Re: CAPTCHA over smtp (yet another spam solution to discuss)

Seth Goodman wrote:

>> > Mailing lists, for one reason.
>>
>> These would go away.  To avoid confusion, they would be replaced by 
>> ordinary mailboxes with read-only IMAP access or RSS or whatever.  
>> The mailing list administrator makes the decision who gets to post. 
>> Either by whitelisting, or by requiring captchas solved, or by 
>> requiring a security deposit.  The latter has the benefit that they 
>> could seize the bond and reimburse subscribers.
>
>An interesting idea.  Several issues:

Not new, though: http://code.dogmap.org/posterity/

>1) Read-only IMAP is a more modern version of news.

News (NNTP?) is /not/ read-only.

>That's been around a very long time,

"That" == News?

>but I'd still guess that more people read them via email than news.

"them" == mailing lists?

>For better or worse, bandwidth is less of an issue than convenience for 
>many people.

I think you may be saying, "it's more convenient for users to subscribe 
to mailing lists by entering their email address into some web form 
rather than figuring out how to make their MUA subscribe to RSS feeds or 
to read-only IMAP folders."

>2) Posting a bond to an mailing list you wish to post mail to would be 
>seen by many people as exclusionary.  Several mailing lists that I read 
>are unwilling to even require posters to register, as that is seen as 
>too restrictive.

My point again:  Make all restrictions configurable, for the recipient.  
Where in this case, the recipient is the mailing list.

>Though I don't agree with them, I mention it to show there is a wide 
>variety of opinion on what is too restrictive and I doubt that 
>requiring a cash bond before posting would be widely accepted.

It'd be okay if it were accepted where it matters.  Like for mailing 
lists with thousands of subscribers, which you bring up next...

>3) Captcha's as a barrier to mailing list posting would be even less of 
>a barrier than for ordinary mail.  You pay someone in a poor country a 
>few cents to defeat the captcha for a mailing list that gets 
>distributed to a few thousand recipients.  That's irresistible.

Administrators of large-scale mailing lists have an incentive to keep 
their lists non-polluted.  Let them decide themselves what to do with 
that incentive.  They might moderate, they might require bonds, they 
might want to have captchas solved.  Why impose a one-size-fits-all 
pseudo-solution?


Seth Goodman wrote:

>> No; "repudiable" is correct.  The verifier sends a challenge, and 
>> computing a valid reponse requires the private key.  But the prover 
>> can repudiate ever having "signed" (in the offline sense) because it 
>> is trivial to manufacture, given any response, a corresponding 
>> challenge.
>
>I agree with Brian that there is probably some confusion here.  One of 
>the strengths of public key signatures such as GPG is that they are 
>non-repudiable.

For an offline system, that's not a strength, it's a core feature.

In an online system, the sender issues a zero-knowledge proof of their 
knowledge of a valid signature.  The recipient does not have the ability 
to prove anything to anyone, although they have the entire transcript of 
the communication.

>GPG signatures do give all future recipients that ability, though I 
>would suggest it is unnecessary for ordinary email.

It's not just unnecessary, it's a major impediment for sender 
authentication to work.  Even if PKI worked (think: identity-based 
encryption), even if MUAs came with built-in crypto support and made its 
usage easy, people will feel uneasy about non-repudiably signing their 
every words, although most won't admit it.


--Joachim