Add DKIM Ed25519 capability (#276)

gen-smtp · Sep 25, 2021 · 5ce941b · 5ce941b
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diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
@@ -15,11 +15,11 @@ jobs:
 
     strategy:
       matrix:
-        otp_vsn: [21.3, 22.3, 23.3, 24.0]
+        otp_vsn: [21.3, 22.3, 23.3, 24.1]
         os: [ubuntu-latest]
         profile: [test, ranch18, ranch20]
         exclude:
-            - otp_vsn: 24.0     # ranch 1.7 does not work on OTP24+
+            - otp_vsn: 24.1     # ranch 1.7 does not work on OTP24+
               profile: test
 
     container:

diff --git a/README.markdown b/README.markdown
@@ -90,13 +90,18 @@ The `send` method variants `send/2, send/3, send_blocking/2` take an `Options` a
 
 ### DKIM signing of outgoing emails
 
-You may wish to configure DKIM signing [RFC6376](http://tools.ietf.org/html/rfc6376) of outgoing emails
-for better security. To do that you need public and private RSA keys, which can be generated by
+You may wish to configure DKIM signing [RFC6376](https://datatracker.ietf.org/doc/html/rfc5672) or [RFC8463](https://datatracker.ietf.org/doc/html/rfc8463) (Ed25519) of outgoing emails
+for better security. To do that you need public and private keys, which can be generated by
 following commands:
 
 ```bash
+# RSA
 openssl genrsa -out private-key.pem 1024
 openssl rsa -in private-key.pem -out public-key.pem -pubout
+
+# Ed25519 - Erlang/OTP 24.1+ only!
+openssl genpkey -algorithm ed25519 -out private-key.pem
+openssl pkey -in private-key.pem -out public-key.pem -pubout
 ```
 
 To send DKIM-signed email:
@@ -120,12 +125,20 @@ SignedMailBody = \
 gen_smtp_client:send({"whatever@example.com", ["andrew@hijacked.us"], SignedMailBody}, []).
 ```
 
-Don't forget to put your public key to `foo.bar._domainkey.example.com` TXT DNS record as smth like
+For using Ed25519 you need to set the option `{a, 'ed25519-sha256'}`.
+
+Don't forget to put your public key to `foo.bar._domainkey.example.com` TXT DNS record as something like
 
+RSA:
 ```
 v=DKIM1; g=*; k=rsa; p=MIGfMA0GCSqGSIb3DQEBA......
 ```
 
+Ed25519:
+```
+v=DKIM1; g=*; k=ed25519; p=MIGfMA0GCSqGSIb3DQEBA......
+```
+
 See RFC6376 for more details.
 
 ## Server Example

diff --git a/src/mimemail.erl b/src/mimemail.erl
@@ -105,6 +105,7 @@
                          {t, now | calendar:datetime()} |
                          {x, calendar:datetime()} |
                          {c, {simple | relaxed, simple | relaxed}} |
+                         {a, 'rsa-sha256' | 'ed25519-sha256'} |
                          {private_key, dkim_priv_key()}].
 -type options() :: [{encoding, binary()} |
                     {decode_attachment, boolean()} |
@@ -1070,11 +1071,18 @@ encode_extra_utf_bytes(Bytes, [C|T], AccOut) when C >= 128 andalso C =< 191 ->
 %% be located in "foo.bar._domainkey.example.com" (see RFC-6376 #3.6.2.1).
 %% `t' - signature timestamp: 'now' or UTC {Date, Time}
 %% `x' - signatue expiration time: UTC {Date, Time}
+%% `a` - signing algorithm (default: `rsa-sha256`):
 %% `private_key' - private key, to sign emails. May be of 2 types: encrypted and
 %% plain in PEM format:
+%% RSA
 %% `{pem_plain, KeyBinary}' - generated by <code>openssl genrsa -out out-file.pem 1024</code>
 %% `{pem_encrypted, KeyBinary, Password}' - generated by, eg
 %%  <code>openssl genrsa -des3 -out out-file.pem 1024</code>
+%% RFC8463
+%% Ed25519 - Erlang/OTP 24.1+ only!
