The Base16, Base32, and Base64 Data Encodings
draft-josefsson-rfc3548bis-04
The information below is for an old version of the document that is already published as an RFC.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 4648.
|
|
|---|---|---|---|
| Author | Simon Josefsson | ||
| Last updated | 2020-01-21 (Latest revision 2006-05-12) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Reviews | |||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 4648 (Proposed Standard) | |
| Action Holders |
(None)
|
||
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Ted Hardie | ||
| Send notices to | (None) |
draft-josefsson-rfc3548bis-04
Network Working Group S. Josefsson
Internet-Draft SJD
Obsoletes: 3548 (if approved) May 11, 2006
Expires: November 12, 2006
The Base16, Base32, and Base64 Data Encodings
draft-josefsson-rfc3548bis-04
Status of this Memo
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This Internet-Draft will expire on November 12, 2006.
Copyright Notice
Copyright (C) The Internet Society (2006).
Keywords
Base Encoding, Base64, Base32, Base16, Hex.
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, use of different encoding alphabets, and
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canonical encodings.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in this Document . . . . . . . . . . . . . . 3
3. Implementation Discrepancies . . . . . . . . . . . . . . . . . 3
3.1. Line Feeds In Encoded Data . . . . . . . . . . . . . . . . 3
3.2. Padding Of Encoded Data . . . . . . . . . . . . . . . . . 4
3.3. Interpretation Of Non-Alphabet Characters In Encoded
data . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.4. Choosing The Alphabet . . . . . . . . . . . . . . . . . . 4
3.5. Canonical Encoding . . . . . . . . . . . . . . . . . . . . 6
4. Base 64 Encoding . . . . . . . . . . . . . . . . . . . . . . . 6
5. Base 64 Encoding With URL And Filename Safe Alphabet . . . . . 9
6. Base 32 Encoding . . . . . . . . . . . . . . . . . . . . . . . 10
7. Base 32 Encoding With Extended Hex Alphabet . . . . . . . . . 11
8. Base 16 Encoding . . . . . . . . . . . . . . . . . . . . . . . 13
9. Illustrations And Examples . . . . . . . . . . . . . . . . . . 14
10. Test Vectors . . . . . . . . . . . . . . . . . . . . . . . . . 15
11. ISO C99 Implementation Of Base64 . . . . . . . . . . . . . . . 16
11.1. Prototypes: base64.h . . . . . . . . . . . . . . . . . . . 16
11.2. Implementation: base64.c . . . . . . . . . . . . . . . . . 18
12. Security Considerations . . . . . . . . . . . . . . . . . . . 27
13. Changes Since RFC 3548 . . . . . . . . . . . . . . . . . . . . 27
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
15. Copying Conditions . . . . . . . . . . . . . . . . . . . . . . 28
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
16.1. Normative References . . . . . . . . . . . . . . . . . . . 28
16.2. Informative References . . . . . . . . . . . . . . . . . . 29
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 30
Intellectual Property and Copyright Statements . . . . . . . . . . 31
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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 [1] 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. Multipurpose Internet Mail Extensions (MIME) [4] 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.
2. Conventions Used in this Document
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 [2].
3. 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.
3.1. Line Feeds In Encoded Data
MIME [4] 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 Privacy Enhanced Mail (PEM) [3] 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 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.
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3.2. 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.
The base64 and base32 alphabets use padding, as described below in
section 4 and 6, but the base16 alphabet does not need it, see
section 8.
3.3. 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 encoded data 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 adjacent carriage return/
line feed (CRLF) characters constitute "non-alphabet characters" and
are ignored. Furthermore, such specifications MAY ignore the pad
character, "=", treating it as non-alphabet data, if it is present
before the end of the encoded data. 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 MAY also be
ignored.
3.4. 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:
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o Handled by humans. Characters "0", "O" are easily confused, as
well as "1", "l" and "I". In the base32 alphabet below, where 0
(zero) and 1 (one) are 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.)
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o Encoded into structures that mandate 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. For an example of a highly specialized variant, see
IMAP [8]. In this document, we document and name some currently used
alphabets.
