This should do it in C#: string Vigenere(string text, string key) => string.Join("", text.Select((p, k) => p < 'A' || p > 'Z' ? p : (char)((p + key[k % key.L

This should do it in C#: string Vigenere(string text, string key) => string.Join("", text.Select((p, k) => p < 'A' || p > 'Z' ? p : (char)((p + key[k % key.Length]) % 26 + 85)));