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I think the core disagreement is whether forwarding an unmodified signed email should be considered "from the original author" or "from the nearest authenticate

I think the core disagreement is whether forwarding an unmodified signed email should be considered "from the original author" or "from the nearest authenticated hop". You say that the email should be considered to be from YouTube, because it was originally created and signed by YouTube. In your model, the fact it was received from some unknown third party is non-notable. I think it's the most recent hop that matters. Just because the original content of the email came from YouTube does not mean that it should be allowed to claim a @youtube.com From address. It should have a @robtoledoyour.com address, because that was the nearest hop in the forwarding chain that Gmail can verify (e.g. via TLS). And then, since the email is "from" robtoledoyour.com but claims to be from YouTube, it should be discarded (or at least sent to spam). As you note, this would break use cases that depend on DKIM to allow relaying email through unrelated third-party servers. I think that's fine, because I don't think third-party relays should be allowed to claim the identity of the original server.

GauntletWizard 4y ago
Here's the interesting thing: DKIM verifies the message contents. You can add many hops through many servers and still verify the DKIM - It's a design feature, actually, because of Similarly to GPG, where an authenticated e-mail can be forwareded but you can still read and verify the original author's signature, DKIM hashes over the message contents and not the headers, so you can add remailer headers and still verify the original domain. It appears that the message contents are indeed from youtube.com, even if it was forwarded. This is the property that we want from mailing lists, where you can verify the original sender's domain authorized the letter even though you received it from a different domain. Why someone is forwarding a legit google e-mail on and adding their own address, I cannot say. There's probably security implications, and ways to abuse it, but the message itself and the DKIM check are legit.
md_ 4y ago
I don't think that's the core disagreement at all. You're being too unspecific about what "from" you are talking about. :) All the YouTube.com DKIM signature says is that YouTube.com did in fact author this content. Which is true! ARC and SPF (both present on this message!) indicate that the last hop was not YouTube.com, and who the last hop was! So all the information you were asking for is in fact present here. What you are suggesting is just a different phrasing of your prior suggestion: that people should not be allowed to forward email. And given this message has an ARC header that indicates it was forwarded, and from whom, it seems really strange to me to say that instead of messages that indicate they were forwarded, you think we should just ban forwarding entirely. Maybe the problem is on Gmail's MUA: maybe this should be more prominently shown to the user as a forwarded email?
- jmillikin 4y ago
  I'll try using different terms, in hopes that'll help: * The author of the email is <no-reply@youtube.com>, verified by DKIM. * The sender of the email is <postalerts@robtoledoyour.com>, verified by TLS. If the author and the sender are the same, then there's no problem -- that's who the email is from. If the author and the sender are different, you think the email is "from" the author, and I think the email is "from" the sender. IMO whether the content was originally authored by YouTube might be interesting in some vague abstract chain-of-trust sense, but it's not useful to me as a consumer of email. I want to know (1) why some message came to arrive in my inbox and (2) who sent it to me. The desired UX is something like this: [postalerts@robtoledoyour.com] Updates to YouTube's Terms of Service (ideally also tagged as spam due to the mismatch between author and sender) An analogy might be physical mail. If I receive a copy of Dante's Inferno in my mailbox, then I expect the return address to describe the person who put my address on the shipment. It would be beyond useless for the return address to say "Dante Alighieri, Italy" even though that is an accurate description of the work's author.
  - md_ 4y ago
    As a nitpick, TLS does not indicate the sender of the email. You can try using different terms, but the standard terminology in the mail space is "envelope From" vs "header From", as I used above. These are different things, and there's enough information in this email for you to know which is which. Some MUAs--like Outlook--would render this email as "Sent by postalerts@robtoledoyour.com on behalf of no-reply@youtube.com" As I said, it may be that you want your MUA to do that, in which case...use Outlook? But this has nothing to do with limitations of SMTP.

GauntletWizard 4y ago

Here's the interesting thing: DKIM verifies the message contents. You can add many hops through many servers and still verify the DKIM - It's a design feature, actually, because of Similarly to GPG, where an authenticated e-mail can be forwareded but you can still read and verify the original author's signature, DKIM hashes over the message contents and not the headers, so you can add remailer headers and still verify the original domain. It appears that the message contents are indeed from youtube.com, even if it was forwarded. This is the property that we want from mailing lists, where you can verify the original sender's domain authorized the letter even though you received it from a different domain. Why someone is forwarding a legit google e-mail on and adding their own address, I cannot say. There's probably security implications, and ways to abuse it, but the message itself and the DKIM check are legit.

md_ 4y ago

I don't think that's the core disagreement at all. You're being too unspecific about what "from" you are talking about. :) All the YouTube.com DKIM signature says is that YouTube.com did in fact author this content. Which is true! ARC and SPF (both present on this message!) indicate that the last hop was not YouTube.com, and who the last hop was! So all the information you were asking for is in fact present here. What you are suggesting is just a different phrasing of your prior suggestion: that people should not be allowed to forward email. And given this message has an ARC header that indicates it was forwarded, and from whom, it seems really strange to me to say that instead of messages that indicate they were forwarded, you think we should just ban forwarding entirely. Maybe the problem is on Gmail's MUA: maybe this should be more prominently shown to the user as a forwarded email?

jmillikin 4y ago
I'll try using different terms, in hopes that'll help: * The author of the email is <no-reply@youtube.com>, verified by DKIM. * The sender of the email is <postalerts@robtoledoyour.com>, verified by TLS. If the author and the sender are the same, then there's no problem -- that's who the email is from. If the author and the sender are different, you think the email is "from" the author, and I think the email is "from" the sender. IMO whether the content was originally authored by YouTube might be interesting in some vague abstract chain-of-trust sense, but it's not useful to me as a consumer of email. I want to know (1) why some message came to arrive in my inbox and (2) who sent it to me. The desired UX is something like this: [postalerts@robtoledoyour.com] Updates to YouTube's Terms of Service (ideally also tagged as spam due to the mismatch between author and sender) An analogy might be physical mail. If I receive a copy of Dante's Inferno in my mailbox, then I expect the return address to describe the person who put my address on the shipment. It would be beyond useless for the return address to say "Dante Alighieri, Italy" even though that is an accurate description of the work's author.
- md_ 4y ago
  As a nitpick, TLS does not indicate the sender of the email. You can try using different terms, but the standard terminology in the mail space is "envelope From" vs "header From", as I used above. These are different things, and there's enough information in this email for you to know which is which. Some MUAs--like Outlook--would render this email as "Sent by postalerts@robtoledoyour.com on behalf of no-reply@youtube.com" As I said, it may be that you want your MUA to do that, in which case...use Outlook? But this has nothing to do with limitations of SMTP.