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rfc:rfc3548

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
 encoded data unless the specification referring to this document
 explicitly states otherwise.

2.3. Interpretation of non-alphabet characters in encoded data

 Base encodings use a specific, reduced, alphabet to encode binary
 data.  Non alphabet characters could exist within base encoded data,
 caused by data corruption or by design.  Non alphabet characters may
 be exploited as a "covert channel", where non-protocol data can be
 sent for nefarious purposes.  Non alphabet characters might also be
 sent in order to exploit implementation errors leading to, e.g.,
 buffer overflow attacks.
 Implementations MUST reject the encoding if it contains characters
 outside the base alphabet when interpreting base encoded data, unless
 the specification referring to this document explicitly states
 otherwise.  Such specifications may, as MIME does, instead state that
 characters outside the base encoding alphabet should simply be
 ignored when interpreting data ("be liberal in what you accept").
 Note that this means that any CRLF constitute "non alphabet
 characters" and are ignored.  Furthermore, such specifications may
 consider the pad character, "=", as not part of the base alphabet
 until the end of the string.  If more than the allowed number of pad
 characters are found at the end of the string, e.g., a base 64 string
 terminated with "===", the excess pad characters could be ignored.

2.4. Choosing the alphabet

 Different applications have different requirements on the characters
 in the alphabet.  Here are a few requirements that determine which
 alphabet should be used:
 o   Handled by humans.  Characters "0", "O" are easily interchanged,
     as well "1", "l" and "I".  In the base32 alphabet below, where 0
     (zero) and 1 (one) is not present, a decoder may interpret 0 as
     O, and 1 as I or L depending on case.  (However, by default it
     should not, see previous section.)

Josefsson Informational [Page 3] RFC 3548 The Base16, Base32, and Base64 Data Encodings July 2003

 o   Encoded into structures that place other requirements.  For base
     16 and base 32, this determines the use of upper- or lowercase
     alphabets.  For base 64, the non-alphanumeric characters (in
     particular "/") may be problematic in file names and URLs.
 o   Used as identifiers.  Certain characters, notably "+" and "/" in
     the base 64 alphabet, are treated as word-breaks by legacy text
     search/index tools.
 There is no universally accepted alphabet that fulfills all the
 requirements.  In this document, we document and name some currently
 used alphabets.

3. Base 64 Encoding

 The following description of base 64 is due to [2], [3], [4] and [5].
 The Base 64 encoding is designed to represent arbitrary sequences of
 octets in a form that requires case sensitivity but need not be
 humanly readable.
 A 65-character subset of US-ASCII is used, enabling 6 bits to be
 represented per printable character.  (The extra 65th character, "=",
 is used to signify a special processing function.)
 The encoding process represents 24-bit groups of input bits as output
 strings of 4 encoded characters.  Proceeding from left to right, a
 24-bit input group is formed by concatenating 3 8-bit input groups.
 These 24 bits are then treated as 4 concatenated 6-bit groups, each
 of which is translated into a single digit in the base 64 alphabet.
 Each 6-bit group is used as an index into an array of 64 printable
 characters.  The character referenced by the index is placed in the
 output string.

Josefsson Informational [Page 4] RFC 3548 The Base16, Base32, and Base64 Data Encodings July 2003

