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The following is an excerpt from RFC 1341 which defines Base64 encoding.
Network Working Group N. Borenstein, Bellcore
Request for Comments: 1341 N. Freed, Innosoft
June 1992
MIME (Multipurpose Internet Mail Extensions):
Mechanisms for Specifying and Describing
the Format of Internet Message Bodies
Excerpt Defining Base64 Encoding
5.2 Base64 Content-Transfer-Encoding
The Base64 Content-Transfer-Encoding is designed to
represent arbitrary sequences of octets in a form that is
not humanly readable. The encoding and decoding algorithms
are simple, but the encoded data are consistently only about
33 percent larger than the unencoded data. This encoding is
based on the one used in Privacy Enhanced Mail applications,
as defined in RFC 1113. The base64 encoding is adapted
from RFC 1113, with one change: base64 eliminates the "*"
mechanism for embedded clear text.
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.)
NOTE: This subset has the important property that it is
represented identically in all versions of ISO 646,
including US ASCII, and all characters in the subset are
also represented identically in all versions of EBCDIC.
Other popular encodings, such as the encoding used by the
UUENCODE utility and the base85 encoding specified as part
of Level 2 PostScript, do not share these properties, and
thus do not fulfill the portability requirements a binary
transport encoding for mail must meet.
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 base64 alphabet. When
encoding a bit stream via the base64 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 byte, and the eighth
bit will be the low-order bit in the first byte, and so on.
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. These characters, identified
in Table 1, below, are selected so as to be universally
representable, and the set excludes characters with
particular significance to SMTP (e.g., ".", "CR", "LF") and
to the encapsulation boundaries defined in this document
(e.g., "-").
Table 1: The Base64 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
The output stream (encoded bytes) must be represented in
lines of no more than 76 characters each. All line breaks
or other characters not found in Table 1 must be ignored by
decoding software. In base64 data, characters other than
those in Table 1, line breaks, and other white space
probably indicate a transmission error, about which a
warning message or even a message rejection might be
appropriate under some circumstances.
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 body.
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. Output character positions
which are not required to represent actual input data are
set to the character "=". Since all base64 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.
Care must be taken to use the proper octets for line breaks
if base64 encoding is applied directly to text material that
has not been converted to canonical form. In particular,
text line breaks should be converted into CRLF sequences
prior to base64 encoding. The important thing to note is
that this may be done directly by the encoder rather than in
a prior canonicalization step in some implementations.
NOTE: There is no need to worry about quoting apparent
encapsulation boundaries within base64-encoded parts of
multipart entities because no hyphen characters are used in
the base64 encoding.
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