Merge pull request #251 from zkemail/optional_bh_check

Add skipBodyHash check flag in verifyDkimSignature function

packages/helpers/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@zk-email/helpers",
-  "version": "6.3.2",
+  "version": "6.4.2",
   "license": "MIT",
   "main": "dist",
   "scripts": {

packages/helpers/src/dkim/index.ts

@@ -30,6 +30,7 @@ export interface DKIMVerificationResult {
  * @param enableSanitization If true, email will be applied with various sanitization to try and pass DKIM verification
  * @param fallbackToZKEmailDNSArchive If true, ZK Email DNS Archive (https://archive.prove.email/api-explorer) will
  *                                    be used to resolve DKIM public keys if we cannot resolve from HTTP DNS
+ * @param skipBodyHash If true, it bypass the dkim body hash check
  * @returns
  */
 export async function verifyDKIMSignature(
@@ -37,17 +38,18 @@ export async function verifyDKIMSignature(
   domain: string = '',
   enableSanitization: boolean = true,
   fallbackToZKEmailDNSArchive: boolean = false,
+  skipBodyHash = false,
 ): Promise<DKIMVerificationResult> {
   const emailStr = email.toString();
 
-  let dkimResult = await tryVerifyDKIM(email, domain, fallbackToZKEmailDNSArchive);
+  let dkimResult = await tryVerifyDKIM(email, domain, fallbackToZKEmailDNSArchive, skipBodyHash);
 
   // If DKIM verification fails, try again after sanitizing email
   let appliedSanitization;
   if (dkimResult.status.comment === 'bad signature' && enableSanitization) {
     const results = await Promise.all(
       sanitizers.map((sanitize) =>
-        tryVerifyDKIM(sanitize(emailStr), domain, fallbackToZKEmailDNSArchive).then((result) => ({
+        tryVerifyDKIM(sanitize(emailStr), domain, fallbackToZKEmailDNSArchive, skipBodyHash).then((result) => ({
           result,
           sanitizer: sanitize.name,
         })),
@@ -98,6 +100,7 @@ async function tryVerifyDKIM(
   email: Buffer | string,
   domain: string = '',
   fallbackToZKEmailDNSArchive: boolean = false,
+  skipBodyHash = false,
 ) {
   const resolver = async (name: string, type: string) => {
     try {
@@ -115,6 +118,7 @@ async function tryVerifyDKIM(
 
   const dkimVerifier = new DkimVerifier({
     resolver,
+    skipBodyHash,
   });
 
   await writeToStream(dkimVerifier, email as any);

packages/helpers/src/lib/mailauth/dkim-verifier.ts

@@ -50,12 +50,14 @@ export class DkimVerifier extends MessageParser {
   private arc: { chain: false };
   private seal: { bodyHash: string };
   private sealBodyHashKey: string = '';
+  private skipBodyHash: boolean = false;
   constructor(options: Record<string, any>) {
     super();
 
     this.options = options || {};
     this.resolver = this.options.resolver;
     this.minBitLength = this.options.minBitLength;
+    this.skipBodyHash = this.options.skipBodyHash 
 
     this.results = [];
 
@@ -180,7 +182,7 @@ export class DkimVerifier extends MessageParser {
 
   async finalChunk() {
     try {
-      if (!this.headers || !this.bodyHashes.size) {
+      if (!this.headers || (!this.skipBodyHash && !this.bodyHashes.size)) {
         return;
       }
 
@@ -242,7 +244,7 @@ export class DkimVerifier extends MessageParser {
 
         let bodyHashObj = this.bodyHashes.get(signatureHeader.bodyHashKey);
         let bodyHash = bodyHashObj?.hash;
-        if (signatureHeader.parsed?.bh?.value !== bodyHash) {
+        if ((signatureHeader.parsed?.bh?.value !== bodyHash) && !this.skipBodyHash) {
           status.result = 'neutral';
           status.comment = `body hash did not verify`;
         } else {
@@ -332,7 +334,8 @@ export class DkimVerifier extends MessageParser {
 
         if (
           typeof signatureHeader.maxBodyLength === 'number' &&
-          signatureHeader.maxBodyLength !== signatureHeader.bodyHashedBytes
+          signatureHeader.maxBodyLength !== signatureHeader.bodyHashedBytes &&
+          !this.skipBodyHash
         ) {
           status.result = 'fail';
           status.comment = `invalid body length ${signatureHeader.bodyHashedBytes}`;

packages/helpers/tests/dkim.test.ts

@@ -72,6 +72,28 @@ describe('DKIM signature verification', () => {
       expect(e.message).toBe('DKIM signature not found for domain domain.com');
     }
   });
+
+  it('should skip body-hash verification for body-less emails', async () => {
+    // From address domain is icloud.com
+    const email = fs.readFileSync(path.join(__dirname, 'test-data/email-bodyless.eml'));
+
+    // Should pass with default domain
+    const result = await verifyDKIMSignature(email, "", true, false, true);
+    expect.assertions(1);
+    expect(result.signingDomain).toBe('icloud.com');
+  });
+
+  it('should pass for tampered body if skipBodyHash=true', async () => {
+    const email = fs.readFileSync(path.join(__dirname, 'test-data/email-body-tampered.eml'));
+
+    try {
+      await verifyDKIMSignature(email, '', true, false, true);
+    } catch (e) {
+      expect(e.message).toBe(
+        'DKIM signature verification failed for domain icloud.com. Reason: body hash did not verify',
+      );
+    }
+  });
 });
 
 it('should fallback to ZK Email Archive if DNS over HTTP fails', async () => {

packages/helpers/tests/test-data/email-bodyless.eml

@@ -0,0 +1,15 @@
+DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=icloud.com; s=1a1hai; t=1693038337; bh=7xQMDuoVVU4m0W0WRVSrVXMeGSIASsnucK9dJsrc+vU=; h=from:Content-Type:Mime-Version:Subject:Message-Id:Date:to; b=EhLyVPpKD7d2/+h1nrnu+iEEBDfh6UWiAf9Y5UK+aPNLt3fAyEKw6Ic46v32NOcZD
+	 M/zhXWucN0FXNiS0pz/QVIEy8Bcdy7eBZA0QA1fp8x5x5SugDELSRobQNbkOjBg7Mx
+	 VXy7h4pKZMm/hKyhvMZXK4AX9fSoXZt4VGlAFymFNavfdAeKgg/SHXLds4lOPJV1wR
+	 2E21g853iz5m/INq3uK6SQKzTnz/wDkdyiq90gC0tHQe8HpDRhPIqgL5KSEpuvUYmJ
+	 wjEOwwHqP6L3JfEeROOt6wyuB1ah7wgRvoABOJ81+qLYRn3bxF+y1BC+PwFd5yFWH5
+	 Ry43lwp1/3+sA==
+from: runnier.leagues.0j@icloud.com
+Content-Type: text/plain; charset=us-ascii
+Content-Transfer-Encoding: 7bit
+Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3731.500.231\))
+Subject: Hello
+Message-Id: <8F819D32-B6AC-489D-977F-438BBC4CAB27@me.com>
+Date: Sat, 26 Aug 2023 12:25:22 +0400
+to: zkewtest@gmail.com
+