                 Table 1: The Base 64 Alphabet
    Value Encoding  Value Encoding  Value Encoding  Value Encoding
        0 A            17 R            34 i            51 z
        1 B            18 S            35 j            52 0
        2 C            19 T            36 k            53 1
        3 D            20 U            37 l            54 2
        4 E            21 V            38 m            55 3
        5 F            22 W            39 n            56 4
        6 G            23 X            40 o            57 5
        7 H            24 Y            41 p            58 6
        8 I            25 Z            42 q            59 7
        9 J            26 a            43 r            60 8
       10 K            27 b            44 s            61 9
       11 L            28 c            45 t            62 +
       12 M            29 d            46 u            63 /
       13 N            30 e            47 v
       14 O            31 f            48 w         (pad) =
       15 P            32 g            49 x
       16 Q            33 h            50 y
 Special processing is performed if fewer than 24 bits are available
 at the end of the data being encoded.  A full encoding quantum is
 always completed at the end of a quantity.  When fewer than 24 input
 bits are available in an input group, zero bits are added (on the
 right) to form an integral number of 6-bit groups.  Padding at the
 end of the data is performed using the '=' character.  Since all base
 64 input is an integral number of octets, only the following cases
 can arise:
 (1) the final quantum of encoding input is an integral multiple of 24
 bits; here, the final unit of encoded output will be an integral
 multiple of 4 characters with no "=" padding,
 (2) the final quantum of encoding input is exactly 8 bits; here, the
 final unit of encoded output will be two characters followed by two
 "=" padding characters, or
 (3) the final quantum of encoding input is exactly 16 bits; here, the
 final unit of encoded output will be three characters followed by one
 "=" padding character.

Josefsson Informational [Page 5] RFC 3548 The Base16, Base32, and Base64 Data Encodings July 2003

4. Base 64 Encoding with URL and Filename Safe Alphabet

 The Base 64 encoding with an URL and filename safe alphabet has been
 used in [8].
 An alternative alphabet has been suggested that used "~" as the 63rd
 character.  Since the "~" character has special meaning in some file
 system environments, the encoding described in this section is
 recommended instead.
 This encoding should not be regarded as the same as the "base64"
 encoding, and should not be referred to as only "base64".  Unless
 made clear, "base64" refer to the base 64 in the previous section.
 This encoding is technically identical to the previous one, except
 for the 62:nd and 63:rd alphabet character, as indicated in table 2.
       Table 2: The "URL and Filename safe" Base 64 Alphabet
  Value Encoding  Value Encoding  Value Encoding  Value Encoding
     0 A            17 R            34 i            51 z
     1 B            18 S            35 j            52 0
     2 C            19 T            36 k            53 1
     3 D            20 U            37 l            54 2
     4 E            21 V            38 m            55 3
     5 F            22 W            39 n            56 4
     6 G            23 X            40 o            57 5
     7 H            24 Y            41 p            58 6
     8 I            25 Z            42 q            59 7
     9 J            26 a            43 r            60 8
    10 K            27 b            44 s            61 9
    11 L            28 c            45 t            62 - (minus)
    12 M            29 d            46 u            63 _ (understrike)
    13 N            30 e            47 v
    14 O            31 f            48 w         (pad) =
    15 P            32 g            49 x
    16 Q            33 h            50 y

5. Base 32 Encoding

 The following description of base 32 is due to [7] (with
 corrections).
 The Base 32 encoding is designed to represent arbitrary sequences of
 octets in a form that needs to be case insensitive but need not be
 humanly readable.

Josefsson Informational [Page 6] RFC 3548 The Base16, Base32, and Base64 Data Encodings July 2003

 A 33-character subset of US-ASCII is used, enabling 5 bits to be
 represented per printable character.  (The extra 33rd character, "=",
 is used to signify a special processing function.)
 The encoding process represents 40-bit groups of input bits as output
 strings of 8 encoded characters.  Proceeding from left to right, a
 40-bit input group is formed by concatenating 5 8bit input groups.
 These 40 bits are then treated as 8 concatenated 5-bit groups, each
 of which is translated into a single digit in the base 32 alphabet.
 When encoding a bit stream via the base 32 encoding, the bit stream
 must be presumed to be ordered with the most-significant-bit first.
 That is, the first bit in the stream will be the high-order bit in
 the first 8bit byte, and the eighth bit will be the low-order bit in
 the first 8bit byte, and so on.
 Each 5-bit group is used as an index into an array of 32 printable
 characters.  The character referenced by the index is placed in the
 output string.  These characters, identified in Table 2, below, are
 selected from US-ASCII digits and uppercase letters.
                 Table 3: The Base 32 Alphabet
      Value Encoding  Value Encoding  Value Encoding  Value Encoding
          0 A             9 J            18 S            27 3
          1 B            10 K            19 T            28 4
          2 C            11 L            20 U            29 5
          3 D            12 M            21 V            30 6
          4 E            13 N            22 W            31 7
          5 F            14 O            23 X
          6 G            15 P            24 Y         (pad) =
          7 H            16 Q            25 Z
          8 I            17 R            26 2
 Special processing is performed if fewer than 40 bits are available
 at the end of the data being encoded.  A full encoding quantum is
 always completed at the end of a body.  When fewer than 40 input bits
 are available in an input group, zero bits are added (on the right)
 to form an integral number of 5-bit groups.  Padding at the end of
 the data is performed using the "=" character.  Since all base 32
 input is an integral number of octets, only the following cases can
 arise:
 (1) the final quantum of encoding input is an integral multiple of 40
 bits; here, the final unit of encoded output will be an integral
 multiple of 8 characters with no "=" padding,

