fix(core): enforce explicit tag length and validation in file keychain

Author: luisfelipe-alt

packages/core/src/services/fileKeychain.ts

@@ -27,8 +27,15 @@ export class FileKeychain implements Keychain {
   }
 
   private encrypt(text: string): string {
-    const iv = crypto.randomBytes(16);
-    const cipher = crypto.createCipheriv('aes-256-gcm', this.encryptionKey, iv);
+    const iv = crypto.randomBytes(12);
+    const cipher = crypto.createCipheriv(
+      'aes-256-gcm',
+      this.encryptionKey,
+      iv,
+      {
+        authTagLength: 16,
+      },
+    );
 
     let encrypted = cipher.update(text, 'utf8', 'hex');
     encrypted += cipher.final('hex');
@@ -48,10 +55,19 @@ export class FileKeychain implements Keychain {
     const authTag = Buffer.from(parts[1], 'hex');
     const encrypted = parts[2];
 
+    if (iv.length !== 12 && iv.length !== 16) {
+      throw new Error('Invalid IV length: Must be 12 or 16 bytes');
+    }
+
+    if (authTag.length !== 16) {
+      throw new Error('Invalid authentication tag length: Must be 16 bytes');
+    }
+
     const decipher = crypto.createDecipheriv(
       'aes-256-gcm',
       this.encryptionKey,
       iv,
+      { authTagLength: 16 },
     );
     decipher.setAuthTag(authTag);
 
@@ -67,30 +83,28 @@ export class FileKeychain implements Keychain {
   }
 
   private async loadData(): Promise<Record<string, Record<string, string>>> {
+    let data: string;
     try {
-      const data = await fs.readFile(this.tokenFilePath, 'utf-8');
-      const decrypted = this.decrypt(data);
-      // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
-      return JSON.parse(decrypted) as Record<string, Record<string, string>>;
+      data = await fs.readFile(this.tokenFilePath, 'utf-8');
     } catch (error: unknown) {
       // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
-      const err = error as NodeJS.ErrnoException & { message?: string };
+      const err = error as NodeJS.ErrnoException;
       if (err.code === 'ENOENT') {
         return {};
       }
-      if (
-        err.message?.includes('Invalid encrypted data format') ||
-        err.message?.includes(
-          'Unsupported state or unable to authenticate data',
-        )
-      ) {
-        throw new Error(
-          `Corrupted credentials file detected at: ${this.tokenFilePath}\n` +
-            `Please delete or rename this file to resolve the issue.`,
-        );
-      }
       throw error;
     }
+
+    try {
+      const decrypted = this.decrypt(data);
+      // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
+      return JSON.parse(decrypted) as Record<string, Record<string, string>>;
+    } catch {
+      throw new Error(
+        `Corrupted credentials file detected at: ${this.tokenFilePath}\n` +
+          `Please delete or rename this file to resolve the issue.`,
+      );
+    }
   }
 
