Converting 15 digit salesforce ids to 18 digit ids

/*
 * A while back I overheard one of my coworkers grumbling about converting 15 digit salesforce ids to their
 * 18 digit variant. I had no idea what he was talking about, but goolge and Alan Shanahan did. Below is my
 * attempt at implementing the algorithm Alan described in this blog post
 * https://astadiaemea.wordpress.com/2010/06/21/15-or-18-character-ids-in-salesforce-com-%E2%80%93-do-you-know-how-useful-unique-ids-are-to-your-development-effort/
 *
 * The following code has not been used anywhere or really tested for that matter. I just wanted to better
 * understand what was being discussed on the other side of my cube.
 */

/// <summary>
/// Convert a case sensitive Salesforce id to the case insensitive format.
/// 
/// The algorithm for converting a case sensitive id to a case insensitive id is as follows.
/// 
///     1 - Split the 15 character id into three five character chunks.
///     2 - Reverse the order of the five character chunks.
///     3 - Iterate over the three chunks and replace all capital letters with 1 and all lower case letters and
///         numeric characters with 0.
///     4 - Each chunk is now a five digit binary number.
///     5 - Convert the binary values to decimal.
///     6 - The decimal value is used to look up a value in the following zero indexed array.
///
///              char[] map = "ABCDEFGHIJKLMNOPQRSTUVWXYZ012345"
/// 
///     7 - Append the three mapped characters to the end of the 15 character id.
/// 
///     For Example:
///             -> "00Do0000000HpVI"
///             -> ["00Do0", "00000", "0HpVI"]
///             -> ["0oD00", "00000", "IVpH0"]
///             -> ["00100", "00000", "11010"]
///             -> [4, 0, 26]
///             -> ["E", "A", "0"]
///             -> "00Do0000000HpVIEA0"
/// </summary>
/// <param name="id"></param>
/// <returns></returns>
private string ToCaseInsensitive(string id)
{
    const int chunkSize = 5;
    var eighteenDigit = new StringBuilder(id);

    for (int chunkStartIndex = 0; chunkStartIndex < id.Length; chunkStartIndex = chunkStartIndex + chunkSize)
    {
        eighteenDigit.Append(ParityBit(id.Substring(chunkStartIndex, chunkSize)));
    }

    return eighteenDigit.ToString();
}

/// <summary>
/// Given a five character block of alphanumeric characters produce a parity character.
/// </summary>
/// <param name="block"></param>
/// <returns></returns>
private char ParityBit(string block)
{
    var asBinary = new StringBuilder();
    char[] map = "ABCDEFGHIJKLMNOPQRSTUVWXYZ012345".ToCharArray();
    char[] reversedBlock = block.Reverse().ToArray();

    foreach (char c in reversedBlock)
    {
        asBinary.Append(Regex.IsMatch(c.ToString(), @"^[A-Z]$") ? "1" : "0");
    }

    int index = Convert.ToInt32(asBinary.ToString(), fromBase: 2);

    return map[index];
}