import crypto from "crypto"

// AES-256-GCM encryption for secrets at rest (OAuth tokens). The key is derived
// from ACCOUNTING_ENCRYPTION_KEY, falling back to BETTER_AUTH_SECRET, via SHA-256
// so no additional configuration is required.
let warnedAboutFallbackKey = false

function getKey(): Buffer {
  const dedicated = process.env.ACCOUNTING_ENCRYPTION_KEY
  const secret = dedicated || process.env.BETTER_AUTH_SECRET
  if (!secret) throw new Error("No encryption key configured (BETTER_AUTH_SECRET)")

  // Riding on BETTER_AUTH_SECRET works, but couples two independent rotation
  // schedules: rotating the auth secret would silently make every stored OAuth
  // token undecryptable, with no migration path and no error until a user's
  // next accounting sync fails. Warn once so this is caught before that
  // happens rather than after.
  if (!dedicated && !warnedAboutFallbackKey) {
    warnedAboutFallbackKey = true
    console.warn(
      "[crypto] ACCOUNTING_ENCRYPTION_KEY is not set — deriving the at-rest key " +
        "from BETTER_AUTH_SECRET. Rotating BETTER_AUTH_SECRET will make all " +
        "stored OAuth tokens undecryptable. Set a dedicated key in production."
    )
  }

  return crypto.createHash("sha256").update(secret).digest()
}

/** Encrypt a UTF-8 string → "iv:tag:ciphertext" (all base64). */
export function encrypt(plaintext: string): string {
  const iv = crypto.randomBytes(12)
  const cipher = crypto.createCipheriv("aes-256-gcm", getKey(), iv)
  const enc = Buffer.concat([cipher.update(plaintext, "utf8"), cipher.final()])
  const tag = cipher.getAuthTag()
  return [iv.toString("base64"), tag.toString("base64"), enc.toString("base64")].join(":")
}

/** Decrypt a value produced by encrypt(). */
export function decrypt(payload: string): string {
  const [ivB64, tagB64, dataB64] = payload.split(":")
  const decipher = crypto.createDecipheriv("aes-256-gcm", getKey(), Buffer.from(ivB64, "base64"))
  decipher.setAuthTag(Buffer.from(tagB64, "base64"))
  return Buffer.concat([decipher.update(Buffer.from(dataB64, "base64")), decipher.final()]).toString("utf8")
}