   private async saveData(

packages/core/src/services/gcmTruncationReproduction.test.ts

@@ -0,0 +1,201 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
+import { promises as fs } from 'node:fs';
+import * as path from 'node:path';
+import * as os from 'node:os';
+import * as crypto from 'node:crypto';
+import { FileKeychain } from './fileKeychain.js';
+
+describe('AES-GCM Tag Length Verification', () => {
+  let tempDir: string;
+
+  beforeEach(async () => {
+    // Create a unique temporary directory for test isolation
+    tempDir = await fs.mkdtemp(path.join(os.tmpdir(), 'gemini-test-keychain-'));
+    vi.stubEnv('GEMINI_CLI_HOME', tempDir);
+  });
+
+  afterEach(async () => {
+    vi.unstubAllEnvs();
+    // Clean up the temporary directory
+    await fs.rm(tempDir, { recursive: true, force: true });
+  });
+
+  it('should use a secure 128-bit (16-byte) AES-GCM authentication tag and standard 12-byte IV', async () => {
+    const keychain = new FileKeychain();
+    const service = 'test-service';
+    const account = 'test-account';
+    const password = 'secure-password-123';
+
+    // 1. Save credentials to trigger encryption and file write
+    await keychain.setPassword(service, account, password);
+
+    // 2. Read the raw encrypted file from disk
+    const credentialsFilePath = path.join(
+      tempDir,
+      '.gemini',
+      'gemini-credentials.json',
+    );
+    const rawEncryptedData = await fs.readFile(credentialsFilePath, 'utf-8');
+
+    // 3. Parse the encrypted data format (iv:authTag:encrypted)
+    const parts = rawEncryptedData.split(':');
+    expect(parts).toHaveLength(3);
+
+    const ivHex = parts[0];
+    const authTagHex = parts[1];
+
+    // 4. Verify the lengths of the components
+    const ivBuffer = Buffer.from(ivHex, 'hex');
+    const authTagBuffer = Buffer.from(authTagHex, 'hex');
+
+    // IV should be exactly 12 bytes (96 bits) by default
+    expect(ivBuffer.length).toBe(12);
+    expect(ivHex.length).toBe(24);
+
+    // Authentication Tag should be exactly 16 bytes (128 bits)
+    expect(authTagBuffer.length).toBe(16);
+    expect(authTagHex.length).toBe(32); // 32 hex characters
+
+    // Assert that the tag is NOT truncated to 4 bytes (32 bits)
+    expect(authTagBuffer.length).not.toBe(4);
+    expect(authTagHex.length).not.toBe(8); // 8 hex characters
+
+    // 5. Verify that decryption works correctly with the 16-byte tag
+    const decryptedPassword = await keychain.getPassword(service, account);
+    expect(decryptedPassword).toBe(password);
+  });
+
+  it('should support both 12-byte and 16-byte IVs for backward compatibility', async () => {
+    const keychain = new FileKeychain();
+    const service = 'test-service';
+    const account = 'test-account';
+    const password = 'secure-password-123';
+
+    // 1. Save credentials to trigger encryption and file write (generates 12-byte IV)
+    await keychain.setPassword(service, account, password);
+
+    // 2. Verify 12-byte IV decryption works
+    let decryptedPassword = await keychain.getPassword(service, account);
+    expect(decryptedPassword).toBe(password);
+
+    // 3. Manually simulate a legacy 16-byte IV credentials file
+    const credentialsFilePath = path.join(
+      tempDir,
+      '.gemini',
+      'gemini-credentials.json',
+    );
+    const legacyIv = crypto.randomBytes(16);
+    const encryptionKey = (keychain as unknown as { encryptionKey: Buffer })
+      .encryptionKey;
+    const cipher = crypto.createCipheriv(
+      'aes-256-gcm',
+      encryptionKey,
+      legacyIv,
+      {
+        authTagLength: 16,
+      },
+    );
+
+    let encrypted = cipher.update(
+      JSON.stringify({ [service]: { [account]: password } }),
+      'utf8',
+      'hex',
+    );
+    encrypted += cipher.final('hex');
+    const authTag = cipher.getAuthTag();
+
+    const legacyPayload =
+      legacyIv.toString('hex') +
+      ':' +
+      authTag.toString('hex') +
+      ':' +
+      encrypted;
+    await fs.writeFile(credentialsFilePath, legacyPayload, 'utf-8');
+
+    // 4. Verify 16-byte IV decryption works successfully (backward compatibility)
+    decryptedPassword = await keychain.getPassword(service, account);
+    expect(decryptedPassword).toBe(password);
+  });
+
+  it('should reject decryption of a credentials file with a truncated tag', async () => {
+    const keychain = new FileKeychain();
+    const service = 'test-service';
+    const account = 'test-account';
+    const password = 'secure-password-123';
+
+    // 1. Save credentials to trigger encryption and file write
+    await keychain.setPassword(service, account, password);
+
+    // 2. Read the raw encrypted file from disk
+    const credentialsFilePath = path.join(
+      tempDir,
+      '.gemini',
+      'gemini-credentials.json',
+    );
+    const rawEncryptedData = await fs.readFile(credentialsFilePath, 'utf-8');
+
+    // 3. Parse the encrypted data format (iv:authTag:encrypted)
+    const parts = rawEncryptedData.split(':');
+    expect(parts).toHaveLength(3);
+
+    const ivHex = parts[0];
+    const authTagHex = parts[1];
+    const encryptedHex = parts[2];
+
+    // 4. Create a truncated 4-byte tag (8 hex characters)
+    const truncatedTagHex = authTagHex.substring(0, 8);
+    const truncatedEncryptedData = `${ivHex}:${truncatedTagHex}:${encryptedHex}`;
+
+    // 5. Overwrite the credentials file with the truncated-tag payload
+    await fs.writeFile(credentialsFilePath, truncatedEncryptedData, 'utf-8');
+
+    // 6. Attempt to retrieve the password and verify it throws a clear, handled validation error
+    await expect(keychain.getPassword(service, account)).rejects.toThrow(
+      'Corrupted credentials file detected',
+    );
+  });
+
+  it('should reject decryption of a credentials file with a truncated IV', async () => {
+    const keychain = new FileKeychain();
+    const service = 'test-service';
+    const account = 'test-account';
+    const password = 'secure-password-123';
+
+    // 1. Save credentials to trigger encryption and file write
+    await keychain.setPassword(service, account, password);
+
+    // 2. Read the raw encrypted file from disk
+    const credentialsFilePath = path.join(
+      tempDir,
+      '.gemini',
+      'gemini-credentials.json',
+    );
+    const rawEncryptedData = await fs.readFile(credentialsFilePath, 'utf-8');
+
+    // 3. Parse the encrypted data format (iv:authTag:encrypted)
+    const parts = rawEncryptedData.split(':');
+    expect(parts).toHaveLength(3);
+
+    const ivHex = parts[0];
+    const authTagHex = parts[1];
+    const encryptedHex = parts[2];
+
+    // 4. Create a truncated 4-byte IV (8 hex characters)
+    const truncatedIvHex = ivHex.substring(0, 8);
+    const truncatedEncryptedData = `${truncatedIvHex}:${authTagHex}:${encryptedHex}`;
+
+    // 5. Overwrite the credentials file with the truncated-IV payload
+    await fs.writeFile(credentialsFilePath, truncatedEncryptedData, 'utf-8');
+
+    // 6. Attempt to retrieve the password and verify it throws a clear, handled validation error
+    await expect(keychain.getPassword(service, account)).rejects.toThrow(
+      'Corrupted credentials file detected',
+    );
+  });
+});