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Can someone explain why DKIM isn't sufficient? I really don't get the point of SPF/DMARC/ARC everything else when DKIM exists. Why can't everyone just set up D

Can someone explain why DKIM isn't sufficient? I really don't get the point of SPF/DMARC/ARC everything else when DKIM exists. Why can't everyone just set up DKIM and consider the problem solved?

teddyh 3y ago
Well, at least DMARC is also necessary, since if you recieve an email lacking a DKIM signature, you can’t know if the email should have had a DKIM signature unless there is a DMARC record configured for the sender’s domain.
- dataflow 3y ago
  Why not take the stance that every email should have a DKIM signature? Just like how we moved to "every website should use https". If you don't, your untrusted by default.
  - joveian 3y ago
    Google is slowly moving to reject mail that doesn't pass DMARC no matter the domain settings; it sounds like they are currently rejecting a percentage of messages that fail DMARC. https://support.google.com/mail/answer/81126 https://support.google.com/mail/answer/81126 The issue mentioned in the paper related to SPF is due to adding third parties to the SPF record who then allow spoofing (see table 3 at the top of page 9 in the paper for a summary of the four issues they describe). Hopefully, those third parties will fix that and more mail admins will be aware of the potential negative effects of adding a third party to SPF. It doesn't seem like SPF causes much trouble and if it was possible for DMARC to specify that both SPF and DKIM must validate then that would help in the case of issues with the DKIM signature cryptography. DKIM has a replay issue, although that is not as bad if the to and date fields are signed.
    - dataflow 3y ago
      If they're doing that, that seems like a move intended to prevent forwarding rather than one that's actually necessary for DKIM? Again, I don't see why DKIM isn't enough here. Like you said, replay attacks only work if you don't do DKIM properly, i.e. if you avoid signing some of the initial headers. So how about just... require them to be signed? Sign the message ID, sign the date, sign the addressees, sign the body... sign everything. If you don't, you're assumed to be risky, just like with http:// http://. Problem solved, no?
      - joveian 3y ago
        DMARC is DKIM or SPF so requiring DMARC part of the way to what you are suggesting. There is still a replay possibility with DKIM even if limited to replaying the exact same message (unless providers or email clients track already seen messages until DKIM expiration, which could happen but I don't think is happening at this point). I'm not an expert just someone trying to find a security focused email provider with minimally reasonable privacy policies that doesn't randomly drop email (which unfortunately doesn't seem to exist :( ) so I'm not sure why things are the way they are. Why SPF exists might just be due to being earlier and there not being any strong reason to stop using it. Even Google and Microsoft aren't able to simply require whatever they want without a long transition period. I'm fairly sure Microsoft would like to make email entirely proprietary (and some of the issues almost only apply to them so they aren't likely to be the ones to improve email security). As best I can tell the people with the most direct financial interest in email are the marketers, which isn't a great situation for real improvement (BIMI seems primarily designed to replace the lost income from browsers dropping EV certificates while having similar issues). And security is a hard sell for anything it seems. So all in all I think we are lucky that Google has been doing as much as they have been. A number of old timers already complain about how it isn't really email any more. If Google did want to eventually require DKIM the path to get there would likely still look like what they have been doing. There has also been recent activity just this year to improve the S/MIME situation (best practices document) and in a lot of ways requiring email to be signed by the sender rather than the sender's provider would be more helpful than requiring DKIM over SPF (although it isn't ideal to tie email to the CA system). Possibly we will see a push in that direction at some point.
  - teddyh 3y ago
    Note that the analogous thing to HTTPS for SMTP would be SSMTP (SMTP over TLS, enforced by DANE). Not DKIM.

Can someone explain why DKIM isn't sufficient? I really don't get the point of SPF/DMARC/ARC everything else when DKIM exists. Why can't everyone just set up DKIM and consider the problem solved?

teddyh 3y ago

Well, at least DMARC is also necessary, since if you recieve an email lacking a DKIM signature, you can’t know if the email should have had a DKIM signature unless there is a DMARC record configured for the sender’s domain.

dataflow 3y ago
Why not take the stance that every email should have a DKIM signature? Just like how we moved to "every website should use https". If you don't, your untrusted by default.
- joveian 3y ago
  Google is slowly moving to reject mail that doesn't pass DMARC no matter the domain settings; it sounds like they are currently rejecting a percentage of messages that fail DMARC. https://support.google.com/mail/answer/81126 https://support.google.com/mail/answer/81126 The issue mentioned in the paper related to SPF is due to adding third parties to the SPF record who then allow spoofing (see table 3 at the top of page 9 in the paper for a summary of the four issues they describe). Hopefully, those third parties will fix that and more mail admins will be aware of the potential negative effects of adding a third party to SPF. It doesn't seem like SPF causes much trouble and if it was possible for DMARC to specify that both SPF and DKIM must validate then that would help in the case of issues with the DKIM signature cryptography. DKIM has a replay issue, although that is not as bad if the to and date fields are signed.
  - dataflow 3y ago
    If they're doing that, that seems like a move intended to prevent forwarding rather than one that's actually necessary for DKIM? Again, I don't see why DKIM isn't enough here. Like you said, replay attacks only work if you don't do DKIM properly, i.e. if you avoid signing some of the initial headers. So how about just... require them to be signed? Sign the message ID, sign the date, sign the addressees, sign the body... sign everything. If you don't, you're assumed to be risky, just like with http:// http://. Problem solved, no?
    - joveian 3y ago
      DMARC is DKIM or SPF so requiring DMARC part of the way to what you are suggesting. There is still a replay possibility with DKIM even if limited to replaying the exact same message (unless providers or email clients track already seen messages until DKIM expiration, which could happen but I don't think is happening at this point). I'm not an expert just someone trying to find a security focused email provider with minimally reasonable privacy policies that doesn't randomly drop email (which unfortunately doesn't seem to exist :( ) so I'm not sure why things are the way they are. Why SPF exists might just be due to being earlier and there not being any strong reason to stop using it. Even Google and Microsoft aren't able to simply require whatever they want without a long transition period. I'm fairly sure Microsoft would like to make email entirely proprietary (and some of the issues almost only apply to them so they aren't likely to be the ones to improve email security). As best I can tell the people with the most direct financial interest in email are the marketers, which isn't a great situation for real improvement (BIMI seems primarily designed to replace the lost income from browsers dropping EV certificates while having similar issues). And security is a hard sell for anything it seems. So all in all I think we are lucky that Google has been doing as much as they have been. A number of old timers already complain about how it isn't really email any more. If Google did want to eventually require DKIM the path to get there would likely still look like what they have been doing. There has also been recent activity just this year to improve the S/MIME situation (best practices document) and in a lot of ways requiring email to be signed by the sender rather than the sender's provider would be more helpful than requiring DKIM over SPF (although it isn't ideal to tie email to the CA system). Possibly we will see a push in that direction at some point.
- teddyh 3y ago
  Note that the analogous thing to HTTPS for SMTP would be SSMTP (SMTP over TLS, enforced by DANE). Not DKIM.