There’s a number that you can’t post, though you can find many links to it, because it’s an HD-DVD processing key that got into the wild. This is a great example of the sheer absurdity of modern copyright law: it can be illegal to post a number, because someone else used it as a key. You might, by that logic, be prohibited from saying your birthdate is x/x/y because that’s someone’s locker combination.
But here’s the other thing. Let’s say I generate a similar number, at random:
32 40 04 1A 2B 07 16 07 39 01 0C 15 0B 04 37 0A 21 37 11
for instance. Or
0A 14 25 35 40 15 2F 17 2A 27 1B 40 0F 03 1F 14 3F 3E 11
1C 0C 0F 2E 1A 2B 0C 0B 15 00 1D 0A 1A 26 0B 11 26 22 40
3D 1C 28 1D 06 05 1D 1E 10 3A 25 14 21 3F 29 12 23 0C 36
04 35 08 25 12 0B 15 00 16 22 16 21 21 08 20 0F 2B 24 14
These numbers are not, as far as anyone knows now, illegal. But they’re either potentially illegal in the future or they’re not, but it’s potentially illegal to discuss them in the future.
Option 1: those numbers are potentially illegal. If it’s possible for them to sue someone for posting a number that does something in particular, then it’s possible for them to sue me for posting it too, even if I did it earlier.
Option 2: it’s not illegal to post numbers. This makes the most sense, but it’s not what we’ve seen so far.
It’s legal for me to generate random numbers. It’s not legal for me to try them to see if they’re processing keys or not. But if they are – let’s say they decide to sue me for posting one of them – then even though I don’t and can’t have any knowledge now that they’re illegal, it becomes illegal for me to have posted them.
And if it’s legal to know numbers, as long as you don’t know their uses, then we could just post all the possible key combinations somewhere now, and then later you could legally say “boy, I sure do like
0B 0D 3C 06 1E 3C 04 01 2C 09 3E 23 1C 20 10 27 3D 27 1D
… it’s the best number of all” but not “… it’s one of the numbers used as a decryption key.”
If it is indeed, illegal to know particular numbers, we’ve reached the height of absurdity. This is beyond even “it’s illegal to know a fact”. This is “it’s illegal to know that there are numbers of a certain size.” Everyone would need to be taught in school that there are an infinite number of numbers, but you should stay away from ones of certain sizes, because they belong to people who will sue you.