NCode.Base64Url

A high-performance .NET library for encoding and decoding base64url data as defined in RFC 4648 Section 5. Built with modern memory-efficient APIs including Span<T>, ReadOnlySequence<T>, and IBufferWriter<T>.

Features

• URL-Safe Encoding — Uses - and _ instead of + and /, making encoded data safe for URLs, filenames, and identifiers
• No Padding — Omits trailing = padding characters (decoder accepts both padded and unpadded input)
• Zero-Allocation Options — TryEncode/TryDecode methods write directly to caller-provided buffers
• Streaming Support — Full support for ReadOnlySequence<T> for processing fragmented/pipelined data
• Buffer Writer Integration — Direct output to IBufferWriter<T> for high-throughput scenarios
• High Performance — Optimized with unsafe code, lookup tables, and aggressive inlining

Installation

dotnet add package NCode.Base64Url

Quick Start

using NCode.Encoders;

// Encoding
byte[] data = { 0x48, 0x65, 0x6C, 0x6C, 0x6F }; // "Hello" in ASCII
string encoded = Base64Url.Encode(data);         // Returns "SGVsbG8"

// Decoding
byte[] decoded = Base64Url.Decode("SGVsbG8");    // Returns original bytes

API Summary

Encoding Methods

Method	Description
GetCharCountForEncode(int)	Calculates the output character count for a given byte count
Encode(ReadOnlySpan<byte>)	Encodes bytes to a base64url string
Encode(ReadOnlySequence<byte>)	Encodes a byte sequence to a base64url string
Encode(ReadOnlySpan<byte>, IBufferWriter<char>)	Encodes bytes directly to a buffer writer
Encode(ReadOnlySequence<byte>, IBufferWriter<char>)	Encodes a byte sequence directly to a buffer writer
TryEncode(ReadOnlySpan<byte>, Span<char>, out int)	Attempts to encode bytes to a provided character buffer
TryEncode(ReadOnlySequence<byte>, Span<char>, out int)	Attempts to encode a byte sequence to a provided character buffer

Decoding Methods

Method	Description
GetByteCountForDecode(int)	Calculates the output byte count for a given character count
Decode(ReadOnlySpan<char>)	Decodes a base64url string to a byte array
Decode(ReadOnlySpan<char>, IBufferWriter<byte>)	Decodes a base64url string directly to a buffer writer
Decode(ReadOnlySequence<char>, IBufferWriter<byte>)	Decodes a character sequence directly to a buffer writer
TryDecode(ReadOnlySpan<char>, Span<byte>, out int)	Attempts to decode a base64url string to a provided byte buffer
TryDecode(ReadOnlySequence<char>, Span<byte>, out int)	Attempts to decode a character sequence to a provided byte buffer

Advanced Usage

Zero-Allocation Encoding

byte[] data = GetData();
int charCount = Base64Url.GetCharCountForEncode(data.Length);
Span<char> buffer = stackalloc char[charCount];

if (Base64Url.TryEncode(data, buffer, out int charsWritten))
{
    // Use buffer[..charsWritten]
}

Zero-Allocation Decoding

ReadOnlySpan<char> encoded = "SGVsbG8";
int byteCount = Base64Url.GetByteCountForDecode(encoded.Length);
Span<byte> buffer = stackalloc byte[byteCount];

if (Base64Url.TryDecode(encoded, buffer, out int bytesWritten))
{
    // Use buffer[..bytesWritten]
}

Using with IBufferWriter

var writer = new ArrayBufferWriter<char>();
int charsWritten = Base64Url.Encode(data, writer);

Target Frameworks

• .NET 8.0
• .NET 10.0

Release Notes

Version	Changes
v1.0.0	Initial release
v1.1.0	Added support for ReadOnlySequence<T>
v1.1.1	Added sequence overloads without buffer writer
v1.1.2	Optimized single-segment sequence handling
v2.0.0	Target .NET 8.0+ only
v2.0.1	Updates to xmldoc and readme

License

Licensed under the Apache License, Version 2.0.