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Base64 Encoding
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This tutorial helps you understand:
- Base64 Encoding Algorithm
- W3C Implementation
- Sun Implementation
Base64 Encoding Algorithm
Base64 algorithm is designed to encode any binary data, an stream of bytes, into
a stream of 64-printable characters.
Base64 encoding algorithm was first presented in "RFC 1421 - Privacy Enhancement
for Internet Electronic Mail:
Part I: Message Encryption and Authentication Procedures" in 1993 by John Linn.
It was later modified slightly in "RFC 1521 - MIME (Multipurpose Internet Mail Extensions)
Part One: Mechanisms for Specifying and
Describing the Format of Internet Message Bodies" in September 1993 by N. Borenstein, et al..
The 64 printable characters used by Base64:
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
15 P 32 g 49 x
16 Q 33 h 50 y
The encoding process is to:
- Divid the input bytes stream into blocks of 3 bytes.
- Divid the 24 bits of a 3-byte block into 4 groups of 6 bits.
- Map each group of 6 bits to 1 printable character, based on the 6-bit value.
- If the last 3-byte block has only 1 byte of input data, pad 2 bytes of zero (\x0000).
After encoding it as a normal block, override the last 2 characters with 2 equal signs (==),
so the decoding process knows 2 bytes of zero were padded.
- If the last 3-byte block has only 2 bytes of input data, pad 1 byte of zero (\x00).
After encoding it as a normal block, override the last 1 character with 1 equal signs (=),
so the decoding process knows 1 byte of zero was padded.
- Carriage return (\r) and new line (\n) are inserted into the output character stream.
They will be ignored by the decoding process.
Example 1: Input data, 1 byte, "A". Encoded output, 4 characters, "QQ=="
Input Data A
Input Bits 01000001
Padding 01000001 00000000 00000000
\ \ \
Bit Groups 010000 010000 000000 000000
Mapping Q Q A A
Overriding Q Q = =
Example 2: Input data, 2 bytes, "AB". Encoded output, 4 characters, "QUI="
Input Data A B
Input Bits 01000001 01000010
Padding 01000001 01000010 00000000
\ \ \
Bit Groups 010000 010100 001000 000000
Mapping Q U I A
Overriding Q U I =
Example 3: Input data, 3 bytes, "ABC". Encoded output, 4 characters, "QUJD"
Input Data A B C
Input Bits 01000001 01000010 01000011
\ \ \
Bit Groups 010000 010100 001001 000011
Mapping Q U J D
W3C Implementation
One of the Java implementations of Base64 algorithm available on the Internet is
from the Jigsaw project at w3c.org. The Base64 algorithm is implemented into 2 classes,
Base64Encoder and Base64Decoder in the org.w3c.tools.codec package. Here is how to
download this package, and make it available to your Java environment.
- Go to http://www.w3.org/Jigsaw/, and follow the instruction to download jigsaw_2.2.2.zip.
- Unzip jigsaw_2.2.2.zip to \local\jigsaw, and add \local\jigsaw\classes\jigsaw.jar to your
Java class path.
(Continued on next part...)
Part:
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