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 that need not be human 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 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.

(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.

---