import { createCipheriv, createDecipheriv, createHash, randomBytes } from 'node:crypto'
import { readFileSync } from 'node:fs'

const ALGORITHM = 'aes-256-gcm'
const IV_BYTES = 12
const TAG_BYTES = 16

/**
 * The key of the secrets: the SHA-256 of ENCRYPTION_KEY, else of the content of the file ENCRYPTION_KEY_FILE names
 * without its final newline; null when neither is set. A file that cannot be read throws with the reason.
 */
export function encryptionKey(): Buffer | null {
  let secret = process.env.ENCRYPTION_KEY
  if (!secret && process.env.ENCRYPTION_KEY_FILE) {
    try {
      secret = readFileSync(process.env.ENCRYPTION_KEY_FILE, 'utf8').replace(/\r?\n$/, '')
    } catch (error) {
      throw new Error(`ENCRYPTION_KEY_FILE ${process.env.ENCRYPTION_KEY_FILE} cannot be read: ${(error as Error).message}`)
    }
  }
  return secret ? createHash('sha256').update(secret, 'utf8').digest() : null
}

/** The text encrypted with AES-256-GCM: base64 of a random IV, the ciphertext and the authentication tag. */
export function encrypt(text: string, key: Buffer): string {
  const iv = randomBytes(IV_BYTES)
  const cipher = createCipheriv(ALGORITHM, key, iv)
  const data = Buffer.concat([cipher.update(text, 'utf8'), cipher.final()])
  return Buffer.concat([iv, data, cipher.getAuthTag()]).toString('base64')
}

/** The text of encrypt's answer; another key, or a changed answer, throws. */
export function decrypt(encrypted: string, key: Buffer): string {
  const bytes = Buffer.from(encrypted, 'base64')
  const decipher = createDecipheriv(ALGORITHM, key, bytes.subarray(0, IV_BYTES))
  decipher.setAuthTag(bytes.subarray(bytes.length - TAG_BYTES))
  return Buffer.concat([decipher.update(bytes.subarray(IV_BYTES, bytes.length - TAG_BYTES)), decipher.final()]).toString('utf8')
}