Vegenere Decipher

gistfile1.cs

static string DecipherVeginere(string text, string key)
{
    StringBuilder result = new StringBuilder();
    int keyLength = key.Length;
    int diff;
    char decoded;

    for (int i = 0; i < text.Length; i++)
    {
        diff = text[i] - key[i%keyLength];

        //diff should never be more than 25 or less than -25
        if(diff < 0)
            diff += 26;

        decoded = (char)(diff + 'a') ;
        result.Append(decoded);
    }

    return result.ToString();
}