Network Working Group S. Josefsson (editor)
Internet-Draft Extundo
Expires: November 2, 2001 May 4, 2001
Base Encodings
draft-josefsson-base-encoding-02.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on November 2, 2001.
Distribution of this document is unlimited. Comments and
suggestions on this document are encouraged.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This draft contain descriptions of the commonly used base 16, base
32, and base 64 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 (where applicable).
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 [3].
Josefsson (editor) Expires November 2, 2001 [Page 1]
Internet-Draft Base Encodings May 2001
Table of Contents
1. Implementation variances . . . . . . . . . . . . . . . . . . . 3
1.1 Line feeds in encoded data . . . . . . . . . . . . . . . . . . 3
1.2 Padding of encoded data . . . . . . . . . . . . . . . . . . . 3
1.3 Interpretation of non-alphabet characters in encoded data . . 3
2. Base 64 Encoding . . . . . . . . . . . . . . . . . . . . . . . 4
3. Base 32 Encoding . . . . . . . . . . . . . . . . . . . . . . . 6
4. Base 16 Encoding . . . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 10
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 11
Josefsson (editor) Expires November 2, 2001 [Page 2]
Internet-Draft Base Encodings May 2001
1. Implementation variances
Base encodings have historically been implemented with some minor
variances. This section's purpose is to describe these variances,
and to mandate a default behaviour, to reduce the possibility for
ambiguity in other documents using base encodings.
1.1 Line feeds in encoded data
RFC 2045 [4] is often used as a reference for base64 encoding.
However, RFC2045 does not define "base64" per se, but rather a
"base64 Content-Transfer-Encoding" for use within MIME. RFC 2045
enforces a limit on line length of base64 encode data to 76
characters.
Implementation of specifications using this document as reference
for base encodings MUST NOT add line feeds to the encoded data,
unless explicitely stated and handled otherwise in said
specifications.
1.2 Padding of encoded data
In some circumstances, the use of padding ("=") in base encoded data
is not required nor used.
Implementation of specifications using this document as reference
for base encodings MUST do proper padding to the encoded data,
unless explicitely stated and handled otherwise in said
specifications.
1.3 Interpretation of non-alphabet characters in encoded data
Base encodings use a specific, reduced, alphabet to encode binary
data. Non base alphabet characters may exist within base encoded
data, caused by data corruption or by design.
Implementations of specifications using this document as reference
for base encodings MUST ignore characters outside the base encoding
alphabet when interpreting base encoded data (``be liberal in what
you accept''), unless explicitely stated and handled otherwise in
said specifications.
(Note that this means, e.g. CRLF-padding after 76 characters
constitue "non alphabet characters", and should simply be ignored.
Also, the pad character, "=", should not be regarded as part of the
base alphabet until the end of the string.)
Josefsson (editor) Expires November 2, 2001 [Page 3]
Internet-Draft Base Encodings May 2001
2. Base 64 Encoding
The following description of base64 is due to [1], [4] and [5].
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.
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:
Josefsson (editor) Expires November 2, 2001 [Page 4]
Internet-Draft Base Encodings May 2001
(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 (editor) Expires November 2, 2001 [Page 5]
Internet-Draft Base Encodings May 2001
3. Base 32 Encoding
The following description of base32 is due to [6] (the padding
section has been corrected though).
The Base32 encoding is designed to represent arbitrary sequences of
octets in a form that needs to be case insensitive but need not be
humanly readable.
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
base32 alphabet. When encoding a bit stream via the base32
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 2: The Base32 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
Josefsson (editor) Expires November 2, 2001 [Page 6]
Internet-Draft Base Encodings May 2001
base32 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,
(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.
Because it is used only for padding at the end of the data, the
occurrence of any "=" characters may be taken as evidence that the
end of the data has been reached (without truncation in transit).
No such assurance is possible, however, when the number of octets
transmitted was a multiple of three and no "=" characters are
present.
Josefsson (editor) Expires November 2, 2001 [Page 7]
Internet-Draft Base Encodings May 2001
4. Base 16 Encoding
The following description is original but analogous to previous
descriptions.
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.
This draft describe two alphabets used with Base 16 encoding, the
first is considered to be more common. A variation of the first,
"lowercase hex format", uses lower case characters for values 10-15.
Table 3: The "Hex" 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
Table 4: The Canonical Base 16 Alphabet
Value Encoding Value Encoding Value Encoding Value Encoding
0 A 4 E 8 I 12 M
1 B 5 F 9 J 13 N
2 C 6 G 10 K 14 O
3 D 7 H 11 L 15 P
Unlike base32 and base64, no special padding is necessery since a
full code word is always available.
Josefsson (editor) Expires November 2, 2001 [Page 8]
Internet-Draft Base Encodings May 2001
5. Security Considerations
This draft does not discuss security.
Acknowledgement
I'd like to thank Tony Hansen for comments on the draft.
Josefsson (editor) Expires November 2, 2001 [Page 9]
Internet-Draft Base Encodings May 2001
References
[1] Linn, J., "Privacy Enhancement for Internet Electronic Mail:
Part I -- Message Encipherment and Authentication Procedures",
RFC 1113, August 1989.
[2] Elz, R., "A Compact Representation of IPv6 Addresses", RFC
1924, April 1996.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[4] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
[5] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[6] Myers, J., "SASL GSSAPI mechanisms", draft
draft-ietf-cat-sasl-gssapi-01, May 2000.
Author's Address
Simon Josefsson
Extundo
Drottningholmsvgen 70
Stockholm 112 42
Sweden
Phone: +46 8 6190422
EMail: simon@josefsson.org
Josefsson (editor) Expires November 2, 2001 [Page 10]
Internet-Draft Base Encodings May 2001
Full Copyright Statement
Copyright (C) The Internet Society (2001). 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 assigns.
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.
Josefsson (editor) Expires November 2, 2001 [Page 11]