3.5. Canonical Encoding
The padding step in base 64 and base 32 encoding can, if improperly
implemented, lead to non-significant alterations of the encoded data.
For example, if the input is only one octet for a base 64 encoding,
then all six bits of the first symbol are used, but only the first
two bits of the next symbol are used. These pad bits MUST be set to
zero by conforming encoders, which is described in the descriptions
on padding below. If this property do not hold, there is no
canonical representation of base encoded data, and multiple base
encoded strings can be decoded to the same binary data. If this
property (and others discussed in this document) holds, a canonical
encoding is guaranteed.
In some environments, the alteration is critical and therefor
decoders MAY chose to reject an encoding if the pad bits have not
been set to zero. The specification referring to this may mandate a
specific behaviour.
4. Base 64 Encoding
The following description of base 64 is derived from [3], [4], [5]
and [6]. This encoding may be referred to as "base64".
The Base 64 encoding is designed to represent arbitrary sequences of
octets in a form that allows the use of both upper- and lowercase
letters 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
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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 character 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, bits with value zero 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
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(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.
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5. Base 64 Encoding With URL And Filename Safe Alphabet
The Base 64 encoding with an URL and filename safe alphabet has been
used in [12].
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. The remaining unreserved URI character is ".",
but some file system environments does not permit multiple "." in a
filename, thus making the "." character unattractive as well.
The pad character "=" is typically percent-encoded when used in an
URI [9], but if the data length is known implicitly, this can be
avoided by skipping the padding, see section 3.2.
This encoding may be referred to as "base64url". 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 _
13 N 30 e 47 v (underline)
14 O 31 f 48 w
15 P 32 g 49 x
16 Q 33 h 50 y (pad) =
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6. Base 32 Encoding
The following description of base 32 is derived from [11] (with
corrections). This encoding may be referred to as "base32".
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.
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 character 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 3, 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, bits with value zero 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
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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.
7. Base 32 Encoding With Extended Hex Alphabet
The following description of base 32 is derived from [7]. This
encoding may be referred to as "base32hex". This encoding should not
be regarded as the same as the "base32" encoding, and should not be
referred to as only "base32". This encoding is used by, e.g., NSEC3
[10]
One property with this alphabet, that the base64 and base32 alphabet
lack, is that encoded data maintain its sort order when the encoded
data is compared bit-wise.
This encoding is identical to the previous one, except for the
alphabet. The new alphabet is found in table 4.
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Table 4: The "Extended Hex" Base 32 Alphabet
Value Encoding Value Encoding Value Encoding Value Encoding
0 0 9 9 18 I 27 R
1 1 10 A 19 J 28 S
2 2 11 B 20 K 29 T
3 3 12 C 21 L 30 U
4 4 13 D 22 M 31 V
5 5 14 E 23 N
6 6 15 F 24 O (pad) =
7 7 16 G 25 P
8 8 17 H 26 Q
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8. Base 16 Encoding
The following description is original but analogous to previous
descriptions. Essentially, Base 16 encoding is the 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 character 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.
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9. 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 [5].
+--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
[7]. 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 [5], with corrections.
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Input data: 0x14fb9c03d97e
Hex: 1 4 f b 9 c | 0 3 d 9 7 e
8-bit: 00010100 11111011 10011100 | 00000011 11011001 01111110
6-bit: 000101 001111 101110 011100 | 000000 111101 100101 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 = =
10. Test Vectors
BASE64("") = ""
BASE64("f") = "Zg=="
BASE64("fo") = "Zm8="
BASE64("foo") = "Zm9v"
BASE64("foob") = "Zm9vYg=="
BASE64("fooba") = "Zm9vYmE="
BASE64("foobar") = "Zm9vYmFy"
BASE32("") = ""
BASE32("f") = "MY======"
BASE32("fo") = "MZXQ===="
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BASE32("foo") = "MZXW6==="
BASE32("foob") = "MZXW6YQ="
BASE32("fooba") = "MZXW6YTB"
BASE32("foobar") = "MZXW6YTBOI======"
BASE32-HEX("") = ""
BASE32-HEX("f") = "CO======"
BASE32-HEX("fo") = "CPNG===="
BASE32-HEX("foo") = "CPNMU==="
BASE32-HEX("foob") = "CPNMUOG="
BASE32-HEX("fooba") = "CPNMUOJ1"
BASE32-HEX("foobar") = "CPNMUOJ1E8======"
BASE16("") = ""
BASE16("f") = "66"
BASE16("fo") = "666F"
BASE16("foo") = "666F6F"
BASE16("foob") = "666F6F62"
BASE16("fooba") = "666F6F6261"
BASE16("foobar") = "666F6F626172"
11. ISO C99 Implementation Of Base64
Below is an ISO C99 implementation of Base64 encoding and decoding.
