Enable DKIM signing

nixos/machines/nyarlathotep/configuration.nix

@@ -75,6 +75,18 @@
       group = "stalwart-mail";
       mode = "0440";
     };
+    "dkim_rsa" = {
+      sopsFile = ./dkim.keys.yaml;
+      owner = "stalwart-mail";
+      group = "stalwart-mail";
+      mode = "0440";
+    };
+    "dkim_ed25519" = {
+      sopsFile = ./dkim.keys.yaml;
+      owner = "stalwart-mail";
+      group = "stalwart-mail";
+      mode = "0440";
+    };
     # password for https://stalw.art/docs/auth/authorization/administrator/#fallback-administrator encoded to be supplied in the basic auth header
     stalwartAdmin = {
       sopsFile = ./stalwartAdmin.secrets.yaml;

nixos/machines/nyarlathotep/dkim.keys.yaml

@@ -0,0 +1,40 @@
+dkim_rsa: ENC[AES256_GCM,data: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,iv:QbZVXp5FQhmYZvXxXNxWKrNm5GqM+2P3a5pPk499mlc=,tag:F+KNoPRnoLLhOpEj6Czj6Q==,type:str]
+dkim_ed25519: ENC[AES256_GCM,data:cZHm7bVpQ/VhYLt2CnNk9364k+J5ybgSLrR7Vm1GsCU6JcAvHl8Y5R7mqwgS+gTnHX7K02GuIGXa8909/aEotE0ZMY5irKJ25SGJqTaqQafbiMOz65CRQh5trtcMBF4s4wRYOkDGgz09KkELbkDHyQZFcrGqvgM=,iv:p9ROj/epqR3xtrimXF1onJJHH9JUqNG9z1MxKVu9uPg=,tag:m53rXkcu+ernS5JX+k8YcA==,type:str]
+sops:
+    kms: []
+    gcp_kms: []
+    azure_kv: []
+    hc_vault: []
+    age:
+        - recipient: age1rasjnr2tlv9y70sj0z0hwpgpxdc974wzg5umtx2pnc6z0p05u3js6r8sln
+          enc: |
+            -----BEGIN AGE ENCRYPTED FILE-----
+            YWdlLWVuY3J5cHRpb24ub3JnL3YxCi0+IFgyNTUxOSB6VnhvWHdsZWNHemlueFo4
+            L0xCTGp4NlRuU3YwRWJiSHFBbmtURTNMQkVRCnlSbFc0Q2xINjRvU2tQeStQc1U5
+            VElxcTVuNm9MUm01RkpGYytrYWg0czgKLS0tIHZqUWhkMGRNNjJvUTQrOHBpZXVS
+            NlpjeDQxbVZIRHFCcmNtT1JSVHp1K2sKSNcC0fcOar/KKzs1twaozB8wfdFT9OdB
+            4quV/ycNpJpfs6+2r0RTLBxYFyusybu1swosAni+PJsRXS82+PTXHQ==
+            -----END AGE ENCRYPTED FILE-----
+        - recipient: age1epz92k2rkp43hkrg3u0jgkzhnkwx8y43kag7rvfzwl9wcddelvusyetxl7
+          enc: |
+            -----BEGIN AGE ENCRYPTED FILE-----
+            YWdlLWVuY3J5cHRpb24ub3JnL3YxCi0+IFgyNTUxOSBsUTUzYzZuMkYvcTlrUmRK
+            aStnak5IWitFUSt0eVBQOHIzcTlrMFRFTjA4CmlYUTdobXFUK2tYMWtFekNqNnhp
+            R2RRRFdHc1p6bFVjYU9lbTRBeEM3Y2sKLS0tIHdsRW1wR25pVkZIYU1yMm9sQXpr
+            NFhiN0pyaHVWT1h5eVFXMWZDb0sxUGMKIVkYYheD8F9aaAyCA+m9ZGlV8vKbAW4r
+            H6FUe+ats30abxoYfHZfMJv17BxJtpodksSxWjnPYm0dfRf/EF/vSQ==
+            -----END AGE ENCRYPTED FILE-----
+        - recipient: age1ktwclxa640l89le6yecm8v2z6hmwr4lusd6x9gyzamhv57887szqtqp59a
+          enc: |
+            -----BEGIN AGE ENCRYPTED FILE-----
+            YWdlLWVuY3J5cHRpb24ub3JnL3YxCi0+IFgyNTUxOSBvU3NzY0Uxc0NhY2xJZyti
+            TCtTS1crV3hzMXZNV3k4cm0zUFNuY2tBL0dNCnNpYytoaUI1eERhdG1PUlZ2eE5C
+            R2UrVlBwcXR2L1VNR3RJL1lEQmlTSDgKLS0tIFJyLzhZeG5zejFmL2VkYy8xVEM1
+            U3QwOXlRdU8yd3ozL2hUVzRXNGE0bDQKT7SLAqICsbFmRUF+3s2avpBt0dLUbHLX
+            AgQzx5v6GpMMNwCkCrOnpFX6al7zkRSYHe7hbn03BBORz9mPHek5ew==
+            -----END AGE ENCRYPTED FILE-----
+    lastmodified: "2025-03-02T07:58:00Z"
+    mac: ENC[AES256_GCM,data:OvERjDFfHTJbTfwq9BmXBQy6pjeyIhao6zP4we0KeYL3skbw4+aaMixjUFzjauby0C7nJjEPBSk6pwK3lN+rScS5g7J8tTNtmhfEDQbfsS5zNDKzIQjYxbUbDr2cTPWwCA73gRGMwLbyNvdfuEp46jNV8OJ8km/y2nyG9lDcBb4=,iv:0RSU2MdZWiYEapwXGzevP9/vc/Sk1MS6a0MnCRQyIs8=,tag:vvngXS2IRzH999yzo4JyFQ==,type:str]
+    pgp: []
+    unencrypted_suffix: _unencrypted
+    version: 3.9.4

