Add option to allow SHA1 verification for back compatible

[codecov-commenter]

Codecov Report

Merging #33 into master will increase coverage by 1.06%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
+ Coverage   66.62%   67.68%   +1.06%     
==========================================
  Files           7        7              
  Lines         845      848       +3     
==========================================
+ Hits          563      574      +11     
+ Misses        213      206       -7     
+ Partials       69       68       -1     

Impacted Files	Coverage Δ
dkim/verify.go	65.19% <100.00%> (+4.03%)	:arrow_up:

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[emersion]

Please rebase against our master branch, to only keep the last commit.

https://git-rebase.io/

[huicao]

rebased, thanks!

[emersion]

Thanks for rebasing.

What's the use-case for this patch? The RFC says it "MUST NOT" be used, so I'm not sure it's a good idea to add an option to use it.

DKIM signatures aren't designed to be verified a long time after the email is received. Instead, the receiving mail server is supposed to check the signatures and add an Authentication-Results header.

[huicao]

The traffic received by our email server still has lot of SHA1 used. Google or others still validate them. I understand those servers should not send this, but this is the way how DKIM is adopted.

[emersion]

Hmm. Actually, scratch that. The RFC states: "rsa-sha1 MUST NOT be used for signing or verifying."

I would be fine to add this as an option if the RFC said sha1 MUST NOT be used for signing only. But the RFC explicitly says it MUST NOT be used for verifying too.

I'm afraid I can't accept this PR, sorry. I'd rather make DKIM signature verification fail in this case. Feel free to keep this patch in a fork.

[emersion]

Note, the real solution is to contact the email providers still using sha1 and ask them to upgrade.

[huicao]

I don't think SHA1 will disappear any time soon, so many APIs still uses SHA1 or MD5 to query. I don't have the time or resource to tell all those providers to fix them. Actually, having SHA1 DKIM is better than no DKIM at all.

[foxcpp]

DKIM is used as an input for policy decisions, lack of SHA1 support incentivizes adoption of better algorithms. If everybody continues to support SHA1 - it will indeed not "disappear any time soon".