.NET/C# String Encryption

I recently converted my PHP functions for a .NET project.

C# Code:

private string djEncryptString(string s)
{
	//
	// s is a random string to be encrypted.
	// key is a randomized string that must contain all
	// the characters found in s as an absolute minimum.
	//

	char[] key_chars = PassDecryptKey.ToCharArray();
	char[] string_chars = s.ToCharArray();
	string key_coords = "";
	string key_coords_lengths = "";

	foreach (char c in string_chars) {
		for (int n=0; n<key_chars.Length; n++) {
			if (c == key_chars[n]) {
				key_coords += n.ToString();
				key_coords_lengths += n.ToString().Length.ToString();
			}
		}
	}

	return key_coords +"-"+ key_coords_lengths;
}

private string djDecryptString(string s)
{
	//
	// s is the output from djEncryptString().
	// key is the same key as used to encrypt with.
	//

	char[] key_chars = PassDecryptKey.ToCharArray();
	string key_coords = s.Split('-')[0];
	char[] key_coords_lengths = s.Split('-')[1].ToCharArray();
	string DecryptedString = "";

	foreach (char c in key_coords_lengths) {
		DecryptedString += key_chars[
			Int32.Parse(key_coords.Substring(0,(int)Char.GetNumericValue(c)))
		];
		key_coords = key_coords.Substring((int)Char.GetNumericValue(c));
	}
			
	return DecryptedString;
}