using Sarand.Net.Standard.Base.Core;
using Sarand.Net.Standard.Base.Utils;
using System;
using System.Collections.Generic;
using System.Security.Cryptography;
using System.Text;

namespace Sarand.Net.Standard.Base.Helpers
{
	//=======================================================================
	public static class SacsEncryptionHelper
	{
		//-------------------------------------------------------------------
		/// <summary>Number of byes in key. [32 bytes = 256 bits]</summary>
		public static int KeyLength { get; } = 32;
		/// <summary>Default encoding: UTF8.</summary>
		public static Encoding DefaultEncoding { get; } = Encoding.UTF8;

		//-------------------------------------------------------------------
		// Symmetric Encryption in .NET Core
		// System.Security.Cryptography implements and provides the APIs for encryption in .NET Core.
		// Ref. https://damienbod.com/2020/08/19/symmetric-and-asymmetric-encryption-in-net-core/
		//-------------------------------------------------------------------
		public static bool IsBase64(String value, int expectedLength = 0)
		{
			try
			{
				Byte[] decoded = Convert.FromBase64String(value);
				if (expectedLength > 0 && decoded.Length != expectedLength)
					return false;
			}
			catch (Exception) { return false; }
			return true;
		}
		public static String EncodePlainKey(String plainKey, int keyLength = 32, Encoding encoding = null)
		{
			if (StringUtil.IsEmpty(plainKey))
				throw new NotSupportedException("Plain key must not be null/blank");
			// Ensure key if 32 bytes long.
			while (StringUtil.GetLength(plainKey) < keyLength)
				plainKey += plainKey;
			plainKey = StringUtil.Left(plainKey, keyLength);

			// Encode to base-64.
			encoding = encoding ?? DefaultEncoding;
			return Convert.ToBase64String(encoding.GetBytes(plainKey));
		}

		//-------------------------------------------------------------------
		// Generate random key and IV.
		// NB Can use own key if suitable - 32 character value in base-64 format.
		public static String InitSymmetricEncryptionKey()
		{
			var key = GetEncodedRandomString(KeyLength); // 256 bits.
			return key;
		}
		public static string InitSymmetricEncryptionIV(String key)
		{
			Aes cipher = CreateCipher(key);
			var IVBase64 = Convert.ToBase64String(cipher.IV);
			return IVBase64;
		}
		public static (string Key, string IVBase64) InitSymmetricEncryptionKeyIV()
		{
			var key = InitSymmetricEncryptionKey();
			var IVBase64 = InitSymmetricEncryptionIV(key);
			return (key, IVBase64);
		}

		//-------------------------------------------------------------------
		public static string GetEncodedRandomString(int length)
		{
			var base64 = Convert.ToBase64String(GenerateRandomBytes(length));
			return base64;
		}

		private static Aes CreateCipher(string keyBase64)
		{
			// Default values: Keysize 256 bits, Padding PKC27.
			Aes cipher = Aes.Create();
			// Ensure the integrity of the ciphertext if using CBC
			cipher.Mode = CipherMode.CBC;

			cipher.Padding = PaddingMode.ISO10126;
			byte[] rawKey = Convert.FromBase64String(keyBase64);
			String rawKeyValue = System.Text.Encoding.Default.GetString(rawKey);
			cipher.Key = rawKey;

			return cipher;
		}

		private static byte[] GenerateRandomBytes(int length)
		{
			var byteArray = new byte[length];
			SacsRandomHelper.Fill(byteArray);
			return byteArray;
		}

		//-------------------------------------------------------------------
		public static string Encrypt(string text, string IV, string key)
		{
			Aes cipher = CreateCipher(key);
			cipher.IV = Convert.FromBase64String(IV);

			ICryptoTransform cryptTransform = cipher.CreateEncryptor();
			byte[] plaintext = Encoding.UTF8.GetBytes(text);
			byte[] cipherText = cryptTransform.TransformFinalBlock(plaintext, 0, plaintext.Length);

			return Convert.ToBase64String(cipherText);
		}

		//-------------------------------------------------------------------
		public static string Decrypt(string encryptedText, string IV, string key)
		{
			Aes cipher = CreateCipher(key);
			cipher.IV = Convert.FromBase64String(IV);

			ICryptoTransform cryptTransform = cipher.CreateDecryptor();
			byte[] encryptedBytes = Convert.FromBase64String(encryptedText);
			byte[] plainBytes = cryptTransform.TransformFinalBlock(encryptedBytes, 0, encryptedBytes.Length);

			return Encoding.UTF8.GetString(plainBytes);
		}

		//-------------------------------------------------------------------
	}
	//=======================================================================
}