Add DKIM Ed25519 capability

[cw789]

This pull request depends on erlang/otp#5157.
And so DKIM with Ed25519 signing will only be available with OTP 24.1+.

We only will support this or that (RSA / Ed25519) for now.
Dual signing with RSA & Ed25519 won't be part of this PR.

Close #275

[cw789]

Hello Sergey. I've applied your code suggestions. Thanks for your time in the meanwhile.
I've tried some things again for the problem above, but it still seems to be too deep buried for my Erlang or crypto experience.

[cw789]

Ok things are cleaned up now. Testing + tests for it are still missing.
I don't know how to make this with the -ifdef(OTP_RELEASE). thing.
I would keep this open as draft until OTP 24.1 is out - if ok?

[cw789]

I updated the description with mentioning we won't implement dual signing.

[cw789]

Will something like this in mimemail.erl work?

%% TODO: Remove me once we require Erlang/OTP 24+
-ifdef(OTP_RELEASE).
  -if(?OTP_RELEASE >= 24).
    -define(DKIM_ED25519, true).
  -endif.
-endif.

-ifdef(DKIM_ED25519).
dkim_get_algorithm_digest(Algorithm) ->
	case Algorithm of
		'rsa-sha256' -> sha256;
		'ed25519-sha256' -> none
	end.
-else.
dkim_get_algorithm_digest(Algorithm) ->
	case Algorithm of
		'rsa-sha256' -> sha256;
		'ed25519-sha256' -> throw("DKIM Ed25519 requires Erlang/OTP 24.1+").
	end.
-endif.

Or should we use instead of -if(?OTP_RELEASE >= 24) something likely as -if(version(erlang) >= [24, 1, whatever].

[cw789]

Another open question.
I would like to duplicate dkim_sign_rsa_test_() and make an adapted dkim_sign_ed25519_test_.
How could I make sure this test only runs on Erlang/OTP 24.1+?

[seriyps]

I'm thinking maybe it's better to do some checks at runtime? Smth like lists:member(ed25519, crypto:supports(curves))? But then we might need to also implement the fix backport as I suggested in #276 (comment)
Or we may also check the version of the public_key OTP application application:get_key(public_key, vsn). to see if it includes the bugfix erlang/otp#5157

The problem with -if(?OTP_RELEASE >= 24). is that there is no way to check if you are running 24.0 or 24.1.

I'll try to look in more details later today or tomorrow

[cw789]

Ok. A runtime check for high enough public_key vsn sounds like a good consideration.

[seriyps]

Seems public_key-1.11.2 is the one where the fix is released (actually today): https://erlang.org/download/OTP-24.1.README

--- Improvements and New Features ---

OTP-17609 Application(s): public_key
Related Id(s): GH-5156, GH-5157

           When decoding an 'ECPrivateKey' unwrap the private key.
           For more precise information see RFC 8410, section 7.

[cw789]

It seems that comparing SemVer versions isn't something done out of the box in Erlang. Is it worth to implement a version_compare function or is there any simpler possibility I oversee?

[seriyps]

I think smth similar to this one is the easiest you can do:

https://github.com/gen-smtp/gen_smtp/pull/171/files#diff-870779fb647905a4c2f76a5679211924ddf4ced941daa9594f1b1064a8dceff2R145-R153

So

{ok, VerString} = application:get_key(public_key, vsn),
Ver = lists:map(fun erlang:list_to_integer/1,
					string:tokens(VerString, ".")),
	if Ver < [1, 11, 2] ->
			...not supported...;
	   true ->
			...supported...
	end.

[cw789]

Thanks Sergey. Half of what I already had - but I missed the part of converting the strings to integer.

The test now will still fail. I've not jet managed to setup the rebar3 test thing properly on my machine.

[seriyps]

Really great! Thanks! I added some less critical comments, up to you if you want to fix them or not

[cw789]

Thanks a lot Sergey. Thanks for mentoring me during my first time working with Erlang.
Hope it wasn't too cumbersome, as I've jumped into the cold water without understanding the Erlang syntax beforehand.
I've found out that a lot of the cool things in Elixir were already cool things in Erlang before - very well thought.

This PR is now ready to merge from my side. Tests should pass & my manual test also worked out.

[seriyps]

Cool! Thanks for not giving up on it =)

[mworrell]

And merged. Thank you!