C# Tutorial "Symmetric Cryptography" : Exercise: Decrypt

Symmetric Cryptography

Exercise: Decrypt

Please create function for decrypt message and output to Console plain text
using System;
using System.Linq;
using System.Text;

					
public class Program
{
	public const int BLOCK_SIZE = 4;
	
	public const UInt16 SECRET = 65535;
	
	public const int ROUNDS = 6;
	
	public static void Main()
	{
		var blocks = new uint[] {1074632793,2393460436,1820682509,53228064,3452521679,3660823688,1296079281,1025549298,19687968,721445875,3202418118,3721585831,78447188,1211095104,422738656,271553040,3111137266,152305785,239110881,3170940915,2657138408,1661319489,1817774531,195557908,1778915809,1447756419,2856347641,3380626671,1875413487,411273861,420747897,897633263,4210034662,2869831457,3532921957,67406433,329509508,420747897,897633263,3068331398,606405456,1035516916,3600889486};
		
		var key = "1234";
		
		var result = Feistel(blocks, key);

		var cipherText = Encoding.ASCII.GetString(result.SelectMany(r => BitConverter.GetBytes(r).Reverse()).ToArray());

		
		Console.Write(cipherText);
	}
	
	
	public static uint[] Feistel(uint[] plainText, string password)
	{
		var cipherBlocks = new uint[plainText.Count()];
		
		for (int i = 0; i < plainText.Count(); i++)
		{
			cipherBlocks[i] = F(plainText[i], password);
		}
		
		return cipherBlocks;
	}
	
	public static uint F(uint originalBlock, string key)
	{			
		var bytes = GetBytes(originalBlock);

		UInt16 leftPart = ToUInt16(bytes.Take(2).ToArray());
		UInt16 rightPart = ToUInt16(bytes.Skip(2).Take(2).ToArray());
		
		var keyBytes = Encoding.ASCII.GetBytes(key);

		var key1 = ToUInt16(keyBytes.Take(2).ToArray().ToArray());
		var key2 = ToUInt16(keyBytes.Skip(2).Take(2).ToArray().ToArray());

		for (int i = 0; i < ROUNDS; i++)
		{
			var f = RoundF(leftPart,  i % 2 == 0 ? key1 : key2);

			UInt16 result = (UInt16) (f ^ rightPart);

			if (i < ROUNDS - 1)
			{
				rightPart = leftPart;
				leftPart = result;
			}
			else
			{
				rightPart = result;
			}
		}

		var cipherBytes = new byte[BLOCK_SIZE]
        					{
            	BitConverter.GetBytes(leftPart)[1], BitConverter.GetBytes(leftPart)[0],
				BitConverter.GetBytes(rightPart)[1],BitConverter.GetBytes(rightPart)[0]
				             };

		return ToUInt32(cipherBytes);
	}
	
	public static UInt16 RoundF(UInt16 originalBlock, UInt16 key)
	{
		UInt16 result = (UInt16) (originalBlock ^ key);

		result = (UInt16) (result << 5);
		return result;
	}
	
	
	public static UInt16 ToUInt16(byte[] bytes)
	{
		UInt16 result = 0;

		result += (ushort) (bytes[0]*256 + bytes[1]);

		return result;
	}

	public static UInt32 ToUInt32(byte[] bytes)
	{
		UInt32 result = 0;

		for (int i = 0; i < 4; i++)
		{
			result += (UInt32) Math.Pow(256, 3 - i)*bytes[i];
		}

		return result;
	}
	
	public static uint[] GetBlocks(string text)
	{
		var bytes = Encoding.ASCII.GetBytes(text);

		var blocksCount = (int)Math.Ceiling(bytes.Count() / (double)BLOCK_SIZE);

		var result = new uint[blocksCount];

		for (int i = 0; i < blocksCount; i++)
		{
			result[i] = ToUInt32(bytes.Skip(i * BLOCK_SIZE).Take(BLOCK_SIZE).ToArray());
		}

		return result;
	}
	
	public static byte[] GetBytes(UInt32 originalBlock)
		{
			return BitConverter.GetBytes(originalBlock).Reverse().ToArray();
		}
}