Vigenere Cipher C# Implementation

Code:
static void VigenereEncrypt(ref StringBuilder s, string key)
{
	for (int i = 0; i < s.Length; i++) s[i] = Char.ToUpper(s[i]);
	key = key.ToUpper();
	int j = 0;
	for (int i = 0; i < s.Length; i++)
	{
		if (Char.IsLetter(s[i]))
		{
			s[i] = (char)(s[i] + key[j] - 'A');
			if (s[i] > 'Z') s[i] = (char)(s[i] - 'Z' + 'A' - 1);
		}
		j = j + 1 == key.Length ? 0 : j + 1;
	}
}

static void VigenereDecrypt(ref StringBuilder s, string key)
{
	for (int i = 0; i < s.Length; i++) s[i] = Char.ToUpper(s[i]);
	key = key.ToUpper();
	int j = 0;
	for (int i = 0; i < s.Length; i++)
	{
		if (Char.IsLetter(s[i]))
		{
			s[i] = s[i] >= key[j] ?
					  (char)(s[i] - key[j] + 'A') :
					  (char)('A' + ('Z' - key[j] + s[i] - 'A') + 1);
		}
		j = j + 1 == key.Length ? 0 : j + 1;
	}
}

public static void MainMethod()
{
	StringBuilder s = new StringBuilder("ArkPhaze");
	const string key = "KeyData";
	VigenereEncrypt(ref s, key);
	Console.WriteLine(s);
	VigenereDecrypt(ref s, key);
	Console.WriteLine(s);
}