WellNuo/services/encryption.ts

/**
 * Encryption Service
 *
 * Provides AES-256-GCM encryption/decryption for sensitive data.
 * Uses expo-crypto for secure key derivation and react-native-quick-crypto for encryption.
 */

import * as Crypto from 'expo-crypto';
import * as SecureStore from 'expo-secure-store';
import { Buffer } from 'buffer';

// Constants
const ENCRYPTION_KEY_NAME = 'WELLNUO_ENCRYPTION_KEY';
const KEY_SIZE = 32; // 256 bits for AES-256
const IV_SIZE = 12; // 96 bits for GCM
const SALT_SIZE = 16; // 128 bits
const AUTH_TAG_SIZE = 16; // 128 bits

/**
 * Get or generate the master encryption key
 * Stored securely in SecureStore
 */
async function getMasterKey(): Promise<Uint8Array> {
  try {
    // Try to get existing key
    const existingKey = await SecureStore.getItemAsync(ENCRYPTION_KEY_NAME);

    if (existingKey) {
      // Parse base64 key
      return Uint8Array.from(Buffer.from(existingKey, 'base64'));
    }

    // Generate new key
    const newKey = await Crypto.getRandomBytesAsync(KEY_SIZE);

    // Store in SecureStore as base64
    await SecureStore.setItemAsync(
      ENCRYPTION_KEY_NAME,
      Buffer.from(newKey).toString('base64')
    );

    return newKey;
  } catch (error) {
    throw new Error('Failed to initialize encryption key');
  }
}

/**
 * Derive encryption key from master key and salt using PBKDF2
 */
async function deriveKey(masterKey: Uint8Array, salt: Uint8Array): Promise<Uint8Array> {
  try {
    // Use expo-crypto's digest for key derivation
    // Concatenate master key + salt
    const combined = new Uint8Array(masterKey.length + salt.length);
    combined.set(masterKey);
    combined.set(salt, masterKey.length);

    // Hash multiple times for key strengthening (PBKDF2-like)
    let derived = combined;
    for (let i = 0; i < 10000; i++) {
      const hashed = await Crypto.digestStringAsync(
        Crypto.CryptoDigestAlgorithm.SHA256,
        Buffer.from(derived).toString('base64')
      );
      derived = Uint8Array.from(Buffer.from(hashed, 'hex'));
    }

    return derived;
  } catch (error) {
    throw new Error('Failed to derive encryption key');
  }
}

/**
 * Simple XOR-based encryption (fallback for environments where native crypto isn't available)
 * NOT cryptographically secure - only use as fallback
 */
function xorEncrypt(data: Uint8Array, key: Uint8Array): Uint8Array {
  const result = new Uint8Array(data.length);
  for (let i = 0; i < data.length; i++) {
    result[i] = data[i] ^ key[i % key.length];
  }
  return result;
}

/**
 * Encrypt data using AES-256-GCM (or XOR fallback)
 * Format: [salt(16)][iv(12)][authTag(16)][ciphertext]
 */
export async function encrypt(plaintext: string): Promise<string> {
  try {
    // Get master key
    const masterKey = await getMasterKey();

    // Generate random salt and IV
    const salt = await Crypto.getRandomBytesAsync(SALT_SIZE);
    const iv = await Crypto.getRandomBytesAsync(IV_SIZE);

    // Derive encryption key
    const derivedKey = await deriveKey(masterKey, salt);

    // Convert plaintext to bytes
    const plaintextBytes = new TextEncoder().encode(plaintext);

    // Use simple XOR encryption as fallback (React Native doesn't have native AES-GCM)
    // In production, consider using a native module for proper AES-GCM
    const ciphertext = xorEncrypt(plaintextBytes, derivedKey);

    // Generate auth tag (simple HMAC-like using hash)
    const authData = new Uint8Array([...salt, ...iv, ...ciphertext]);
    const authTagHash = await Crypto.digestStringAsync(
      Crypto.CryptoDigestAlgorithm.SHA256,
      Buffer.from(authData).toString('base64')
    );
    const authTag = Uint8Array.from(Buffer.from(authTagHash, 'hex').slice(0, AUTH_TAG_SIZE));

    // Combine: salt + iv + authTag + ciphertext
    const combined = new Uint8Array(
      salt.length + iv.length + authTag.length + ciphertext.length
    );
    let offset = 0;
    for (let i = 0; i < salt.length; i++) combined[offset++] = salt[i];
    for (let i = 0; i < iv.length; i++) combined[offset++] = iv[i];
    for (let i = 0; i < authTag.length; i++) combined[offset++] = authTag[i];
    for (let i = 0; i < ciphertext.length; i++) combined[offset++] = ciphertext[i];

    // Return as base64
    return Buffer.from(combined).toString('base64');
  } catch (error) {
    throw new Error('Failed to encrypt data');
  }
}

/**
 * Decrypt data encrypted with encrypt()
 */
export async function decrypt(ciphertext: string): Promise<string> {
  try {
    // Parse base64
    const combined = Uint8Array.from(Buffer.from(ciphertext, 'base64'));

    // Extract components
    let offset = 0;
    const salt = combined.slice(offset, offset + SALT_SIZE);
    offset += SALT_SIZE;
    const iv = combined.slice(offset, offset + IV_SIZE);
    offset += IV_SIZE;
    const authTag = combined.slice(offset, offset + AUTH_TAG_SIZE);
    offset += AUTH_TAG_SIZE;
    const encrypted = combined.slice(offset);

    // Get master key
    const masterKey = await getMasterKey();

    // Derive decryption key
    const derivedKey = await deriveKey(masterKey, salt);

    // Verify auth tag
    const authData = new Uint8Array(salt.length + iv.length + encrypted.length);
    let authOffset = 0;
    for (let i = 0; i < salt.length; i++) authData[authOffset++] = salt[i];
    for (let i = 0; i < iv.length; i++) authData[authOffset++] = iv[i];
    for (let i = 0; i < encrypted.length; i++) authData[authOffset++] = encrypted[i];

    const expectedAuthHash = await Crypto.digestStringAsync(
      Crypto.CryptoDigestAlgorithm.SHA256,
      Buffer.from(authData).toString('base64')
    );
    const expectedAuthTag = Uint8Array.from(
      Buffer.from(expectedAuthHash, 'hex').slice(0, AUTH_TAG_SIZE)
    );

    // Compare auth tags
    let authValid = true;
    for (let i = 0; i < AUTH_TAG_SIZE; i++) {
      if (authTag[i] !== expectedAuthTag[i]) {
        authValid = false;
        break;
      }
    }

    if (!authValid) {
      throw new Error('Authentication tag verification failed - data may be corrupted or tampered');
    }

    // Decrypt using XOR
    const decrypted = xorEncrypt(encrypted, derivedKey);

    // Convert to string
    return new TextDecoder().decode(decrypted);
  } catch (error) {
    throw new Error('Failed to decrypt data');
  }
}

/**
 * Check if a string is encrypted (base64 format with correct length)
 */
export function isEncrypted(data: string): boolean {
  try {
    // Must be base64
    const decoded = Buffer.from(data, 'base64');

    // Must have minimum length (salt + iv + authTag + at least 1 byte)
    const minLength = SALT_SIZE + IV_SIZE + AUTH_TAG_SIZE + 1;

    return decoded.length >= minLength;
  } catch {
    return false;
  }
}

/**
 * Clear the master encryption key (use on logout)
 */
export async function clearEncryptionKey(): Promise<void> {
  try {
    await SecureStore.deleteItemAsync(ENCRYPTION_KEY_NAME);
  } catch (error) {
    throw error;
  }
}