Fixed a bug in retrieving the DKIM public key

packages/helpers/src/lib/mailauth/DoH.ts

@@ -0,0 +1,97 @@
+// DoH servers list
+export enum DoHServer {
+    // Google Public DNS
+    Google = "https://dns.google/resolve",
+    // Cloudflare DNS
+    Cloudflare = "https://cloudflare-dns.com/dns-query",
+}
+
+/**
+ * DNS over HTTPS (DoH) resolver
+ *
+ * @export
+ * @class DoH
+ */
+export class DoH {
+
+    // DNS response codes
+    static DoHStatusNoError = 0;
+    // DNS RR types
+    static DoHTypeTXT = 16;
+
+    /**
+     * Resolve DKIM public key from DNS
+     *
+     * @static
+     * @param {string} name DKIM record name (e.g. 20230601._domainkey.gmail.com)
+     * @param {string} DNSServer DNS over HTTPS API URL
+     * @return {*}  {(Promise<string | null>)} DKIM public key or null if not found
+     * @memberof DoH
+     */
+    public static async resolveDKIMPublicKey(name: string, DNSServer: string): Promise<string | null> {
+        if (!DNSServer.startsWith('https://')) {
+            DNSServer = 'https://' + DNSServer;
+        }
+        if (DNSServer.endsWith('/')) {
+            DNSServer = DNSServer.slice(0, -1);
+        }
+        const resp = await fetch(
+            DNSServer + "?" +
+            new URLSearchParams({
+                name: name,
+                // DKIM public key record type is TXT
+                type: DoH.DoHTypeTXT.toString(),
+            }),
+            {
+                headers: {
+                    "accept": "application/dns-json",
+                }
+            }
+        );
+        if (resp.status === 200) {
+            const out = await resp.json();
+            if (typeof out === 'object' && out !== null && 'Status' in out && 'Answer' in out) {
+                const resp = out as DoHResponse;
+                if (resp.Status === DoH.DoHStatusNoError && resp.Answer.length > 0) {
+                    for (const ans of resp.Answer) {
+                        if (ans.type === DoH.DoHTypeTXT) {
+                            let DKIMRecord = ans.data;
+                            /*
+                                Remove all double quotes
+                                Some DNS providers wrap TXT records in double quotes, 
+                                and others like Cloudflare may include them. According to 
+                                TXT (potentially multi-line) and DKIM (Base64 data) standards,
+                                we can directly remove all double quotes from the DKIM public key.
+                            */
+                            DKIMRecord = DKIMRecord.replace(/"/g, '');
+                            return DKIMRecord;
+                        }
+                    }
+                }
+            }
+        }
+        return null;
+    }
+}
+
+interface DoHResponse {
+    Status: number;       // NOERROR - Standard DNS response code (32 bit integer).
+    TC: boolean;          // Whether the response is truncated
+    AD: boolean;          // Whether all response data was validated with DNSSEC
+    CD: boolean;          // Whether the client asked to disable DNSSEC
+    Question: Question[];
+    Answer: Answer[];
+    Comment: string;
+}
+
+interface Question {
+    name: string;          // FQDN with trailing dot
+    type: number;          // A - Standard DNS RR type. 5:CNAME, 16:TXT
+}
+
+interface Answer {
+    name: string;          // Always matches name in the Question section
+    type: number;          // A - Standard DNS RR type. 5:CNAME, 16:TXT
+    TTL: number;           // Record's time-to-live in seconds
+    data: string;          // Record data
+}

packages/helpers/src/lib/mailauth/tools.ts

@@ -9,6 +9,7 @@ import crypto, { KeyObject } from "crypto";
 import parseDkimHeaders from "./parse-dkim-headers";
 import { DkimVerifier } from "./dkim-verifier";
 import type { Parsed, SignatureType } from "./dkim-verifier";
+import { DoH, DoHServer } from './DoH';
 
 const IS_BROWSER = typeof window !== "undefined";
 
@@ -247,16 +248,17 @@ export const formatSignatureHeaderLine = (
 };
 
 async function resolveDNSHTTP(name: string, type: string) {
-  const resp = await fetch(
-    "https://dns.google/resolve?" +
-      new URLSearchParams({
-        name: name,
-        type: type,
-      })
-  );
-  const out = await resp.json();
-  // For some DNS, the Answer response here contains more than 1 element in the array. The last element is the one containing the public key
-  return [out.Answer[out.Answer.length - 1].data];
+  if (type !== "TXT") {
+    throw new Error("DKIM record type is not TXT");
+  }
+  const DKIMRecord = await DoH.resolveDKIMPublicKey(name, DoHServer.Google);
+  if (!DKIMRecord) {
+    throw new CustomError("No DKIM record found", "ENODATA");
+  }
+  if (DKIMRecord !== await DoH.resolveDKIMPublicKey(name, DoHServer.Cloudflare)) {
+    console.error("DKIM record mismatch!");
+  }
+  return [DKIMRecord];
 }
 
 // from https://developers.google.com/web/updates/2012/06/How-to-convert-ArrayBuffer-to-and-from-String
@@ -301,9 +303,7 @@ export const getPublicKey = async (
   resolver: (...args: [name: string, type: string]) => Promise<any>
 ) => {
   minBitLength = minBitLength || 1024;
-  if (!IS_BROWSER) {
-    resolver = resolver || require("dns").promises.resolve;
-  } else {
+  if (!resolver) {
     resolver = resolveDNSHTTP;
   }