Avatar of JeffDun
JeffDun
 asked on

Decrypt RC4 values

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 asp.net 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),
					1).ToCharArray()[0]);

				// 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.
			RC4Initialize(password);

			// 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()
					[0];

				// 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;
		}

	}
}

Open in new window

RC4.txt
Encryption

Avatar of undefined
Last Comment
Robert Schutt

8/22/2022 - Mon
ASKER CERTIFIED SOLUTION
Robert Schutt

THIS SOLUTION ONLY AVAILABLE TO MEMBERS.
View this solution by signing up for a free trial.
Members can start a 7-Day free trial and enjoy unlimited access to the platform.
See Pricing Options
Start Free Trial
GET A PERSONALIZED SOLUTION
Ask your own question & get feedback from real experts
Find out why thousands trust the EE community with their toughest problems.
I started with Experts Exchange in 2004 and it's been a mainstay of my professional computing life since. It helped me launch a career as a programmer / Oracle data analyst
William Peck