the government sees the risk of kids seeing porn as greater, because it is “detrimental to children’s development.”