+%% `{pem_plain, KeyBinary}' - generated by <code>openssl genpkey -algorithm ed25519 -out out-file.pem</code>
+%% `{pem_encrypted, KeyBinary, Password}' - generated by, eg
+%%  <code>openssl genpkey -des3 -algorithm ed25519 -out out-file.pem</code>
 %%  3rd paramerter is password to decrypt the key.
 -spec dkim_sign_email([binary()], binary(), dkim_options()) -> [binary()].
 dkim_sign_email(Headers, Body, Opts) ->
@@ -1089,21 +1097,22 @@ dkim_sign_email(Headers, Body, Opts) ->
 									   end
 							   end, [], [t, x]),
 	{HdrsCanT, BodyCanT} = Can = proplists:get_value(c, Opts, {relaxed, simple}),
+	Algorithm = proplists:get_value(a, Opts, 'rsa-sha256'),
 	PrivateKey = proplists:get_value(private_key, Opts),
 
 	%% hash body
 	CanBody = dkim_canonicalize_body(Body, BodyCanT),
 	BodyHash = dkim_hash_body(CanBody),
 	Tags = [%% {b, <<>>},
-			{v, 1}, {a, <<"rsa-sha256">>}, {bh, BodyHash}, {c, Can},
+			{v, 1}, {a, Algorithm}, {bh, BodyHash}, {c, Can},
 			{d, SDID}, {h, HeadersToSign}, {s, Selector} | OptionalTags],
 	%% hash headers
 	Headers1 = dkim_filter_headers(Headers, HeadersToSign),
 	CanHeaders = dkim_canonicalize_headers(Headers1, HdrsCanT),
 	[DkimHeaderNoB] = dkim_canonicalize_headers([dkim_make_header([{b, undefined} | Tags])], HdrsCanT),
 	DataHash = dkim_hash_data(CanHeaders, DkimHeaderNoB),
 	%% sign
-	Signature = dkim_sign(DataHash, PrivateKey),
+	Signature = dkim_sign(DataHash, Algorithm, PrivateKey),
 	DkimHeader = dkim_make_header([{b, Signature} | Tags]),
 	[DkimHeader | Headers].
 
@@ -1147,15 +1156,37 @@ dkim_hash_data(CanonicHeaders, DkimHeader) ->
 	JoinedHeaders = << <<Hdr/binary, "\r\n">> || Hdr <- CanonicHeaders>>,
 	crypto:hash(sha256, <<JoinedHeaders/binary, DkimHeader/binary>>).
 
-dkim_sign(DataHash, {pem_plain, PrivBin}) ->
+%% TODO: Remove once we require Erlang/OTP 24.1+
+%% Related Erlang/OTP bug: https://github.com/erlang/otp/pull/5157
+ed25519_supported() ->
+	{ok, PublicKeyAppVersionString} = application:get_key(public_key, vsn),
+	PublicKeyAppVersionList =
+		lists:map(fun erlang:list_to_integer/1, string:tokens(PublicKeyAppVersionString, ".")),
+	PublicKeyAppVersionList >= [1, 11, 2].
+
+dkim_get_algorithm_digest(Algorithm) ->
+	case Algorithm of
+		'rsa-sha256' ->
+			sha256;
+		'ed25519-sha256' ->
+			case ed25519_supported() of
+				true ->
+					none;
+				false ->
+					throw("DKIM with Ed25519 requires Erlang/OTP 24.1+")
+			end
+	end.
