c# - Code for decoding/encoding a modified base64 URL

Question

I want to base64 encode data to put it in a URL and then decode it within my HttpHandler.

I have found that Base64 Encoding allows for a '/' character which will mess up my UriTemplate matching. Then I found that there is a concept of a "modified Base64 for URL" from wikipedia:

A modified Base64 for URL variant exists, where no padding '=' will be used, and the '+' and '/' characters of standard Base64 are respectively replaced by '-' and '_', so that using URL encoders/decoders is no longer necessary and has no impact on the length of the encoded value, leaving the same encoded form intact for use in relational databases, web forms, and object identifiers in general.

Using .NET I want to modify my current code from doing basic base64 encoding and decoding to using the "modified base64 for URL" method. Has anyone done this?

To decode, I know it starts out with something like:

string base64EncodedText = base64UrlEncodedText.Replace('-', '+').Replace('_', '/');

// Append '=' char(s) if necessary - how best to do this?

// My normal base64 decoding now uses encodedText

But, I need to potentially add one or two '=' chars to the end which looks a little more complex.

My encoding logic should be a little simpler:

// Perform normal base64 encoding
byte[] encodedBytes = Encoding.UTF8.GetBytes(unencodedText);
string base64EncodedText = Convert.ToBase64String(encodedBytes);

// Apply URL variant
string base64UrlEncodedText = base64EncodedText.Replace("=", String.Empty).Replace('+', '-').Replace('/', '_');

I have seen the Guid to Base64 for URL StackOverflow entry, but that has a known length and therefore they can hardcode the number of equal signs needed at the end.

Answer 1

Also check class HttpServerUtility with UrlTokenEncode and UrlTokenDecode methods that is handling URL safe Base64 encoding and decoding.

Note 1: The result is not a valid Base64 string. Some unsafe characters for URL are replaced.

Note 2: The result differs from the base64url algorithm in RFC4648.

///<summary>
/// Base 64 Encoding with URL and Filename Safe Alphabet using UTF-8 character set.
///</summary>
///<param name="str">The origianl string</param>
///<returns>The Base64 encoded string</returns>
public static string Base64ForUrlEncode(string str)
{
    byte[] encbuff = Encoding.UTF8.GetBytes(str);
    return HttpServerUtility.UrlTokenEncode(encbuff);
}
///<summary>
/// Decode Base64 encoded string with URL and Filename Safe Alphabet using UTF-8.
///</summary>
///<param name="str">Base64 code</param>
///<returns>The decoded string.</returns>
public static string Base64ForUrlDecode(string str)
{
    byte[] decbuff = HttpServerUtility.UrlTokenDecode(str);
    return Encoding.UTF8.GetString(decbuff);
}