Sanitizing Strings according to Windows Client Certificate Enrollment Protocol [MS-WCCE]

StringExtensions.Wcce.cs

public static partial class StringExtensions
{
  /// <summary>
  /// Sanitizes the string according to Windows Client Certificate Enrollment 
  /// Protocol: http://msdn.microsoft.com/en-us/library/cc249879.aspx
  /// </summary>
  /// <param name="input">The input.</param>
  /// <returns>System.String.</returns>
  /// <remarks>
  /// All disallowed characters in the original name MUST be replaced with 
  /// the appropriate replacements values as specified in section 
  /// 3.1.1.4.1.1.2: http://msdn.microsoft.com/en-us/library/cc249743.aspx.
  /// The sanitized name MUST be truncated to no more than 51 characters in 
  /// total length. The truncated name MUST NOT exceed 51 characters. If an 
  /// incomplete sanitized character sequence remains at the end of the 
  /// string (for example, !002 instead of !0023), the incomplete sequence 
  /// MUST be truncated completely.
  /// The characters that were removed or truncated from the sanitized string 
  /// in the preceding bulleted item MUST be hashed according to the rules 
  /// specified in section 3.1.1.4.1.1.1. The resultant hash MUST be 
  /// converted to a 5-character string. The string MUST be five characters 
  /// in total length and MUST be padded with leading zeros on the left to 
  /// ensure a total length of five characters.
  /// A minus sign (–) MUST be appended to the truncated sanitized name 
  /// followed by the 5-character string that contains the hash value.
  /// </remarks>
  public static string ToWcceSanitizedString(this string input)
  {
    var sanitizedString = string.Empty;
    var hashString = string.Empty;
    var exclusions =
      new List<char>
      {
        '!',
        '"',
        '#',
        '%',
        '&',
        '\'',
        '(',
        ')',
        '*',
        '+',
        ',',
        '/',
        ':',
        ';',
        '<',
        '=',
        '>',
        '?',
        '[',
        '\\',
        ']',
        '^',
        '`',
        '{',
        '|',
        '}',
      };
    foreach (var c in input)
    {
      var charCode = (int)c;
      if (charCode >= 0x20 && 
        charCode <= 0x7F && 
        exclusions.IndexOf(c) == -1)
      {
        if (sanitizedString.Length + 1 > 51)
        {
          hashString = hashString + c;
          continue;
        }
        sanitizedString = sanitizedString + c;

        continue;
      }
      if (sanitizedString.Length + 5 > 51)
      {
        hashString = hashString + "!00" + charCode.ToString("X2");
        continue;
      }
      sanitizedString = sanitizedString + "!00" + charCode.ToString("X2");
    }

    if (!string.IsNullOrWhiteSpace(hashString))
    {
      UInt16 hashSeed = 0;
      const ushort value = 0x8000;

      foreach (var c in hashString)
      {
        var lowBit = (UInt16)(((value & hashSeed) != 0) ? 1 : 0);
        hashSeed = (UInt16)(((hashSeed << 1) | lowBit) + c);
      }

      sanitizedString +=
        "-" + 
        hashSeed.ToString(CultureInfo.InvariantCulture).PadLeft(5, '0');
    }

    return sanitizedString.ToLowerInvariant();
  }
}