Splitting Vigenere cipher into Caesar groups

gistfile1.cs

static List<string> SplitTextWithKeyLen(string text, int keyLen)
{
     List<string> result = new List<string>();
     StringBuilder[] sb = new StringBuilder[keyLen];

     for (int i = 0; i < keyLen; i++)
         sb[i] = new StringBuilder();

     for (int i = 0; i < text.Length; i++)
         sb[i % keyLen].Append(text[i]);

     foreach (var item in sb)
         result.Add(item.ToString());

     result;
}