Base64 Encoding Algorithm

Data Encoding Tutorials - Herong's Tutorial Examples

∟Base64 Encoding Algorithm

This section describes the Base64 encoding algorithm with some simple encoding examples.

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 character set map used by Base64 encoding is listed below:

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 Base64 encoding process is to:

Divid the input bytes stream into blocks of 3 bytes.
Divid 24 bits of each 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 using the Base64 character set map.
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

Table of Contents

About This Book

►Base64 Encoding

►Base64 Encoding Algorithm

RFC 1421 - Privacy Enhancement for Email

RFC 1521 - MIME (Multipurpose Internet Mail Extensions)

W3C Implementation of Base64 in Java

Sun Implementation of Base64 in Java

Sun Implementation of Base64 in Java - Test

Goetz' Implementation of Base64 in JavaScript

Goetz' Implementation of Base64 in JavaScript - Test

Base64 Encoding and Decoding Tools

Base64URL - URL Safe Base64 Encoding

Base32 Encoding

URL Encoding, URI Encoding, or Percent Encoding

UUEncode Encoding

References

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