feat: Add AES-256-GCM encrypted cookie support

[brunorodmoreira]

Feature Request

Summary

The current cookie helper only supports signed cookies (HMAC-SHA256), which provides integrity but not confidentiality — the cookie value remains readable in plaintext.

This request adds encrypted cookie support using AES-256-GCM via the Web Crypto API, providing both confidentiality and integrity for cookie values.

Motivation

• Signed cookies (setSignedCookie) prove the value wasn't tampered with, but the value is still visible (e.g., session=user-id-123.signature).
• Encrypted cookies make the value opaque — useful for storing sensitive data like session tokens, user preferences, or any data that should not be readable client-side.
• AES-256-GCM is the standard choice: authenticated encryption with a single pass.

Proposed API

Mirror the existing signed cookie API:

import {
  setEncryptedCookie,
  getEncryptedCookie,
  generateEncryptedCookie,
} from 'hono/cookie'

// Set
await setEncryptedCookie(c, 'session', 'sensitive-data', secret)

// Get (returns string | undefined | false)
const value = await getEncryptedCookie(c, secret, 'session')

// Generate without setting on response
const cookie = await generateEncryptedCookie('session', 'sensitive-data', secret)

Design

• Algorithm: AES-256-GCM via crypto.subtle (multi-runtime: Node, Deno, Bun, CF Workers)
• Key derivation: HKDF-SHA256 from string | BufferSource secret (same type as signed cookies)
• IV: Random 12 bytes per encryption via crypto.getRandomValues()
• AAD: Cookie name bound as additional authenticated data (prevents copying encrypted values between cookie names)
• Cookie format: base64(iv || ciphertext || authTag), URL-encoded
• Prefix support: Full __Secure- / __Host- prefix support
• Return semantics: false on decryption failure (tampered/wrong key), same as getSignedCookie

Checklist

• API mirrors existing signed cookie pattern
• Uses Web Crypto API for multi-runtime compatibility
• Supports cookie prefixes (secure, host)
• All standard cookie options (domain, path, maxAge, expires, httpOnly, sameSite, etc.)