CryptoKit

CryptoKit provides end-to-end AES-128-GCM encryption for HTTP request/response bodies. It protects sensitive API calls between the browser and Traveloka's backend services. A companion middleware integrates it into the FetchInterceptor pipeline.

Components

Component	File	Purpose
CryptoKit	crypto-kit.ts	Static encryption/decryption utilities
createCryptoKitMiddleware	crypto-kit-middleware.ts	FetchInterceptor middleware factory

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CryptoKit Class

All methods are static — no instantiation required.

Encryption Parameters

Parameter	Value
Algorithm	AES-GCM
Key length	128 bits
IV length	12 bytes
Compression	deflate (pako)
Key derivation	SHA-256 digest of the secret string

Secret Construction

CryptoKit.buildSecret(secretArr: Array<string>): string
// Joins array elements with ':::'
// e.g. ['desktop', 'fsp', '/api/foo', 'device123'] → 'desktop:::fsp:::/api/foo:::device123'

The secret components are derived from request metadata:

• x-client-interface header
• x-domain header
• Request URL
• x-did (device ID, if present)

Request Encryption (handleRequest)

plaintext = `${timestamp}###${requestBody}`
    → TextEncoder.encode
    → deflate (pako compression)
    → AES-GCM encrypt (random 12-byte IV)

Wire format: [IV (12 bytes)][ciphertext]

The timestamp prefix provides replay protection.

Modified headers:

• x-ck-id: 1.0.0 (version marker)
• Content-Type: application/octet-stream

Response Decryption (handleResponse)

Wire format: [IV (12 bytes)][ciphertext]
    → AES-GCM decrypt
    → inflate (pako decompression)
    → TextDecoder.decode → plaintext string

Modified headers:
    → Remove x-ck-id
    → Content-Type: text/plain

Protection Support Check

CryptoKit.isProtectionSupported() verifies all required browser APIs:

• TextEncoder / TextDecoder
• crypto.subtle.encrypt / crypto.subtle.decrypt
• crypto.getRandomValues

Returns false (and logs cryptokit.protection_unavailable) if any are missing. This check gates the entire Sentinel initialisation.

Route-Level Encryption

shouldEncrypt({ url, headers }) determines if a specific request should be encrypted based on a static route map:

protectedRoutes = {
  fsp: [/.*\/dfp\/.*/], // fingerprint-related endpoints
};

In development environments, encryption is disabled by default but can be toggled via localStorage (lsCmnCryptoKitEnabled = '1').

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CryptoKit Middleware

createCryptoKitMiddleware() returns a FetchInterceptorMiddleware with both onRequest and onResponse handlers.

Request Phase

Encrypts the request body when either:

• shouldProtectResource is true (Sentinel config-driven), OR
• CryptoKit.shouldEncrypt() is true (static route map)

The encryption secret is computed from request headers and stored in context.state.requestSecret for response decryption.

Response Phase

Decrypts the response when:

• requestSecret exists in context state
• Response headers contain x-ck-id: 1.0.0 and Content-Type: application/octet-stream

A new Response object is constructed with the decrypted body and cleaned headers.

Development Bypass

In non-production environments, encryption can be disabled by setting localStorage key lsCmnCryptoKitDisabled to '1'. This aids debugging and local development.

Error Handling

Log Key	Scenario
cryptokit.protection_unavailable	Browser doesn't support required crypto APIs
cryptokit.handle_request_error	Encryption failed (thrown as Error('Invalid request'))
cryptokit.handle_response_error	Decryption failed (thrown as Error('Invalid response'))

Security Considerations

• Keys are derived per-request from contextual data (headers + URL + device ID), providing request isolation.
• The IV is generated via crypto.getRandomValues ensuring non-deterministic ciphertext.
• The timestamp embedded in the plaintext enables server-side replay detection.
• AES-GCM provides both confidentiality and authenticity (integrity via the authentication tag).
• Compression before encryption (deflate → encrypt) reduces ciphertext size without leaking plaintext structure in this context.

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Last updated: 2026-02-24