The code assume that the US-ASCII characters are encoding inside
'char' with values below 255, which holds for all POSIX platforms,
but should otherwise be portable. This code is not intended as a
normative specification of base64.
11.1. Prototypes: base64.h
/* base64.h -- Encode binary data using printable characters.
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Copyright (C) 2004, 2005, 2006 Free Software Foundation, Inc.
Written by Simon Josefsson.
This program is free software; you can redistribute it
and/or modify it under the terms of the GNU Lesser
General Public License as published by the Free Software
Foundation; either version 2.1, or (at your option) any
later version.
This program is distributed in the hope that it will be
useful, but WITHOUT ANY WARRANTY; without even the
implied warranty of MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE. See the GNU Lesser General Public
License for more details.
You can retrieve a copy of the GNU Lesser General Public
License from http://www.gnu.org/licenses/lgpl.txt; or by
writing to the Free Software Foundation, Inc., 51
Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */
#ifndef BASE64_H
# define BASE64_H
/* Get size_t. */
# include <stddef.h>
/* Get bool. */
# include <stdbool.h>
/* This uses that the expression (n+(k-1))/k means the
smallest integer >= n/k, i.e., the ceiling of n/k. */
# define BASE64_LENGTH(inlen) ((((inlen) + 2) / 3) * 4)
extern bool isbase64 (char ch);
extern void base64_encode (const char *restrict in,
size_t inlen,
char *restrict out,
size_t outlen);
extern size_t base64_encode_alloc (const char *in,
size_t inlen,
char **out);
extern bool base64_decode (const char *restrict in,
size_t inlen,
char *restrict out,
size_t *outlen);
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extern bool base64_decode_alloc (const char *in,
size_t inlen,
char **out,
size_t *outlen);
#endif /* BASE64_H */
11.2. Implementation: base64.c
/* base64.c -- Encode binary data using printable characters.
Copyright (C) 1999, 2000, 2001, 2004, 2005, 2006 Free Software
Foundation, Inc.
This program is free software; you can redistribute it
and/or modify it under the terms of the GNU Lesser
General Public License as published by the Free Software
Foundation; either version 2.1, or (at your option) any
later version.
This program is distributed in the hope that it will be
useful, but WITHOUT ANY WARRANTY; without even the
implied warranty of MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE. See the GNU Lesser General Public
License for more details.
You can retrieve a copy of the GNU Lesser General Public
License from http://www.gnu.org/licenses/lgpl.txt; or by
writing to the Free Software Foundation, Inc., 51
Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */
/* Written by Simon Josefsson. Partially adapted from GNU
* MailUtils (mailbox/filter_trans.c, as of 2004-11-28).
* Improved by review from Paul Eggert, Bruno Haible, and
* Stepan Kasal.
*
* Be careful with error checking. Here is how you would
* typically use these functions:
*
* bool ok = base64_decode_alloc (in, inlen, &out, &outlen);
* if (!ok)
* FAIL: input was not valid base64
* if (out == NULL)
* FAIL: memory allocation error
* OK: data in OUT/OUTLEN
*
* size_t outlen = base64_encode_alloc (in, inlen, &out);
* if (out == NULL && outlen == 0 && inlen != 0)
* FAIL: input too long
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* if (out == NULL)
* FAIL: memory allocation error
* OK: data in OUT/OUTLEN.