+
+dkim_sign(DataHash, Algorithm, {pem_plain, PrivBin}) ->
 	[PrivEntry] = public_key:pem_decode(PrivBin),
+	Digest = dkim_get_algorithm_digest(Algorithm),
 	Key = public_key:pem_entry_decode(PrivEntry),
-	public_key:sign({digest, DataHash}, sha256, Key);
-dkim_sign(DataHash, {pem_encrypted, EncPrivBin, Passwd}) ->
+	public_key:sign({digest, DataHash}, Digest, Key);
+dkim_sign(DataHash, Algorithm, {pem_encrypted, EncPrivBin, Passwd}) ->
 	[EncPrivEntry] = public_key:pem_decode(EncPrivBin),
+	Digest = dkim_get_algorithm_digest(Algorithm),
 	Key = public_key:pem_entry_decode(EncPrivEntry, Passwd),
-	public_key:sign({digest, DataHash}, sha256, Key).
-
+	public_key:sign({digest, DataHash}, Digest, Key).
 
 dkim_make_header(Tags) ->
 	RevTags = lists:reverse(Tags),				%so {b, ...} became last tag
@@ -1166,9 +1197,9 @@ dkim_make_header(Tags) ->
 dkim_encode_tag(v, 1) ->
 	%% version
 	<<"v=1">>;
-dkim_encode_tag(a, <<"rsa-sha256">>) ->
+dkim_encode_tag(a, Algorithm) ->
 	%% algorithm
-	<<"a=rsa-sha256">>;
+	<<"a=", (atom_to_binary(Algorithm, utf8))/binary>>;
 dkim_encode_tag(b, undefined) ->
 	%% signature (when hashing with no digest)
 	<<"b=">>;
@@ -2377,7 +2408,7 @@ dkim_canonicalization_test_() ->
 						  dkim_canonicalize_headers(Hdrs, relaxed))
 	 end}].
 
-dkim_sign_test_() ->
+dkim_sign_rsa_test_() ->
 	%% * sign using test/fixtures/dkim*.pem
 	{ok, PrivKey} = file:read_file("test/fixtures/dkim-rsa-private.pem"),
 	[{"Sign simple",
@@ -2430,7 +2461,6 @@ dkim_sign_test_() ->
 								 {private_key, {pem_plain, PrivKey}}]}],
 
 			  Enc = encode(Email, Options),
-			  file:write_file("/home/seriy/relaxed-signed.eml", Enc),
 			  {_, _, [{DkimHdrName, DkimHdrVal} | _], _, _} = decode(Enc),
 			  ?assertEqual(<<"DKIM-Signature">>, DkimHdrName),
 			  ?assertEqual(
@@ -2441,4 +2471,115 @@ dkim_sign_test_() ->
 				   "T97QadH42PT6XmO2v01q5nhMgNE4yQyf9DBJs=">>, DkimHdrVal)
 	  end}].
 
+dkim_sign_ed25519_test_() ->
+	case ed25519_supported() of
+		true ->
+			%% * sign using test/fixtures/dkim*.pem
+			{ok, PrivKey} = file:read_file("test/fixtures/dkim-ed25519-private.pem"),
+			[{"Sign simple",
+			fun() ->
+					Email = {<<"text">>, <<"plain">>,
+							[{<<"From">>, <<"me@example.com">>},
+								{<<"Subject">>, <<"Hello world!">>},
+								{<<"Date">>, <<"Thu, 28 Nov 2013 04:15:44 +0400">>},
+								{<<"Message-ID">>, <<"the-id">>},
