[creata]

> you'd use a proof-of-work scheme

I thought the general belief (e.g., '“Proof-of-Work” Proves Not to Work') was that proof-of-work isn't very good anti-spam.

> or a Captcha

Aren't bots better at those than humans by now?

And making people do captchas in an instant messenger is a great way to make people not use that instant messenger.

> or an invite-code system like lobste.rs or early Gmail.

That's not a long-term option if you want to make something mainstream.

[kragen]

There are people who believe that proof-of-work isn't very effective, but none of them have succeeded in spamming the Bitcoin network with blocks they've mined, driving the other miners out of business, nor (for the last several years) with spamming the Bitcoin network with dust transactions they've signed, so I don't think we should take their opinions very seriously.

Bots may be better than humans at Captchas now, although I'm not certain of that, but they certainly weren't when Signal was designed.

I don't see why invite codes would be a problem for mainstream use.

[Ieghaehia9]

> There are people who believe that proof-of-work isn't very effective, but none of them have succeeded in spamming the Bitcoin network with blocks they've mined, driving the other miners out of business, nor (for the last several years) with spamming the Bitcoin network with dust transactions they've signed, so I don't think we should take their opinions very seriously.

Different system. The parent and GP are talking about proof-of-work being used directly for account creation. If a chat service required mining-levels of PoW (and hence any prospective new users to have an ASIC), it would not be very popular. Nor would it be very popular if it used a relative difficulty system and the spammers used dedicated servers while the legitimate users had to compete using only their phones.

[creata]

> none of them have succeeded in spamming the Bitcoin network with blocks they've mined

I'm not saying you're wrong, but I have no idea what you're getting at, because the sentence sounds kind of absurd. As a result, I'm not sure if it addresses your point, but just to throw it out there: Bitcoin and anti-spam are different applications of proof of work. Anti-spam has to strike a compromise between being cheap for the user (who is often on relatively low-powered mobile hardware), and yet annoying enough to deter the spammer. It's not unreasonable to believe that such a compromise does not exist.

> Bots may be better than humans at Captchas now, although I'm not certain of that, but they certainly weren't when Signal was designed.

Fair point, but again, even in 2014, an instant messenger with captchas would have much more friction than every other messenger. And captchas aren't just bad because they introduce enough friction to drive away pretty much everybody: they also make users feel like they're being treated as potential criminals.

> I don't see why invite codes would be a problem for mainstream use.

Can you elaborate? Invite codes blocking access to the service itself "like lobste.rs" mean that no one can use your service unless they've been transitively blessed by you. That's obviously going to limit its reach...

[kragen]

Bitcoin had a spam transaction problem ("dust transactions") which was a bigger problem than email spam, because every transaction is received by every node. It was easy to solve because Bitcoins are minted by proof of work.

I don't think a Captcha for signup would have been much friction. Certainly less than providing a phone number.

Why would someone want to use a closed messaging service like Signal unless they knew an existing user? I don't think that the requirement for that existing user to invite them would be a significant barrier. So I think it's not going to limit its reach.

[alkindiffie]

> That's not a long-term option if you want to make something mainstream.

Groups in messaging apps rarely contain more than 100 users. So invite codes can work well for messaging apps.