This problem is relatively straightforward. Simply loop through the message's letters and shift them by some amount.

The example solution uses the following string extension method to encrypt a string:

// Use a Caesar cipher to encrypt the plaintext.
public static string CaesarEncrypt(this string plaintext, int shift)
{
    plaintext = plaintext.StripText();

    // Encrypt.
    char[] chars = new char[plaintext.Length];
    for (int i = 0; i < plaintext.Length; i++)
    {
        int ch = plaintext[i] - 'A';
        ch = (ch + shift + 26) % 26;
        chars[i] = (char)('A' + ch);
    }
    return new string(chars).ToFiveGrams();
}

This method calls the StripText extension method described shortly to remove any non-letter characters from the string. It then loops through the message's letters, adding the shift to each and storing the results in a char array. When it has finished processing the letters, the method converts the char array into a string, calls the ToFiveGrams extension method (also described shortly), and returns the result.

The following code shows the StripText helper extension method:

// Convert to uppercase, and remove punctuation and spaces.
public static string StripText(this string text)
{
    text = text.ToUpper();
    text = new string(text.Where(
        ch => (ch >= 'A') && (ch <= 'Z')).ToArray());
    return text;
}

This method converts the message into uppercase. It uses a lambda expression to extract the characters between A and Z, and converts those characters into a new string. That removes any non-letter characters from the string. The method then returns the result.

The following code shows the ToFiveGrams helper method:

// Separate the string into five-character pieces.
public static string ToFiveGrams(this string text)
{
    StringBuilder result = new StringBuilder();
    for (int i = 0; i < text.Length; i += 5)
    {
        int length = Math.Min(5, text.Length - i);
        result.Append(" " + text.Substring(i, length));
    }
    result.Remove(0, 1);
    return result.ToString();
}

The code first creates a StringBuilder to hold the result. It breaks the text into groups of five characters and adds the groups preceded by a space character to the StringBuilder. After it has finished processing the text, the code removes the first space character, converts the rest of the StringBuilder contents into a string, and returns the result.

After you write the method to encrypt a string, decrypting a string is easy because decrypting a Caesar cipher message is the same as encrypting it with a negative shift. For example, if you encrypt a plaintext message with the shift 4, then you can decrypt it by encrypting the ciphertext with a shift of -4. The CaesarDecrypt method shown in the following code does that:

// Use a Caesar cipher to decrypt the ciphertext.
public static string CaesarDecrypt(this string ciphertext, int shift)
{
    return ciphertext.CaesarEncrypt(-shift);
}

This method simply calls the CaesarEncrypt method described earlier to encrypt the ciphertext with a negative shift value.

Download the CaesarCipher example solution to see additional details.