+								{<<"Content-Type">>, <<"text/plain; charset=utf-8">>}],
+							#{},
+							<<"123">>},
+					Options = [{dkim, [{s, <<"foo.bar">>},
+										{d, <<"example.com">>},
+										{c, {simple, simple}},
+										{a, 'ed25519-sha256'},
+										{t, {{2014, 2, 4}, {23, 15, 00}}},
+										{x, {{2114, 2, 4}, {23, 15, 00}}},
+										{private_key, {pem_plain, PrivKey}}]}],
+
+					Enc = encode(Email, Options),
+					%% This `Enc' value can be verified, for example, by Python script
+					%% https://launchpad.net/dkimpy like:
+					%% >>> pubkey = ''.join(open("test/fixtures/dkim-ed25519-public.pem").read().splitlines()[1:-1])
+					%% >>> dns_mock = lambda *args: 'v=DKIM1; g=*; k=ed25519; p=' + pubkey
+					%% >>> import dkim
+					%% >>> d = dkim.DKIM(mime_message) % pass `Enc' value as 1'st argument
+					%% >>> d.verify(dnsfunc=dns_mock)
+					%% True
+					{_, _, [{DkimHdrName, DkimHdrVal} | _], _, _} = decode(Enc),
+					?assertEqual(<<"DKIM-Signature">>, DkimHdrName),
+					?assertEqual(<<"t=1391555700; x=4547229300; s=foo.bar; h=from:to:subject:date; d=example.com; c=simple/simple; "
+									"bh=Afm/S7SaxS19en1h955RwsupTF914DQUPqYU8Nh7kpw=; a=ed25519-sha256; v=1; "
+									"b=bFPndkFlgpFbfVKBF9HiVkQQF/3ojOQT7ycrZYp0yYe4oyItUQexlvd+Q7BviiHv/seLVBESpBjLbthbfb5HDA==">>,
+									DkimHdrVal)
+			end},
+			{"Sign relaxed headers, simple body",
+			fun() ->
+					Email = {<<"text">>, <<"plain">>,
+							[{<<"From">>, <<"me@example.com">>},
+								{<<"Subject">>, <<"Hello world!">>},
+								{<<"Date">>, <<"Thu, 28 Nov 2013 04:15:44 +0400">>},
+								{<<"Message-ID">>, <<"the-id-relaxed">>},
+								{<<"Content-Type">>, <<"text/plain; charset=utf-8">>}],
+							#{},
+							<<"123">>},
+					Options = [{dkim, [{s, <<"foo.bar">>},
+										{d, <<"example.com">>},
+										{c, {relaxed, simple}},
+										{a, 'ed25519-sha256'},
+										{private_key, {pem_plain, PrivKey}}]}],
+
+					Enc = encode(Email, Options),
+					{_, _, [{DkimHdrName, DkimHdrVal} | _], _, _} = decode(Enc),
+					?assertEqual(<<"DKIM-Signature">>, DkimHdrName),
+					?assertEqual(<<"s=foo.bar; h=from:to:subject:date; d=example.com; c=relaxed/simple; "
+						"bh=Afm/S7SaxS19en1h955RwsupTF914DQUPqYU8Nh7kpw=; a=ed25519-sha256; v=1; "
+						"b=f7wORU/qmPr4q891m5zmZMadPm9n9e596mBJHBD6tE51PAl4pHdpw9xRC1kwLGmxPTEK5SiQluPVTbDHVhVZBQ==">>,
+						DkimHdrVal)
+			end}];
+		false ->
+			[]
+	end.
