How to Implement Modified Base64 URL Encoding and Decoding in ASP.NET?

ASP.NET Framework: URL-Safe Base64 Encoding and Decoding

This technique modifies standard Base64 encoding to create URL-friendly strings, eliminating padding and characters like ' ' and '/' that can interfere with URI templates. Here's an ASP.NET implementation:

/// <summary>
/// URL-safe Base64 encoding using UTF-8.
/// </summary>
/// <param name="str">The string to encode.</param>
/// <returns>The URL-safe Base64 encoded string.</returns>
public static string UrlSafeBase64Encode(string str)
{
    byte[] encodedBytes = Encoding.UTF8.GetBytes(str);
    return HttpServerUtility.UrlTokenEncode(encodedBytes);
}

/// <summary>
/// URL-safe Base64 decoding using UTF-8.
/// </summary>
/// <param name="str">The URL-safe Base64 encoded string.</param>
/// <returns>The decoded string.</returns>
public static string UrlSafeBase64Decode(string str)
{
    byte[] decodedBytes = HttpServerUtility.UrlTokenDecode(str);
    return Encoding.UTF8.GetString(decodedBytes);
}

To utilize this URL-safe Base64 encoding, simply replace your existing Base64 encoding and decoding methods with the functions provided above.

Important Considerations:

• The output isn't strictly compliant with standard Base64, as URL-unsafe characters are substituted.
• This differs from the RFC 4648 base64url standard; padding ('=') is replaced with '0', '1', or '2' based on the number of padding characters removed.

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