Decrypt RC4 values

Posted on 2012-12-24
Last Modified: 2013-01-10
I'm unable to decrypt RC4 encrypted querystring values from a .net web application.  I have the key and the encrypted text but I'm not able to get the original value.  The values are encoded before being sent in a querystring and I'm using URLDecode on the parameters before trying to decrypt.

When I attempt to decrypt, I get unreadable characters instead of the original text.  The data is being sent from a java application and I'm using an application to decrypt.

I'm using the attached sample code below to encrypt and decrypt data.  There is one function to encrypt and decrypt.

Does anyone know know how to get the original value from the encrypted text?  Could the encrypted text be in a different format?
using System;
using System.Text;
using Microsoft.VisualBasic;

namespace main
	public class rc4encrypt
		protected int[] sbox = new int[256];
		protected int[] key = new int[256];

		protected string plaintext, password;

		public string PlainText
			set { plaintext = value; }
			get { return plaintext; }

		public string Password
			set { password = value; }
			get { return password; }

		private void RC4Initialize(string strPwd)
			// Get the length of the password
			// Instead of Len(), we need to use the Length property
			// of the string
			int intLength = strPwd.Length;

			// Set up our for loop.  In C#, we need to change our syntax.

			// The first argument is the initializer.  Here we declare a
			// as an integer and set it equal to zero.

			// The second argument is expression that is used to test
			// for the loop termination.  Since our arrays have 256
			// elements and are always zero based, we need to loop as long
			// as a is less than or equal to 255.

			// The third argument is an iterator used to increment the
			// value of a by one each time through the loop.  Note that
			// we can use the ++ increment notation instead of a = a + 1
			for (int a = 0; a <= 255; a++)
				// Since we don't have Mid()  in C#, we use the C#
				// equivalent of Mid(), String.Substring, to get a
				// single character from strPwd.  We declare a character
				// variable, ctmp, to hold this value.

				// A couple things to note.  First, the Mod keyword we
				// used in VB need to be replaced with the %
				// operator C# uses.  Next, since the return type of
				// String.Substring is a string, we need to convert it to
				// a char using String.ToCharArray() and specifying that
				// we want the first value in the array, [0].

				char ctmp = (strPwd.Substring((a % intLength),

				// We now have our character and need to get the ASCII
				// code for it.  C# doesn't have the  VB Asc(), but that
				// doesn't mean we can't use it.  In the beginning of our
				// code, we imported the Microsoft.VisualBasic namespace.
				// This allows us to use many of the native VB functions
				// in C#
				// Note that we need to use [] instead of () for our
				// array members.
				key[a] = Microsoft.VisualBasic.Strings.Asc(ctmp);
				sbox[a] = a;

			// Declare an integer x and initialize it to zero.
			int x = 0;

			// Again, create a for loop like the one above.  Note that we
			// need to use a different variable since we've already
			// declared a above.
			for (int b = 0; b <= 255; b++)
				x = (x + sbox[b] + key[b]) % 256;
				int tempSwap = sbox[b];
				sbox[b] = sbox[x];
				sbox[x] = tempSwap;
		public string EnDeCrypt()
			int i = 0;
			int j = 0;
			string cipher = "";

			// Call our method to initialize the arrays used here.

			// Set up a for loop.  Again, we use the Length property
			// of our String instead of the Len() function

			for (int a = 1; a <= plaintext.Length; a++)
				// Initialize an integer variable we will use in this loop
				int itmp = 0;

				// Like the RC4Initialize method, we need to use the %
				// in place of Mod
				i = (i + 1) % 256;
				j = (j + sbox[i]) % 256;
				itmp = sbox[i];
				sbox[i] = sbox[j];
				sbox[j] = itmp;

				int k = sbox[(sbox[i] + sbox[j]) % 256];

				// Again, since the return type of String.Substring is a
				// string, we need to convert it to a char using
				// String.ToCharArray() and specifying that we want the
				// first value, [0].

				char ctmp = plaintext.Substring(a - 1, 1).ToCharArray()

				// Use Asc() from the Microsoft.VisualBasic namespace
				itmp = Microsoft.VisualBasic.Strings.Asc(ctmp);

				// Here we need to use ^ operator that C# uses for Xor
				int cipherby = itmp ^ k;

				// Use Chr() from the Microsoft.VisualBasic namespace                
				cipher += Microsoft.VisualBasic.Strings.Chr(cipherby);

			// Return the value of cipher as the return value of our
			// method
			return cipher;


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Question by:JeffDun
1 Comment
LVL 35

Accepted Solution

Robert Schutt earned 500 total points
ID: 38719383
There's a javascript version here: 

The trick they use is base64 encoding after encryption because the 'string' you return from EnDeCrypt() does not by definition consist of printable characters. The downside is of course that decryption has to be done by first translating the base64 encoded string back to the original encrypted string.

I made a little test program with the key in textBox1 and the text in textBox2, encrypting would be done like this:
main.rc4encrypt rc4 = new main.rc4encrypt();
rc4.Password = textBox1.Text;
rc4.PlainText = textBox2.Text;
textBox2.Text = Convert.ToBase64String(enc.GetBytes(rc4.EnDeCrypt()));

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and decryption:
main.rc4encrypt rc4 = new main.rc4encrypt();
rc4.Password = textBox1.Text;
rc4.PlainText = enc.GetString(Convert.FromBase64String(textBox2.Text));
textBox2.Text = rc4.EnDeCrypt();

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where you could use for both for example:
Encoding enc = Encoding.Default;

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EDIT: actually in keeping with your Chr/Asc usage you should probably use Encoding.ASCII

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