[DavidMcLaughlin]

One:

The anti-spam systems work because they are based on content of emails and properties across the providers entire user-base. Every time you click "Mark as spam" you are contributing data for all users in the service. In a decentralised service, even if people agreed to submit all their emails and information for the greater good (which they probably wouldn't), the data still needs to be centralised somewhere and secured by experts. The blacklist/whitelist of notorious spammers and servers needs to be maintained somewhere. You end up having a committee to do that, an elected/trusted group of people and they need to deal with appeals, etc.

Two:

If the logic for blocking spam were public, don't you think that would make it much easier for spammers to circumvent?

Edit - I can't reply to the user below. Must be some HN feature. But the logic for accepting an email is essentially a decision tree, it is based on data and evolves over time. It is a very different problem from something like encryption.

[edanm]

Fyi - On HN, after a message is posted, there's a delay before anyone can reply. The farther "down" the message is, the longer the delay. The logic is that this delay will prevent uninteresting back-and-forth flamewars. I'm guessing that's the HN feature you were talking about.

[yuvadam]

One makes lots of sense. Two makes none.

By analogy: "the logic for encrypted two-way communication (e.g. RSA) is public, don't you think that makes it much easier for hackers to intercept your credit card details?".

Enough has been said about security - or spam filtering in this case - by obscurity.

[billswift]

You are drawing an invalid analogy between cryptography and filtering. The only reason cryptography works with open algorithms is that the keys can be kept secret. To a very large extent in filtering the specific algorithms are as analogous to cryptographic keys as they are to the other parts of crypto-systems. That is, filtering algorithms are like very primitive cryptography where there was no separation between the system and the keys.

[ehsanu1]

If you can propose a spam-filtering algorithm which would not be circumvented if its exact implementation were known, I'd seriously love to hear it. That would basically be a magic bullet for all spam.