📄 rfc 3548 (rfc3548) - the base16, base32, and base64 data encodings.htm
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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,
(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.
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
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
(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, <A href="http://www.faqs.org/rfcs/rfc2119.html">RFC 2119</A>, 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",
<A href="http://www.faqs.org/rfcs/rfc2045.html">RFC 2045</A>, November 1996.
[4] Callas, J., Donnerhacke, L., Finney, H. and R. Thayer, "OpenPGP
Message Format", <A href="http://www.faqs.org/rfcs/rfc2440.html">RFC 2440</A>, November 1998.
[5] Eastlake, D., "Domain Name System Security Extensions", <A href="http://www.faqs.org/rfcs/rfc2535.html">RFC 2535</A>,
March 1999.
[6] Klyne, G. and L. Masinter, "Identifying Composite Media
Features", <A href="http://www.faqs.org/rfcs/rfc2938.html">RFC 2938</A>, 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 <A href="http://zgp.org/pipermail/p2p-hackers/2001-">http://zgp.org/pipermail/p2p-hackers/2001-</A>
September/000315.html, September 2001.
[9] Cerf, V., "ASCII format for Network Interchange", <A href="http://www.faqs.org/rfcs/rfc20.html">RFC 20</A>, 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.
11. Editor's Address
Simon Josefsson
EMail: <A href="mailto:simon@josefsson.org">simon@josefsson.org</A>
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.
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<P><FONT face=Arial>Comments about this RFC:</FONT></P>
<UL>
<LI><FONT face=Arial><A href="http://www.faqs.org/qa/rfcc-2348.html">RFC
3548: For base32, padding characters are not necessary for representing
discrete...</A> by Jim Garrigues (8/15/2005)</FONT>
<LI><FONT face=Arial><A href="http://www.faqs.org/qa/rfcc-2408.html">RFC
3548: Why are there not easy to use programs? cpay13 </A>by cpay13
(9/3/2005)</FONT>
<LI><FONT face=Arial><A href="http://www.faqs.org/qa/rfcc-2347.html">RFC
3548: I would suggest changing the alphabet for base32 to
0123456789ABCDEFGHJKLMNPQRTU...</A> by Jim Garrigues (8/15/2005)</FONT>
<LI><FONT face=Arial><A href="http://www.faqs.org/qa/rfcc-1940.html">RFC
3548: 1?base64 encoding is 33% more compact than Hex encoding, but
lexicographical...</A> by ChM (4/27/2005)</FONT>
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