Fast and flexible base64 encoder and decoder

Upstream URL



Chaitanya Gupta <mail@chaitanyagupta.com>




qbase64 provides a fast and flexible base64 encoder and decoder for Common Lisp. It provides three interfaces for both encoding and decoding:

  • ENCODE-BYTES and DECODE-STRING are the easiest to use. They allow one to encode a byte vector and decode a base64 string in one go.

  • ENCODE-STREAM and DECODE-STREAM are gray stream classes that allow one to write and read bytes from underlying character streams.

  • ENCODER and DECODER provide the low level interface. The other interfaces are built on top of these.

Table of Contents

Why qbase64?

Given that a couple of Lisp libraries already exist for working with base64 - cl-base64 and s-base64 - why might you want to pick qbase64? There are two reasons:

  1. Stream-based APIs - neither of the alternatives provide a (gray) stream implementation where you can write bytes to a binary stream that is automatically encoded to an underlying character stream and vice-versa.

  2. Performance - qbase64 was written with the objective of being fast while keeping memory consumption independent of the input size. See the Performance section for benchmarks and other details.


Install using quicklisp:

(ql:quickload :qbase64)



The examples below use ENCODE-BYTES and ENCODE-STREAM.

(qbase64:encode-bytes #(1 2 3 4 5 6 7 8))

(with-output-to-string (s)
  (with-open-stream (out (make-instance 'qbase64:encode-stream
                                        :underlying-stream s))
    (write-sequence #(1 2 3 4) out)
    (write-sequence #(5 6 7 8) out)))


The examples below use DECODE-STRING and DECODE-STREAM.

(qbase64:decode-string "AQIDBAUGBwg=")
=> #(1 2 3 4 5 6 7 8)

(with-input-from-string (s "AQIDBAUGBwg=")
  (with-open-stream (in (make-instance 'qbase64:decode-stream
                                       :underlying-stream s))
    (let ((bytes (make-array 4)))
         for position = (read-sequence bytes in)
         do (print (subseq bytes 0 position))
         while (= position (length bytes))))))
; #(1 2 3 4) 
; #(5 6 7 8) 
; #() 


Normally you wouldn't need to use ENCODER and DECODER directly, but if you do (say you want more control over memory management), you can refer to the examples below.

In these examples, fixed length sequences are used for both input and output, and any input buffered by the encoder/decoder is first cleared before further input is fed to it. This allows very tight control over how much memory gets used.

Refer to the doc strings for ENCODER, ENCODE, DECODER and DECODE for more details.

Note that running the following examples requires FLEXI-STREAMS.

(flexi-streams:with-input-from-sequence (in #(1 2 3 4 5 6 7 8))
  (let* ((encoder (qbase64:make-encoder))
         (bytes (make-array 4))
         (string (make-string 5))
         (read-bytes t)
         (buffered nil)
         (eof nil))
       while (or buffered (not eof))
       for end1 = (when read-bytes (read-sequence bytes in))
       if (and read-bytes (< end1 (length bytes))) do (setf eof t)
         (multiple-value-bind (end2 pending)
             (if read-bytes
                 (qbase64:encode encoder bytes string :end1 end1 :finish eof)
                 (qbase64:encode encoder #() string :finish eof))
           (write-string string nil :end end2)
           (setf buffered pending
                 read-bytes (or (not pending) (zerop end2)))))))

(with-input-from-string (in "AQIDBAUGBwg=")
  (let* ((decoder (qbase64:make-decoder))
         (string (make-string 4))
         (bytes (make-array 5))
         (read-string t)
         (buffered nil)
         (eof nil))
       while (or buffered (not eof))
       for end1 = (when read-string (read-sequence string in))
       if (and read-string (< end1 (length string))) do (setf eof t)
         (multiple-value-bind (end2 pending)
             (if read-string
                 (qbase64:decode decoder string bytes :end1 end1)
                 (qbase64:decode decoder "" bytes))
           (print (subseq bytes 0 end2))
           (setf buffered pending
                 read-string (or (not pending) (zerop end2)))))))
; #(1 2 3)
; #(4 5 6)
; #(7 8)
; #()

Limits and Assertions

The library relies on (UNSIGNED-BYTE 8) and fixnum arithmetic to achieve good performance. Consequently,

  • When providing bytes for encoding, ensure that each byte is of type (UNSIGNED-BYTE 8). Although the ARRAY-ELEMENT-TYPE of the byte array can be T, the elements themselves must conform to this restriction.

  • Max length of the byte vector that is used as encoding input or decoding output should never exceed +MAX-BYTES-LENGTH+.

  • Max length of the string that is used as encoding output or decoding input should never exceed +MAX-STRING-LENGTH+.


See this page for CPU and memory benchmarks vs other CL libraries.

Encoding and decoding should be very fast under these conditions:

  • The byte vector is a SIMPLE-ARRAY of element type (UNSIGNED-BYTE 8).

  • The string is a SIMPLE-STRING. Theoretically SIMPLE-BASE-STRING could be even faster.

That said, these are just the optimal conditions. You can safely use any STRING or VECTOR with qbase64 if needed.

Additional Features

Encoding Schemes

Two base64 encoding schemes are supported: original (the default) and URI.

URI encoding scheme is useful when base64 strings are used as GET or POST values in an HTTP request.

The scheme can be set by using the :SCHEME keyword.

(qbase64:encode-bytes #(251 252 253 254 255) :scheme :original)
=> "+/z9/v8="

(qbase64:encode-bytes #(251 252 253 254 255) :scheme :uri)
=> "-_z9_v8="


The encoded base64 stream can broken into multiple lines using the :LINEBREAK keyword. By default it is 0, which means that no newlines are output. Setting it to a positive integer indicates the column number at which lines should be broken.

(princ (qbase64:encode-bytes #(1 2 3 4 5 6 7 8) :linebreak 4))
; Bwg=

During decoding, all whitespace (including newlines) is ignored.

API Reference

At the moment, API reference is available in the form of doc strings for all the exported symbols.

Reporting Bugs

To report a bug in the library, create a Github issue.

Dependencies (4)

  • cl-fad
  • fiveam
  • metabang-bind
  • trivial-gray-streams

Dependents (1)

  • GitHub
  • Quicklisp