Support generating subkeys with 'forwarded communication' flag to dec…

…rypt autoforwarded messages (#8)

These subkeys must not have the standard encryption flags (EtEr) set,
as they are not supposed to be used for direct messages.

Also:
- preserve 'forwarded communication' key flag when reformatting
- fix bug allowing to decrypt forwarded messages by setting
`config.allowInsecureDecryptionWithSigningKeys` instead of
`config.allowForwardedMessages`
- add TS definition for `config.allowForwardedMessages`

openpgp.d.ts

@@ -317,6 +317,7 @@ interface Config {
   aeadProtect: boolean;
   allowUnauthenticatedMessages: boolean;
   allowUnauthenticatedStream: boolean;
+  allowForwardedMessages: boolean;
   checksumRequired: boolean;
   minRSABits: number;
   passwordCollisionCheck: boolean;
@@ -707,6 +708,7 @@ interface SubkeyOptions {
   keyExpirationTime?: number;
   date?: Date;
   sign?: boolean;
+  forwarding?: boolean;
   config?: PartialConfig;
 }
 

src/key/factory.js

@@ -138,7 +138,8 @@ export async function reformat(options, config) {
         helper.getLatestValidSignature(subkey.bindingSignatures, secretKeyPacket, enums.signature.subkeyBinding, dataToVerify, null, config)
       ).catch(() => ({}));
       return {
-        sign: bindingSignature.keyFlags && (bindingSignature.keyFlags[0] & enums.keyFlags.signData)
+        sign: bindingSignature.keyFlags && (bindingSignature.keyFlags[0] & enums.keyFlags.signData),
+        forwarding: bindingSignature.keyFlags && (bindingSignature.keyFlags[0] & enums.keyFlags.forwardedCommunication)
       };
     }));
   }

src/key/helper.js

@@ -93,7 +93,9 @@ export async function createBindingSignature(subkey, primaryKey, options, config
       signatureType: enums.signature.keyBinding
     }, options.date, undefined, undefined, undefined, config);
   } else {
-    signatureProperties.keyFlags = [enums.keyFlags.encryptCommunication | enums.keyFlags.encryptStorage];
+    signatureProperties.keyFlags = options.forwarding ?
+      [enums.keyFlags.forwardedCommunication] :
+      [enums.keyFlags.encryptCommunication | enums.keyFlags.encryptStorage];
   }
   if (options.keyExpirationTime > 0) {
     signatureProperties.keyExpirationTime = options.keyExpirationTime;
@@ -325,6 +327,10 @@ export function sanitizeKeyOptions(options, subkeyDefaults = {}) {
   options.date = options.date || subkeyDefaults.date;
 
   options.sign = options.sign || false;
+  options.forwarding = options.forwarding || false;
+  if (options.sign && options.forwarding) {
+    throw new Error('Incompatible options: "sign" and "forwarding" cannot be set together');
+  }
 
   switch (options.type) {
     case 'ecc':
@@ -383,7 +389,12 @@ export function isValidEncryptionKeyPacket(keyPacket, signature) {
 }
 
 export function isValidDecryptionKeyPacket(signature, config) {
-  if (config.allowInsecureDecryptionWithSigningKeys) {
+  const isSigningKey = !signature.keyFlags ||
+    (signature.keyFlags[0] & enums.keyFlags.sign) !== 0 ||
+    (signature.keyFlags[0] & enums.keyFlags.certifyKeys) !== 0 ||
+    (signature.keyFlags[0] & enums.keyFlags.authentication) !== 0;
+
+  if (isSigningKey && config.allowInsecureDecryptionWithSigningKeys) {
     // This is only relevant for RSA keys, all other signing algorithms cannot decrypt
     return true;
   }

test/general/forwarding.js

@@ -52,4 +52,50 @@ module.exports = () => describe('Forwarding', function() {
     const { data: expectedSerializedKey } = await openpgp.unarmor(charlieKeyArmored);
     expect(serializedKey).to.deep.equal(expectedSerializedKey);
   });
+
+  it('generates subkey with forwarding flag (0x40)', async function() {
+    const { privateKey: armoredKey } = await openpgp.generateKey({ userIDs: { email: '[email protected]' }, subkeys: [{ forwarding: true }, {}] });
+    const privateKey = await openpgp.readKey({ armoredKey });
+
+    expect(privateKey.subkeys[0].bindingSignatures[0].keyFlags[0]).to.equal(openpgp.enums.keyFlags.forwardedCommunication);
+    expect(privateKey.subkeys[1].bindingSignatures[0].keyFlags[0]).to.equal(openpgp.enums.keyFlags.encryptCommunication | openpgp.enums.keyFlags.encryptStorage);
+  });
+
+  it('reformatting a key preserves its forwarding flags (0x40)', async function() {
+    // two subkeys, the first with forwarding flag, the second with standard encryption ones
+    const privateKey = await openpgp.readKey({ armoredKey: `-----BEGIN PGP PRIVATE KEY BLOCK-----
+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+=eC8z
+-----END PGP PRIVATE KEY BLOCK-----` });
+
+    const { privateKey: reformattedKey } = await openpgp.reformatKey({ privateKey, userIDs: { email: '[email protected]' }, format: 'object' });
+
+    expect(reformattedKey.subkeys[0].bindingSignatures[0].keyFlags[0]).to.equal(openpgp.enums.keyFlags.forwardedCommunication);
+    expect(reformattedKey.subkeys[1].bindingSignatures[0].keyFlags[0]).to.equal(openpgp.enums.keyFlags.encryptCommunication | openpgp.enums.keyFlags.encryptStorage);
+  });
+
+  it('refuses to encrypt using encryption key with forwarding flag (0x40)', async function() {
+    const charlieKey = await openpgp.readKey({ armoredKey: charlieKeyArmored });
+
+    await expect(openpgp.encrypt({
+      message: await openpgp.createMessage({ text: 'abc' }),
+      encryptionKeys: charlieKey
+    })).to.be.rejectedWith(/Could not find valid encryption key packet/);
+  });
 });