Josefsson Informational [Page 7] RFC 3548 The Base16, Base32, and Base64 Data Encodings July 2003

 (2) the final quantum of encoding input is exactly 8 bits; here, the
 final unit of encoded output will be two characters followed by six
 "=" padding characters,
 (3) the final quantum of encoding input is exactly 16 bits; here, the
 final unit of encoded output will be four characters followed by four
 "=" padding characters,
 (4) the final quantum of encoding input is exactly 24 bits; here, the
 final unit of encoded output will be five characters followed by
 three "=" padding characters, or
 (5) the final quantum of encoding input is exactly 32 bits; here, the
 final unit of encoded output will be seven characters followed by one
 "=" padding character.

6. Base 16 Encoding

 The following description is original but analogous to previous
 descriptions.  Essentially, Base 16 encoding is the standard standard
 case insensitive hex encoding, and may be referred to as "base16" or
 "hex".
 A 16-character subset of US-ASCII is used, enabling 4 bits to be
 represented per printable character.
 The encoding process represents 8-bit groups (octets) of input bits
 as output strings of 2 encoded characters.  Proceeding from left to
 right, a 8-bit input is taken from the input data.  These 8 bits are
 then treated as 2 concatenated 4-bit groups, each of which is
 translated into a single digit in the base 16 alphabet.
 Each 4-bit group is used as an index into an array of 16 printable
 characters.  The character referenced by the index is placed in the
 output string.
                 Table 5: The Base 16 Alphabet
    Value Encoding  Value Encoding  Value Encoding  Value Encoding
        0 0             4 4             8 8            12 C
        1 1             5 5             9 9            13 D
        2 2             6 6            10 A            14 E
        3 3             7 7            11 B            15 F
 Unlike base 32 and base 64, no special padding is necessary since a
 full code word is always available.

Josefsson Informational [Page 8] RFC 3548 The Base16, Base32, and Base64 Data Encodings July 2003

7. Illustrations and examples

 To translate between binary and a base encoding, the input is stored
 in a structure and the output is extracted.  The case for base 64 is
 displayed in the following figure, borrowed from [4].
       +--first octet--+-second octet--+--third octet--+
       |7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|
       +-----------+---+-------+-------+---+-----------+
       |5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|
       +--1.index--+--2.index--+--3.index--+--4.index--+
 The case for base 32 is shown in the following figure, borrowed from
 [6].  Each successive character in a base-32 value represents 5
 successive bits of the underlying octet sequence.  Thus, each group
 of 8 characters represents a sequence of 5 octets (40 bits).
                      1          2          3
        01234567 89012345 67890123 45678901 23456789
       +--------+--------+--------+--------+--------+
       |< 1 >< 2| >< 3 ><|.4 >< 5.|>< 6 ><.|7 >< 8 >|
       +--------+--------+--------+--------+--------+
                                               <===> 8th character
                                         <====> 7th character
                                    <===> 6th character
                              <====> 5th character
                        <====> 4th character
                   <===> 3rd character
             <====> 2nd character
        <===> 1st character

Josefsson Informational [Page 9] RFC 3548 The Base16, Base32, and Base64 Data Encodings July 2003

