import crypto from 'crypto';

const ALGORITHM = 'aes-256-gcm';
const IV_LENGTH = 16;
const SALT_LENGTH = 16;
const AUTH_TAG_LENGTH = 16;
const KEY_LENGTH = 32;

function deriveKey(secret: string, salt: Buffer): Buffer {
  return crypto.scryptSync(secret, salt, KEY_LENGTH);
}

function getSecret(): string {
  const secret = process.env.SESSION_SECRET;
  if (!secret) {
    throw new Error('SESSION_SECRET environment variable is required for encryption');
  }
  return secret;
}

export function encrypt(text: string): string {
  if (!text) return text;

  const secret = getSecret();
  const salt = crypto.randomBytes(SALT_LENGTH);
  const key = deriveKey(secret, salt);
  const iv = crypto.randomBytes(IV_LENGTH);
  const cipher = crypto.createCipheriv(ALGORITHM, key, iv);

  let encrypted = cipher.update(text, 'utf8', 'hex');
  encrypted += cipher.final('hex');
  const authTag = cipher.getAuthTag();

  // Format: salt:iv:authTag:ciphertext (4 parts — v2)
  return `${salt.toString('hex')}:${iv.toString('hex')}:${authTag.toString('hex')}:${encrypted}`;
}

export function decrypt(encryptedText: string): string {
  if (!encryptedText || !encryptedText.includes(':')) return encryptedText;

  try {
    const secret = getSecret();
    const parts = encryptedText.split(':');

    if (parts.length === 4) {
      // v2 format: salt:iv:authTag:ciphertext
      const salt = Buffer.from(parts[0], 'hex');
      const iv = Buffer.from(parts[1], 'hex');
      const authTag = Buffer.from(parts[2], 'hex');
      const encrypted = parts[3];

      const key = deriveKey(secret, salt);
      const decipher = crypto.createDecipheriv(ALGORITHM, key, iv, { authTagLength: AUTH_TAG_LENGTH });
      decipher.setAuthTag(authTag);

      let decrypted = decipher.update(encrypted, 'hex', 'utf8');
      decrypted += decipher.final('utf8');
      return decrypted;
    }

    if (parts.length === 3) {
      // v1 format: iv:authTag:ciphertext (fixed salt 'salt')
      const key = crypto.scryptSync(secret, 'salt', KEY_LENGTH);
      const iv = Buffer.from(parts[0], 'hex');
      const authTag = Buffer.from(parts[1], 'hex');
      const encrypted = parts[2];

      const decipher = crypto.createDecipheriv(ALGORITHM, key, iv, { authTagLength: AUTH_TAG_LENGTH });
      decipher.setAuthTag(authTag);

      let decrypted = decipher.update(encrypted, 'hex', 'utf8');
      decrypted += decipher.final('utf8');
      return decrypted;
    }

    return encryptedText;
  } catch (error) {
    console.error('Decryption failed:', error);
    return encryptedText;
  }
}