Masking — Obscuring Data While Preserving Length

Summary:
Masking replaces the middle part of a string with repeated mask characters (like *), preserving the string’s total length and showing only a visible prefix and suffix. If the string is too short, it masks the entire string. The mask character can be customized.

Key points:

• Shows a defined number of characters at the start and end of the string.
• Replaces the middle characters with a masking character (default *).
• Supports custom mask characters.
• If the string is too short to be partially masked, it masks the entire string.

Example Implementation:

public static string Mask(string input, int prefixLength = 4, int suffixLength = 4, char maskCharacter = '*')
{
    if (string.IsNullOrWhiteSpace(input)) return new string(maskCharacter, 3);
    if (prefixLength < 0 || suffixLength < 0)
        throw new ArgumentOutOfRangeException("Prefix and suffix lengths cannot be negative.");

    int inputLength = input.Length;

    if (prefixLength + suffixLength >= inputLength)
    {
        return new string(maskCharacter, inputLength);
    }

    string prefix = input.Substring(0, prefixLength);
    string suffix = input.Substring(inputLength - suffixLength);
    string maskedPart = new string(maskCharacter, inputLength - prefixLength - suffixLength);

    return prefix + maskedPart + suffix;
}

Use Case:
Ideal when you need to display partial information while maintaining the same length, such as masking credit card numbers or tokens in a UI.