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private static char Cipher(char ch, int key)
{
    if (!char.IsLetter(ch))
        return ch;

    char offset = char.IsUpper(ch) ? 'A' : 'a';
    return (char) ((((ch + key) - offset) % 26) + offset);
}

public static string Encipher(string input, int key)
{
    string output = string.Empty;
    int st = key;

    foreach (char ch in input)
    {
        output += Cipher(ch, key);
    }

    return output;
}

public static string Decipher(string input, int key)
{
    return Encipher(input, 26 - key);
}