using System;
using System.Collections.Generic;
using System.Linq;
using System.Security.Cryptography;
using System.Text;
using System.Threading.Tasks;

namespace ServiceShared.Crypto
{
    public static class AES
    {
        private static byte[] MemoryMasterKey = null;

        /// <summary>
        /// Sets the memory master key
        /// </summary>
        /// <param name="key">master key that should be set in the memory</param>
        public static void SetMemoryMasterKey(byte[] deriveKey)
        {
            MemoryMasterKey = deriveKey;
        }

        /// <summary>
        /// Encrypts string with password
        /// </summary>
        /// <param name="value">string value that should be encrypted</param>
        /// <param name="deriveKey">shared deriveKey that should be used for encryption(max. 32 bytes)</param>
        /// <returns>returns encrypted base64 string</returns>
        public static string Encrypt(string value, byte[] deriveKey = null)
        {
            string result = null;

            try
            {
                if (deriveKey == null)
                {
                    deriveKey = MemoryMasterKey;
                }

                using (AesGcm aes = new AesGcm(deriveKey))
                {
                    byte[] nonce = new byte[AesGcm.NonceByteSizes.MaxSize];
                    RandomNumberGenerator.Fill(nonce);
                    byte[] plainText = Encoding.UTF8.GetBytes(value);
                    byte[] chiperBuffer = new byte[plainText.Length];
                    byte[] tag = new byte[AesGcm.TagByteSizes.MaxSize];
                    aes.Encrypt(nonce, plainText, chiperBuffer, tag);
                    byte[] resultBuffer = nonce.Concat(chiperBuffer).Concat(tag).ToArray();
                    result = Convert.ToBase64String(resultBuffer);
                }
            }
            catch
            {
                result = null;
            }

            return result;
        }

        /// <summary>
        /// Decrypts encrypted base64 string with password
        /// </summary>
        /// <param name="value">base64 string that should be decrypted</param>
        /// <param name="deriveKey">HKDF.DeriveKey of sharedSecret</param>
        /// <returns>returns plain string</returns>
        public static string Decrypt(string value, byte[] deriveKey = null)
        {
            string result = null;

            try
            {
                if (deriveKey == null)
                {
                    deriveKey = MemoryMasterKey;
                }

                if (!string.IsNullOrEmpty(value) && deriveKey != null && deriveKey.Length == 32)
                {
                    using (AesGcm aes = new AesGcm(deriveKey))
                    {
                        byte[] combined = Convert.FromBase64String(value);
                        byte[] tag = combined.Skip(combined.Length - AesGcm.TagByteSizes.MaxSize).ToArray();
                        byte[] nonce = combined.Take(AesGcm.NonceByteSizes.MaxSize).ToArray();
                        byte[] chiperText = combined.Skip(nonce.Length).Take(combined.Length - nonce.Length - tag.Length).ToArray();
                        byte[] plainTextBuffer = new byte[chiperText.Length];
                        aes.Decrypt(nonce, chiperText, tag, plainTextBuffer);
                        result = Encoding.UTF8.GetString(plainTextBuffer);
                    }
                }
            }
            catch
            {
                result = null;
            }

            return result;
        }

        /// <summary>
        /// Converts the text passed password to the 32 byte array
        /// </summary>
        /// <param name="password">Text based password</param>
        /// <returns></returns>
        public static byte[] GetKey(string password)
        {
            byte[] result = null;

            try
            {
                if (!string.IsNullOrEmpty(password))
                {
                    if (password.Length < 32)
                    {
                        password = password.PadRight(32, '@');
                    }
                    else if (password.Length > 32)
                    {
                        password = MD5.Encrypt(password);
                    }

                    result = Encoding.ASCII.GetBytes(password);
                }
            }
            catch (Exception ex)
            {
                Log.Critical(ex, "ServiceShared.Crypto.AES", "GetKey");
            }

            return result;
        }
    }
}