*
*/
/* Get prototype. */
#include "base64.h"
/* Get malloc. */
#include <stdlib.h>
/* Get UCHAR_MAX. */
#include <limits.h>
/* C89 compliant way to cast 'char' to 'unsigned char'. */
static inline unsigned char
to_uchar (char ch)
{
return ch;
}
/* Base64 encode IN array of size INLEN into OUT array of
size OUTLEN. If OUTLEN is less than
BASE64_LENGTH(INLEN), write as many bytes as possible.
If OUTLEN is larger than BASE64_LENGTH(INLEN), also zero
terminate the output buffer. */
void
base64_encode (const char *restrict in, size_t inlen,
char *restrict out, size_t outlen)
{
static const char b64str[64] =
"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
"abcdefghijklmnopqrstuvwxyz0123456789+/";
while (inlen && outlen)
{
*out++ = b64str[to_uchar (in[0]) >> 2];
if (!--outlen)
break;
*out++ = b64str[((to_uchar (in[0]) << 4)
+ (--inlen ? to_uchar (in[1]) >> 4 : 0))
& 0x3f];
if (!--outlen)
break;
*out++ =
(inlen
? b64str[((to_uchar (in[1]) << 2)
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+ (--inlen ? to_uchar (in[2]) >> 6 : 0))
& 0x3f]
: '=');
if (!--outlen)
break;
*out++ = inlen ? b64str[to_uchar (in[2]) & 0x3f] : '=';
if (!--outlen)
break;
if (inlen)
inlen--;
if (inlen)
in += 3;
}
if (outlen)
*out = '\0';
}
/* Allocate a buffer and store zero terminated base64
encoded data from array IN of size INLEN, returning
BASE64_LENGTH(INLEN), i.e., the length of the encoded
data, excluding the terminating zero. On return, the OUT
variable will hold a pointer to newly allocated memory
that must be deallocated by the caller. If output string
length would overflow, 0 is returned and OUT is set to
NULL. If memory allocation fail, OUT is set to NULL, and
the return value indicate length of the requested memory
block, i.e., BASE64_LENGTH(inlen) + 1. */
size_t
base64_encode_alloc (const char *in, size_t inlen, char **out)
{
size_t outlen = 1 + BASE64_LENGTH (inlen);
/* Check for overflow in outlen computation.
*
* If there is no overflow, outlen >= inlen.
*
* If the operation (inlen + 2) overflows then it yields
* at most +1, so outlen is 0.
*
* If the multiplication overflows, we lose at least half
* of the correct value, so the result is < ((inlen +
* 2) / 3) * 2, which is less than (inlen + 2) * 0.66667,
* which is less than inlen as soon as (inlen > 4).
*/
if (inlen > outlen)
{
*out = NULL;
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return 0;
}
*out = malloc (outlen);
if (*out)
base64_encode (in, inlen, *out, outlen);
return outlen - 1;
}
/* With this approach this file works independent of the
charset used (think EBCDIC). However, it does assume
that the characters in the Base64 alphabet (A-Za-z0-9+/)
are encoded in 0..255. POSIX 1003.1-2001 require that
char and unsigned char are 8-bit quantities, though,
taking care of that problem. But this may be a potential
problem on non-POSIX C99 platforms. */
#define B64(x) \
((x) == 'A' ? 0 \
: (x) == 'B' ? 1 \
: (x) == 'C' ? 2 \
: (x) == 'D' ? 3 \
: (x) == 'E' ? 4 \
: (x) == 'F' ? 5 \
: (x) == 'G' ? 6 \
: (x) == 'H' ? 7 \
: (x) == 'I' ? 8 \
: (x) == 'J' ? 9 \
: (x) == 'K' ? 10 \
: (x) == 'L' ? 11 \
: (x) == 'M' ? 12 \
: (x) == 'N' ? 13 \
: (x) == 'O' ? 14 \
: (x) == 'P' ? 15 \
: (x) == 'Q' ? 16 \
: (x) == 'R' ? 17 \
: (x) == 'S' ? 18 \
: (x) == 'T' ? 19 \
: (x) == 'U' ? 20 \
: (x) == 'V' ? 21 \
: (x) == 'W' ? 22 \
: (x) == 'X' ? 23 \
: (x) == 'Y' ? 24 \
: (x) == 'Z' ? 25 \
: (x) == 'a' ? 26 \
: (x) == 'b' ? 27 \
: (x) == 'c' ? 28 \
: (x) == 'd' ? 29 \
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: (x) == 'e' ? 30 \