 The following example of Base64 data is from [4].
     Input data:  0x14fb9c03d97e
     Hex:     1   4    f   b    9   c     | 0   3    d   9    7   e
     8-bit:   00010100 11111011 10011100  | 00000011 11011001
     11111110
     6-bit:   000101 001111 101110 011100 | 000000 111101 100111
     111110
     Decimal: 5      15     46     28       0      61     37     62
     Output:  F      P      u      c        A      9      l      +
     Input data:  0x14fb9c03d9
     Hex:     1   4    f   b    9   c     | 0   3    d   9
     8-bit:   00010100 11111011 10011100  | 00000011 11011001
                                                     pad with 00
     6-bit:   000101 001111 101110 011100 | 000000 111101 100100
     Decimal: 5      15     46     28       0      61     36
                                                        pad with =
     Output:  F      P      u      c        A      9      k      =
     Input data:  0x14fb9c03
     Hex:     1   4    f   b    9   c     | 0   3
     8-bit:   00010100 11111011 10011100  | 00000011
                                            pad with 0000
     6-bit:   000101 001111 101110 011100 | 000000 110000
     Decimal: 5      15     46     28       0      48
                                                 pad with =      =
     Output:  F      P      u      c        A      w      =      =

8. Security Considerations

 When implementing Base encoding and decoding, care should be taken
 not to introduce vulnerabilities to buffer overflow attacks, or other
 attacks on the implementation.  A decoder should not break on invalid
 input including, e.g., embedded NUL characters (ASCII 0).
 If non-alphabet characters are ignored, instead of causing rejection
 of the entire encoding (as recommended), a covert channel that can be
 used to "leak" information is made possible.  The implications of
 this should be understood in applications that do not follow the
 recommended practice.  Similarly, when the base 16 and base 32
 alphabets are handled case insensitively, alteration of case can be
 used to leak information.
 Base encoding visually hides otherwise easily recognized information,
 such as passwords, but does not provide any computational
 confidentiality.  This has been known to cause security incidents
 when, e.g., a user reports details of a network protocol exchange

Josefsson Informational [Page 10] RFC 3548 The Base16, Base32, and Base64 Data Encodings July 2003

 (perhaps to illustrate some other problem) and accidentally reveals
 the password because she is unaware that the base encoding does not
 protect the password.

9. References

9.1. Normative References

 [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
     Levels", BCP 14, RFC 2119, March 1997.

9.2. Informative References

 [2] Linn, J., "Privacy Enhancement for Internet Electronic Mail:
     Part I: Message Encryption and Authentication Procedures", RFC
     1421, February 1993.
 [3] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
     Extensions (MIME) Part One: Format of Internet Message Bodies",
     RFC 2045, November 1996.
 [4] Callas, J., Donnerhacke, L., Finney, H. and R. Thayer, "OpenPGP
     Message Format", RFC 2440, November 1998.
 [5] Eastlake, D., "Domain Name System Security Extensions", RFC 2535,
     March 1999.
 [6] Klyne, G. and L. Masinter, "Identifying Composite Media
     Features", RFC 2938, September 2000.
 [7] Myers, J., "SASL GSSAPI mechanisms", Work in Progress.
 [8] Wilcox-O'Hearn, B., "Post to P2P-hackers mailing list", World
     Wide Web http://zgp.org/pipermail/p2p-hackers/2001-
     September/000315.html, September 2001.
 [9] Cerf, V., "ASCII format for Network Interchange", RFC 20, October
     1969.

10. Acknowledgements

 Several people offered comments and suggestions, including Tony
 Hansen, Gordon Mohr, John Myers, Chris Newman, and Andrew Sieber.
 Text used in this document is based on earlier RFCs describing
 specific uses of various base encodings.  The author acknowledges the
 RSA Laboratories for supporting the work that led to this document.

Josefsson Informational [Page 11] RFC 3548 The Base16, Base32, and Base64 Data Encodings July 2003

11. Editor's Address

 Simon Josefsson
 EMail: simon@josefsson.org

Josefsson Informational [Page 12] RFC 3548 The Base16, Base32, and Base64 Data Encodings July 2003

12. Full Copyright Statement

 Copyright (C) The Internet Society (2003).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assignees.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Josefsson Informational [Page 13]

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