The Base16, Base32, and Base64 Data Encodings
RFC 3548

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Network Working Group                                  S. Josefsson, Ed.
Request for Comments: 3548                                     July 2003
Category: Informational

             The Base16, Base32, and Base64 Data Encodings

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This document describes the commonly used base 64, base 32, and base
   16 encoding schemes.  It also discusses the use of line-feeds in
   encoded data, use of padding in encoded data, use of non-alphabet
   characters in encoded data, and use of different encoding alphabets.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Implementation discrepancies . . . . . . . . . . . . . . . . .  2
       2.1.  Line feeds in encoded data . . . . . . . . . . . . . . .  2
       2.2.  Padding of encoded data  . . . . . . . . . . . . . . . .  3
       2.3.  Interpretation of non-alphabet characters in encoded
             data . . . . . . . . . . . . . . . . . . . . . . . . . .  3
       2.4.  Choosing the alphabet  . . . . . . . . . . . . . . . . .  3
   3.  Base 64 Encoding . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Base 64 Encoding with URL and Filename Safe Alphabet . . . . .  6
   5.  Base 32 Encoding . . . . . . . . . . . . . . . . . . . . . . .  6
   6.  Base 16 Encoding . . . . . . . . . . . . . . . . . . . . . . .  8
   7.  Illustrations and examples . . . . . . . . . . . . . . . . . .  9
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
       9.1.  Normative References . . . . . . . . . . . . . . . . . . 11
       9.2.  Informative References . . . . . . . . . . . . . . . . . 11
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   11. Editor's Address . . . . . . . . . . . . . . . . . . . . . . . 12
   12. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 13

Josefsson                    Informational                      [Page 1]
RFC 3548     The Base16, Base32, and Base64 Data Encodings     July 2003

1.  Introduction

   Base encoding of data is used in many situations to store or transfer
   data in environments that, perhaps for legacy reasons, are restricted
   to only US-ASCII [9] data.  Base encoding can also be used in new
   applications that do not have legacy restrictions, simply because it
   makes it possible to manipulate objects with text editors.

   In the past, different applications have had different requirements
   and thus sometimes implemented base encodings in slightly different
   ways.  Today, protocol specifications sometimes use base encodings in
   general, and "base64" in particular, without a precise description or
   reference.  MIME [3] is often used as a reference for base64 without
   considering the consequences for line-wrapping or non-alphabet
   characters.  The purpose of this specification is to establish common
   alphabet and encoding considerations.  This will hopefully reduce
   ambiguity in other documents, leading to better interoperability.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].

2.  Implementation discrepancies

   Here we discuss the discrepancies between base encoding
   implementations in the past, and where appropriate, mandate a
   specific recommended behavior for the future.

2.1.  Line feeds in encoded data

   MIME [3] is often used as a reference for base 64 encoding.  However,
   MIME does not define "base 64" per se, but rather a "base 64
   Content-Transfer-Encoding" for use within MIME.  As such, MIME
   enforces a limit on line length of base 64 encoded data to 76
   characters.  MIME inherits the encoding from PEM [2] stating it is
   "virtually identical", however PEM uses a line length of 64
   characters.  The MIME and PEM limits are both due to limits within
   SMTP.

   Implementations MUST NOT not add line feeds to base encoded data
   unless the specification referring to this document explicitly
   directs base encoders to add line feeds after a specific number of
   characters.

Josefsson                    Informational                      [Page 2]
RFC 3548     The Base16, Base32, and Base64 Data Encodings     July 2003

2.2.  Padding of encoded data

   In some circumstances, the use of padding ("=") in base encoded data
   is not required nor used.  In the general case, when assumptions on
   size of transported data cannot be made, padding is required to yield
   correct decoded data.

   Implementations MUST include appropriate pad characters at the end of