+
+dkim_sign_ed25519_encrypted_key_test_() ->
+	case ed25519_supported() of
+		true ->
+			%% * sign using test/fixtures/dkim*.pem
+			{ok, EncryptedPrivKey} =
+				file:read_file("test/fixtures/dkim-ed25519-encrypted-private.pem"),
+			[{"Sign encrypted",
+			fun() ->
+					Email = {<<"text">>, <<"plain">>,
+							[{<<"From">>, <<"me@example.com">>},
+								{<<"Subject">>, <<"Hello world!">>},
+								{<<"Date">>, <<"Thu, 28 Nov 2013 04:15:44 +0400">>},
+								{<<"Message-ID">>, <<"the-id">>},
+								{<<"Content-Type">>, <<"text/plain; charset=utf-8">>}],
+							#{},
+							<<"123">>},
+					Options = [{dkim, [{s, <<"foo.bar">>},
+										{d, <<"example.com">>},
+										{c, {simple, simple}},
+										{a, 'ed25519-sha256'},
+										{t, {{2014, 2, 4}, {23, 15, 00}}},
+										{x, {{2114, 2, 4}, {23, 15, 00}}},
+										{private_key, {pem_encrypted, EncryptedPrivKey, "password"}}]}],
+
+					Enc = encode(Email, Options),
+					%% This `Enc' value can be verified, for example, by Python script
+					%% https://launchpad.net/dkimpy like:
+					%% >>> pubkey = ''.join(open("test/fixtures/dkim-ed25519-public.pem").read().splitlines()[1:-1])
+					%% >>> dns_mock = lambda *args: 'v=DKIM1; g=*; k=ed25519; p=' + pubkey
+					%% >>> import dkim
+					%% >>> d = dkim.DKIM(mime_message) % pass `Enc' value as 1'st argument
+					%% >>> d.verify(dnsfunc=dns_mock)
+					%% True
+					{_, _, [{DkimHdrName, DkimHdrVal} | _], _, _} = decode(Enc),
+					?assertEqual(<<"DKIM-Signature">>, DkimHdrName),
+					?assertEqual(<<"t=1391555700; x=4547229300; s=foo.bar; h=from:to:subject:date; d=example.com; c=simple/simple; "
+						"bh=Afm/S7SaxS19en1h955RwsupTF914DQUPqYU8Nh7kpw=; a=ed25519-sha256; v=1; "
+						"b=JgsuW5OmKPk188YRmxs1cLA8mrAf9FNC+s/PYK7Vat7HF4l7FglcoWWHqm0/Cg7o/V+8bP1RNwes1xDKS8/wDQ==">>,
+						DkimHdrVal)
+			end}];
+		false ->
+			[]
+	end.
+
 -endif.
diff --git a/test/fixtures/dkim-ed25519-encrypted-private.pem b/test/fixtures/dkim-ed25519-encrypted-private.pem
@@ -0,0 +1,5 @@
+-----BEGIN ENCRYPTED PRIVATE KEY-----
+MIGKME4GCSqGSIb3DQEFDTBBMCkGCSqGSIb3DQEFDDAcBAjWxBqVOoAQmQICCAAw
+DAYIKoZIhvcNAgkFADAUBggqhkiG9w0DBwQINxHTI3T4bPEEOFrkHOCl0Y4wOEPa
+TEMzq2vB5tqpSVcbbup6BdRGV1f7yDsk+9l9f08m3pZUIbeNgUy1Y9JmUjxU
+-----END ENCRYPTED PRIVATE KEY-----
diff --git a/test/fixtures/dkim-ed25519-encrypted-public.pem b/test/fixtures/dkim-ed25519-encrypted-public.pem
@@ -0,0 +1,3 @@
+-----BEGIN PUBLIC KEY-----
+MCowBQYDK2VwAyEAHWHDpSxS5ABadBDrOKcpyaImlzV4//pJ3A3UgdLuFMk=
+-----END PUBLIC KEY-----
diff --git a/test/fixtures/dkim-ed25519-private.pem b/test/fixtures/dkim-ed25519-private.pem
@@ -0,0 +1,3 @@
+-----BEGIN PRIVATE KEY-----
+MC4CAQAwBQYDK2VwBCIEINLp5tYtDtUVSeH4BJb3+ygipAjPHFm4eB0QNWlhcUNZ
+-----END PRIVATE KEY-----
diff --git a/test/fixtures/dkim-ed25519-public.pem b/test/fixtures/dkim-ed25519-public.pem
@@ -0,0 +1,3 @@
+-----BEGIN PUBLIC KEY-----
+MCowBQYDK2VwAyEAgxFnePs7aR/rt5KBGSaJU4T+Uh2cIvLtV6cBz5ypIYE=
+-----END PUBLIC KEY-----