using System;

using System.IO;

using System.Linq;

using System.Security.Cryptography;

using Snail.Common.Extensions;



namespace Snail.Common.Security

{

    public class EncryptionService:IEncryptionService

    {

        public byte[] Decode(byte[] encodedData)

        {

            using (var symmetricAlgorithm = CreateSymmetricAlgorithm())

            {

                using (var hashAlgorithm = CreateHashAlgorithm())

                {

                    var iv = new byte[symmetricAlgorithm.BlockSize / 8];

                    var signature = new byte[hashAlgorithm.HashSize / 8];

                    var data = new byte[encodedData.Length - iv.Length - signature.Length];



                    Array.Copy(encodedData, 0, iv, 0, iv.Length);

                    Array.Copy(encodedData, iv.Length, data, 0, data.Length);

                    Array.Copy(encodedData, iv.Length + data.Length, signature, 0, signature.Length);



                    // validate the signature

                    var mac = hashAlgorithm.ComputeHash(iv.Concat(data).ToArray());



                    if (!mac.SequenceEqual(signature))

                    {

                        // message has been tampered

                        throw new ArgumentException();

                    }



                    symmetricAlgorithm.IV = iv;



                    using (var ms = new M