using System;
using System.IO;
using System.Linq;
using System.Security.Cryptography;
using System.Text;

namespace BinaryDad.Extensions
{
    /// <summary>
    /// Set of cryptography helpers using AES and Rijndael cipher
    /// </summary>
    public static class CryptoHelper
    {
        // This constant is used to determine the keysize of the encryption algorithm in bits.
        // We divide this by 8 within the code below to get the equivalent number of bytes.
        private const int DefaultKeySize = 256;

        // This constant determines the number of iterations for the password bytes generation function.
        private const int DerivationIterations = 1000;

        #region Encrypt

        /// <summary>
        /// Encrypts a string with pass key using AES (Rijndael cipher)
        /// </summary>
        /// <param name="plainText"></param>
        /// <param name="passPhrase"></param>
        /// <returns></returns>
        public static string Encrypt(string plainText, string passPhrase)
        {
            return Encrypt(plainText, passPhrase, DefaultKeySize);
        }

        /// <summary>
        /// Encrypts a string with pass key using AES (Rijndael cipher)
        /// </summary>
        /// <param name="plainText"></param>
        /// <param name="passPhrase"></param>
        /// <param name="keySize"></param>
        /// <returns></returns>
        public static string Encrypt(string plainText, string passPhrase, int keySize)
        {
            ValidateKeySize(keySize);

            // Salt and IV is randomly generated each time, but is preprended to encrypted cipher text
            // so that the same Salt and IV values can be used when decrypting.  
            var saltStringBytes = GenerateBitsOfRandomEntropy(keySize);
            var ivStringBytes = GenerateBitsOfRandomEntropy(keySize);
            var plainTextBytes = Encoding.UTF8.GetBytes(plainText);

            using (var password = new Rfc2898DeriveBytes(passPhrase, saltStringBytes, DerivationIterations))
            {
                var keyBytes = password.GetBytes(keySize / 8);

                using (var symmetricKey = new RijndaelManaged())
                {
                    symmetricKey.BlockSize = keySize;
                    symmetricKey.Mode = CipherMode.CBC;
                    symmetricKey.Padding = PaddingMode.PKCS7;

                    using (var encryptor = symmetricKey.CreateEncryptor(keyBytes, ivStringBytes))
                    {
                        using (var memoryStream = new MemoryStream())
                        {
                            using (var cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write))
                            {
                                cryptoStream.Write(plainTextBytes, 0, plainTextBytes.Length);
                                cryptoStream.FlushFinalBlock();

                                // Create the final bytes as a concatenation of the random salt bytes, the random iv bytes and the cipher bytes.
                                var cipherTextBytes = saltStringBytes;

                                cipherTextBytes = cipherTextBytes.Concat(ivStringBytes).ToArray();
                                cipherTextBytes = cipherTextBytes.Concat(memoryStream.ToArray()).ToArray();

                                memoryStream.Close();
                                cryptoStream.Close();

                                return Convert.ToBase64String(cipherTextBytes);
                            }
                        }
                    }
                }
            }
        }

        #endregion

        #region Decrypt

        /// <summary>
        /// Decrypts a string with pass key using AES (Rijndael cipher)
        /// </summary>
        /// <param name="cipherText"></param>
        /// <param name="passPhrase"></param>
        /// <returns></returns>
        public static string Decrypt(string cipherText, string passPhrase)
        {
            return Decrypt(cipherText, passPhrase, DefaultKeySize);
        }

        /// <summary>
        /// Decrypts a string with pass key using AES (Rijndael cipher)
        /// </summary>
        /// <param name="cipherText"></param>
        /// <param name="passPhrase"></param>
        /// <param name="keySize"></param>
        /// <returns></returns>
        public static string Decrypt(string cipherText, string passPhrase, int keySize)
        {
            ValidateKeySize(keySize);

            // Get the complete stream of bytes that represent:
            // [32 bytes of Salt] + [32 bytes of IV] + [n bytes of CipherText]
            var cipherTextBytesWithSaltAndIv = Convert.FromBase64String(cipherText);

            // Get the saltbytes by extracting the first 32 bytes from the supplied cipherText bytes.
            var saltStringBytes = cipherTextBytesWithSaltAndIv.Take(keySize / 8).ToArray();

            // Get the IV bytes by extracting the next 32 bytes from the supplied cipherText bytes.
            var ivStringBytes = cipherTextBytesWithSaltAndIv.Skip(keySize / 8).Take(keySize / 8).ToArray();

            // Get the actual cipher text bytes by removing the first 64 bytes from the cipherText string.
            var cipherTextBytes = cipherTextBytesWithSaltAndIv.Skip((keySize / 8) * 2).Take(cipherTextBytesWithSaltAndIv.Length - ((keySize / 8) * 2)).ToArray();

            using (var password = new Rfc2898DeriveBytes(passPhrase, saltStringBytes, DerivationIterations))
            {
                var keyBytes = password.GetBytes(keySize / 8);

                using (var symmetricKey = new RijndaelManaged())
                {
                    symmetricKey.BlockSize = keySize;
                    symmetricKey.Mode = CipherMode.CBC;
                    symmetricKey.Padding = PaddingMode.PKCS7;

                    using (var decryptor = symmetricKey.CreateDecryptor(keyBytes, ivStringBytes))
                    {
                        using (var memoryStream = new MemoryStream(cipherTextBytes))
                        {
                            using (var cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read))
                            {
                                var plainTextBytes = new byte[cipherTextBytes.Length];
                                var decryptedByteCount = cryptoStream.Read(plainTextBytes, 0, plainTextBytes.Length);

                                memoryStream.Close();
                                cryptoStream.Close();

                                return Encoding.UTF8.GetString(plainTextBytes, 0, decryptedByteCount);
                            }
                        }
                    }
                }
            }
        }

        #endregion

        #region Private Methods

        private static byte[] GenerateBitsOfRandomEntropy(int keySize)
        {
            var randomBytes = new byte[keySize / 8];

            using (var rngCsp = new RNGCryptoServiceProvider())
            {
                // Fill the array with cryptographically secure random bytes.
                rngCsp.GetBytes(randomBytes);
            }

            return randomBytes;
        }

        private static void ValidateKeySize(int keySize)
        {
            if (keySize % 8 != 0)
            {
                throw new ArgumentException("Key size must be a multiple of 8", nameof(keySize));
            }
        }

        #endregion
    }
}