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I'm following an article, https://web.archive.org/web/20210927195845/https://www.4guysfromrolla.com/articles/091802-1.3.aspx, which shows how to convert Mike Shaffer's VB RC4 encryption to C#, however I'm getting different results than the original article, https://web.archive.org/web/20210728063606/https://www.4guysfromrolla.com/webtech/010100-1.shtml.

Using this test link from the original article, https://web.archive.org/web/20210728061038/https://www.4guysfromrolla.com/demos/rc4test.htm, with a password of "abc" and plain text of "testing123", I get "B9 F8 AA 5D 31 B1 8A 42 1E D4". However, when using the C# version, I get something slightly different: "b9 f8 aa 5d 31 b1 160 42 1e d4". I'm getting "160" instead of "8A".

Here's my method which converts the ASCII (final result of C# method) to hex:

public static string ConvertAsciiToHex(string input) { return string.Join(string.Empty, input.Select(c => Convert.ToInt32(c).ToString("X")).ToArray()); }

And here's the C# code I have from the article (modified to static class):

protected static int[] sbox = new int[256]; protected static int[] key = new int[256]; private static string password = "abc"; private static void RC4Initialize(string strPwd) { int intLength = strPwd.Length; for (int a = 0; a <= 255; a++) { char ctmp = (strPwd.Substring((a % intLength), 1).ToCharArray()[0]); key[a] = Microsoft.VisualBasic.Strings.Asc(ctmp); sbox[a] = a; } int x = 0; 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; } } private static string EnDeCrypt(string text) { int i = 0; int j = 0; string cipher = ""; RC4Initialize(password); for (int a = 1; a <= text.Length; a++) { int itmp = 0; 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]; char ctmp = text.Substring(a - 1, 1).ToCharArray() [0]; itmp = Microsoft.VisualBasic.Strings.Asc(ctmp); int cipherby = itmp ^ k; cipher += Microsoft.VisualBasic.Strings.Chr(cipherby); } return cipher; }

I'm calling the method like so:

public static string Encrypt(string text) { return ConvertAsciiToHex(EnDeCrypt(text)); } RC4Encrypt.Encrypt("testing123");

What am I doing wrong?

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1 Answer 1

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It's a difference between calls to Chr and ChrW.

Chr will only take values between 0 and 255 and will return a char whose value is out of that range (at least on my machine as shown below). You were seeing a 138.

 128 != 8364 (?) 130 != 8218 (,) 131 != 402 (ƒ) 132 != 8222 (,) 133 != 8230 (.) 134 != 8224 (+) 135 != 8225 (╪) 136 != 710 (^) 137 != 8240 (%) 138 != 352 (S) 139 != 8249 () 156 != 339 (o) 158 != 382 (z) 159 != 376 (Y)

For a better explanation, a VB.Net dev may be required... ;-)

Given that, however, there's little need for using the Microsoft.VisualBasic calls (hardly a translation when calling VB... ;-) ) because using char will work just fine for what you're doing.

itmp = ctmp; //there's an implicit conversion for char to int int cipherby = itmp ^ k; Console.WriteLine("cipherby = {0}", cipherby); cipher += (char)cipherby; //just cast cipherby to a char
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4 Comments

Thank you - this worked. Do I need to change anything else for the decryption to work? The result isn't matching up. I'm doing return EnDeCrypt(ConvertHexToAscii(text)); to decrypt..
Don't change the output. This should work just fine: EnDeCrypt(EnDeCrypt("testing123")
For decryption? I want to decrypt "B9F8AA5D31B18A421ED4" which should output to "testing123".
You'll need a ConvertHexToChar method (as well as renaming your ConvertHexToAscii to ConvertStringToHex) that is an inversion of your ConvertHexToAscii method.

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