nixos/modules/mail.nix

@@ -108,6 +108,15 @@ in {
             iprev.verify = "relaxed";
             spf.verify.ehlo = "relaxed";
             spf.verify.mail-from = "relaxed";
+
+            # Sign *our* outgoing mails with the configured signatures.
+            dkim.sign = [
+              {
+                "if" = "is_local_domain('', sender_domain) || sender_domain == 'lists.mathebau.de'";
+                "then" = "['rsa-' + sender_domain, 'ed25519-' + sender_domain]";
+              }
+              {"else" = false;}
+            ];
           };
 
           # Forward outgoing mail to HRZ or mail VMs.
@@ -202,12 +211,53 @@ in {
           sieve.trusted.from-addr = "sender"; # set the from-address to the original sender as specified in the MAIL FROM.
           sieve.trusted.from-name = "sender";
           sieve.trusted.return-path = "sender";
+          # If we are the sender, we sign the message with DKIM. Else we leave it alone.
+          sieve.trusted.sign = [
+            {
+              "if" = "is_local_domain('', sender_domain) || sender_domain == 'lists.mathebau.de'";
+              "then" = "['rsa-' + sender_domain, 'ed25519-' + sender_domain]";
+            }
+            {"else" = false;}
+          ];
           sieve.trusted.limits = {
             redirects = 50;
             out-messages = 50;
           };
           session.data.script = "'redirects'";
 
+          # See https://stalw.art/docs/smtp/authentication/dkim/sign
+          # We need two blocks per domain because the domain setting in the blocks does not accept variables like `sender_domain`.
+          signature = let
+            signatureTemplate = domain: {
+              "rsa-${domain}" = {
+                private-key = "%{file:/run/secrets/dkim_rsa}%";
+                domain = "${domain}";
+                selector = "rsa-default";
+                headers = ["From" "To" "Cc" "Date" "Subject" "Message-ID" "Organization" "MIME-Version" "Content-Type" "In-Reply-To" "References" "List-Id" "User-Agent" "Thread-Topic" "Thread-Index"];
+                algorithm = "rsa-sha256";
+                canonicalization = "relaxed/relaxed";
+              };
+              "ed25519-${domain}" = {
+                private-key = "%{file:/run/secrets/dkim_ed25519}%";
+                domain = "${domain}";
+                selector = "ed-default";
+                headers = ["From" "To" "Cc" "Date" "Subject" "Message-ID" "Organization" "MIME-Version" "Content-Type" "In-Reply-To" "References" "List-Id" "User-Agent" "Thread-Topic" "Thread-Index"];
+                algorithm = "ed25519-sha256";
+                canonicalization = "relaxed/relaxed";
+              };
+            };
+          in
+            map signatureTemplate (["lists.mathebau.de"] ++ (map ({domain, ...}: domain) cfg.domains));
+
+          # Sign *our* outgoing mails with the configured signatures.
+          auth.dkim.sign = [
+            {
+              "if" = "is_local_domain('', sender_domain) || sender_domain == 'lists.mathebau.de'";
+              "then" = "['rsa-' + sender_domain, 'ed25519-' + sender_domain]";
+            }
+            {"else" = false;}
+          ];
+
           authentication.fallback-admin = {
             user = "admin";
             # see passwd on azathoth for plaintext or machine secret in encoded format for HTTP Basic AUTH