: (x) == 'f' ? 31 \
: (x) == 'g' ? 32 \
: (x) == 'h' ? 33 \
: (x) == 'i' ? 34 \
: (x) == 'j' ? 35 \
: (x) == 'k' ? 36 \
: (x) == 'l' ? 37 \
: (x) == 'm' ? 38 \
: (x) == 'n' ? 39 \
: (x) == 'o' ? 40 \
: (x) == 'p' ? 41 \
: (x) == 'q' ? 42 \
: (x) == 'r' ? 43 \
: (x) == 's' ? 44 \
: (x) == 't' ? 45 \
: (x) == 'u' ? 46 \
: (x) == 'v' ? 47 \
: (x) == 'w' ? 48 \
: (x) == 'x' ? 49 \
: (x) == 'y' ? 50 \
: (x) == 'z' ? 51 \
: (x) == '0' ? 52 \
: (x) == '1' ? 53 \
: (x) == '2' ? 54 \
: (x) == '3' ? 55 \
: (x) == '4' ? 56 \
: (x) == '5' ? 57 \
: (x) == '6' ? 58 \
: (x) == '7' ? 59 \
: (x) == '8' ? 60 \
: (x) == '9' ? 61 \
: (x) == '+' ? 62 \
: (x) == '/' ? 63 \
: -1)
static const signed char b64[0x100] = {
B64 (0), B64 (1), B64 (2), B64 (3),
B64 (4), B64 (5), B64 (6), B64 (7),
B64 (8), B64 (9), B64 (10), B64 (11),
B64 (12), B64 (13), B64 (14), B64 (15),
B64 (16), B64 (17), B64 (18), B64 (19),
B64 (20), B64 (21), B64 (22), B64 (23),
B64 (24), B64 (25), B64 (26), B64 (27),
B64 (28), B64 (29), B64 (30), B64 (31),
B64 (32), B64 (33), B64 (34), B64 (35),
B64 (36), B64 (37), B64 (38), B64 (39),
B64 (40), B64 (41), B64 (42), B64 (43),
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B64 (44), B64 (45), B64 (46), B64 (47),
B64 (48), B64 (49), B64 (50), B64 (51),
B64 (52), B64 (53), B64 (54), B64 (55),
B64 (56), B64 (57), B64 (58), B64 (59),
B64 (60), B64 (61), B64 (62), B64 (63),
B64 (64), B64 (65), B64 (66), B64 (67),
B64 (68), B64 (69), B64 (70), B64 (71),
B64 (72), B64 (73), B64 (74), B64 (75),
B64 (76), B64 (77), B64 (78), B64 (79),
B64 (80), B64 (81), B64 (82), B64 (83),
B64 (84), B64 (85), B64 (86), B64 (87),
B64 (88), B64 (89), B64 (90), B64 (91),
B64 (92), B64 (93), B64 (94), B64 (95),
B64 (96), B64 (97), B64 (98), B64 (99),
B64 (100), B64 (101), B64 (102), B64 (103),
B64 (104), B64 (105), B64 (106), B64 (107),
B64 (108), B64 (109), B64 (110), B64 (111),
B64 (112), B64 (113), B64 (114), B64 (115),
B64 (116), B64 (117), B64 (118), B64 (119),
B64 (120), B64 (121), B64 (122), B64 (123),
B64 (124), B64 (125), B64 (126), B64 (127),
B64 (128), B64 (129), B64 (130), B64 (131),
B64 (132), B64 (133), B64 (134), B64 (135),
B64 (136), B64 (137), B64 (138), B64 (139),
B64 (140), B64 (141), B64 (142), B64 (143),
B64 (144), B64 (145), B64 (146), B64 (147),
B64 (148), B64 (149), B64 (150), B64 (151),
B64 (152), B64 (153), B64 (154), B64 (155),
B64 (156), B64 (157), B64 (158), B64 (159),
B64 (160), B64 (161), B64 (162), B64 (163),
B64 (164), B64 (165), B64 (166), B64 (167),
B64 (168), B64 (169), B64 (170), B64 (171),
B64 (172), B64 (173), B64 (174), B64 (175),
B64 (176), B64 (177), B64 (178), B64 (179),
B64 (180), B64 (181), B64 (182), B64 (183),
B64 (184), B64 (185), B64 (186), B64 (187),
B64 (188), B64 (189), B64 (190), B64 (191),
B64 (192), B64 (193), B64 (194), B64 (195),
B64 (196), B64 (197), B64 (198), B64 (199),
B64 (200), B64 (201), B64 (202), B64 (203),
B64 (204), B64 (205), B64 (206), B64 (207),
B64 (208), B64 (209), B64 (210), B64 (211),
B64 (212), B64 (213), B64 (214), B64 (215),
B64 (216), B64 (217), B64 (218), B64 (219),
B64 (220), B64 (221), B64 (222), B64 (223),
B64 (224), B64 (225), B64 (226), B64 (227),
B64 (228), B64 (229), B64 (230), B64 (231),
B64 (232), B64 (233), B64 (234), B64 (235),
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B64 (236), B64 (237), B64 (238), B64 (239),
B64 (240), B64 (241), B64 (242), B64 (243),
B64 (244), B64 (245), B64 (246), B64 (247),
B64 (248), B64 (249), B64 (250), B64 (251),
B64 (252), B64 (253), B64 (254), B64 (255)
