Base64 URL Encoding

Note

ASP.NET Core has built-in support for Base64 encoding and decoding via WebEncoders.Base64UrlEncode and WebEncoders.Base64UrlDecode.

JWT serialization involves transforming the three core components of a JWT (Header, Payload, Signature) into a single, compact, URL-safe string. Base64 URL encoding is used instead of standard Base64 because it doesn’t include characters like +, /, or =, making it safe to use directly in URLs and HTTP headers without requiring further encoding.

WebEncoders Encode and Decode

To use the built-in .NET support, ensure you have the following package installed:

dotnet add package Microsoft.AspNetCore.WebUtilities

Then use the following code:

using System.Text;
using Microsoft.AspNetCore.WebUtilities;

var bytes = "hello"u8.ToArray();
var b64url = WebEncoders.Base64UrlEncode(bytes);

bytes = WebEncoders.Base64UrlDecode(b64url);

var text = Encoding.UTF8.GetString(bytes);
Console.WriteLine(text);

IdentityModel’s Base64Url

IdentityModel includes the Base64Url class to help with encoding/decoding:

using System.Text;
using Duende.IdentityModel;

var bytes = "hello"u8.ToArray();
var b64url = Base64Url.Encode(bytes);

bytes = Base64Url.Decode(b64url);

var text = Encoding.UTF8.GetString(bytes);
Console.WriteLine(text);