accounted/lib/auth/oauth-codes.ts

import crypto from 'crypto'

/**
 * Stateless OAuth auth codes.
 * The auth code is an AES-256-GCM encrypted JSON payload containing
 * the user ID, PKCE code_challenge, and expiry.
 *
 * The API key is NOT embedded — it gets created at token exchange
 * after PKCE verification, preventing orphaned keys.
 */

const ALGORITHM = 'aes-256-gcm'
const CODE_TTL_MS = 5 * 60 * 1000 // 5 minutes

function getEncryptionKey(): Buffer {
  const secret = process.env.SUPABASE_SERVICE_ROLE_KEY
  if (!secret) throw new Error('SUPABASE_SERVICE_ROLE_KEY is required')
  return crypto.createHash('sha256').update(secret).digest()
}

export interface AuthCodePayload {
  userId: string
  codeChallenge: string
  redirectUri: string
  exp: number
}

export function createAuthCode(payload: Omit<AuthCodePayload, 'exp'>): string {
  const data: AuthCodePayload = {
    ...payload,
    exp: Date.now() + CODE_TTL_MS,
  }

  const key = getEncryptionKey()
  const iv = crypto.randomBytes(12)
  const cipher = crypto.createCipheriv(ALGORITHM, key, iv)

  const json = JSON.stringify(data)
  const encrypted = Buffer.concat([cipher.update(json, 'utf8'), cipher.final()])
  const tag = cipher.getAuthTag()

  const combined = Buffer.concat([iv, tag, encrypted])
  return combined.toString('base64url')
}

export function decryptAuthCode(code: string): AuthCodePayload | null {
  try {
    const key = getEncryptionKey()
    const combined = Buffer.from(code, 'base64url')

    const iv = combined.subarray(0, 12)
    const tag = combined.subarray(12, 28)
    const encrypted = combined.subarray(28)

    const decipher = crypto.createDecipheriv(ALGORITHM, key, iv)
    decipher.setAuthTag(tag)

    const decrypted = Buffer.concat([decipher.update(encrypted), decipher.final()])
    const payload: AuthCodePayload = JSON.parse(decrypted.toString('utf8'))

    if (Date.now() > payload.exp) return null

    return payload
  } catch {
    return null
  }
}

/**
 * Verify PKCE: SHA256(code_verifier) must equal the stored code_challenge.
 * Only S256 is supported (plain is insecure and not advertised).
 */
export function verifyPkce(
  codeVerifier: string,
  codeChallenge: string
): boolean {
  const hash = crypto
    .createHash('sha256')
    .update(codeVerifier)
    .digest('base64url')
  return hash === codeChallenge
}

/**
 * Hash an auth code for replay tracking.
 */
export function hashAuthCode(code: string): string {
  return crypto.createHash('sha256').update(code).digest('hex')
}