};
#if UCHAR_MAX == 255
# define uchar_in_range(c) true
#else
# define uchar_in_range(c) ((c) <= 255)
#endif
bool
isbase64 (char ch)
{
return uchar_in_range (to_uchar (ch)) && 0 <= b64[to_uchar (ch)];
}
/* Decode base64 encoded input array IN of length INLEN to
output array OUT that can hold *OUTLEN bytes. Return
true if decoding was successful, i.e. if the input was
valid base64 data, false otherwise. If *OUTLEN is too
small, as many bytes as possible will be written to OUT.
On return, *OUTLEN holds the length of decoded bytes in
OUT. Note that as soon as any non-alphabet characters
are encountered, decoding is stopped and false is
returned. This means that, when applicable, you must
remove any line terminators that is part of the data
stream before calling this function. */
bool
base64_decode (const char *restrict in, size_t inlen,
char *restrict out, size_t *outlen)
{
size_t outleft = *outlen;
while (inlen >= 2)
{
if (!isbase64 (in[0]) || !isbase64 (in[1]))
break;
if (outleft)
{
*out++ = ((b64[to_uchar (in[0])] << 2)
| (b64[to_uchar (in[1])] >> 4));
outleft--;
}
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if (inlen == 2)
break;
if (in[2] == '=')
{
if (inlen != 4)
break;
if (in[3] != '=')
break;
}
else
{
if (!isbase64 (in[2]))
break;
if (outleft)
{
*out++ = (((b64[to_uchar (in[1])] << 4) & 0xf0)
| (b64[to_uchar (in[2])] >> 2));
outleft--;
}
if (inlen == 3)
break;
if (in[3] == '=')
{
if (inlen != 4)
break;
}
else
{
if (!isbase64 (in[3]))
break;
if (outleft)
{
*out++ = (((b64[to_uchar (in[2])] << 6) & 0xc0)
| b64[to_uchar (in[3])]);
outleft--;
}
}
}
in += 4;
inlen -= 4;
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}
*outlen -= outleft;
if (inlen != 0)
return false;
return true;
}
/* Allocate an output buffer in *OUT, and decode the base64
encoded data stored in IN of size INLEN to the *OUT
buffer. On return, the size of the decoded data is
stored in *OUTLEN. OUTLEN may be NULL, if the caller is
not interested in the decoded length. *OUT may be NULL
to indicate an out of memory error, in which case *OUTLEN
contain the size of the memory block needed. The
function return true on successful decoding and memory
allocation errors. (Use the *OUT and *OUTLEN parameters
to differentiate between successful decoding and memory
error.) The function return false if the input was
invalid, in which case *OUT is NULL and *OUTLEN is
undefined. */
bool
base64_decode_alloc (const char *in, size_t inlen, char **out,
size_t *outlen)
{
/* This may allocate a few bytes too much, depending on
input, but it's not worth the extra CPU time to compute
the exact amount. The exact amount is 3 * inlen / 4,
minus 1 if the input ends with "=" and minus another 1
if the input ends with "==". Dividing before
multiplying avoids the possibility of overflow. */
size_t needlen = 3 * (inlen / 4) + 2;
*out = malloc (needlen);
if (!*out)
return true;
if (!base64_decode (in, inlen, *out, &needlen))
{
free (*out);
*out = NULL;
return false;
}
if (outlen)
*outlen = needlen;
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return true;
}
12. 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 ignored characters
could also be used for other nefarious purposes, such as to avoid a
string equality comparison or to trigger implementation bugs. The
implications of ignoring non-alphabet characters 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 or
make string equality comparisons fail.
When padding is used, there are some non-significant bits that
warrant security concerns, they may be abused to leak information,
used to bypass string equality comparisons, or to trigger
implementation problems.
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.
Base encoding adds no entropy to the plaintext, but it does increase
the amount of plaintext available and provides a signature for
cryptanalysis in the form of a characteristic probability
distribution.
13. Changes Since RFC 3548
Added the "base32 extended hex alphabet", needed to preserve sort
order of encoded data.
Reference IMAP for the special Base64 encoding used there.
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Fix the example copied from RFC 2440.
Add security consideration about providing a signature for
cryptoanalysis.
Add test vectors and C99 implementation.
Typo fixes.
14. Acknowledgements
Several people offered comments and/or suggestions, including John E.
Hadstate, Tony Hansen, Gordon Mohr, John Myers, Chris Newman and
Andrew Sieber. Text used in this document are 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.
This revised version is based in parts on comments and/or suggestions
made by Roy Arends, Eric Blake, Brian E Carpenter, Elwyn Davies, Bill
Fenner, Sam Hartman, Ted Hardie, Per Hygum, Jelte Jansen, Clement
Kent, Tero Kivinen, Paul Kwiatkowski, and Ben Laurie.
15. Copying Conditions
Copyright (c) 2000-2006 Simon Josefsson
Regarding the abstract and section 1, 3, 8, 10, 12, 13, and 14 of
this document, that were written by Simon Josefsson ("the author",
for the remainder of this section), the author makes no guarantees
and is not responsible for any damage resulting from its use. The
author grants irrevocable permission to anyone to use, modify, and
distribute it in any way that does not diminish the rights of anyone
else to use, modify, and distribute it, provided that redistributed
derivative works do not contain misleading author or version
information and do not falsely purport to be IETF RFC documents.
Derivative works need not be licensed under similar terms.
16. References
16.1. Normative References
[1] Cerf, V., "ASCII format for network interchange", RFC 20,
October 1969.
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[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
16.2. Informative References
[3] Linn, J., "Privacy Enhancement for Internet Electronic Mail:
Part I: Message Encryption and Authentication Procedures",
RFC 1421, February 1993.
[4] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
[5] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
"OpenPGP Message Format", RFC 2440, November 1998.
[6] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[7] Klyne, G. and L. Masinter, "Identifying Composite Media
Features", RFC 2938, September 2000.
[8] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[9] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
January 2005.
[10] Laurie, B., "DNSSEC Hash Authenticated Denial of Existence",
draft-ietf-dnsext-nsec3-04 (work in progress), March 2006.
[11] Myers, J., "SASL GSSAPI mechanisms", Work in
progress draft-ietf-cat-sasl-gssapi-01, May 2000.
[12] 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.
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Author's Address
Simon Josefsson
SJD
Email: simon@josefsson.org
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Intellectual Property Statement
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except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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