Base-64 encoding takes a sequence of 8-bit bytes, breaks the sequence into 6-bit pieces, and assigns each 6-bit piece to one of 64 characters comprising the base-64 alphabet. The 64 possible output characters are common and safe to place in HTTP header fields. The 64 characters include upper- and lowercase letters, numbers, +, and /. The special character = also is used. The base-64 alphabet is shown in Table E-1.

Note that because the base-64 encoding uses 8-bit characters to represent 6 bits of information, base 64-encoded strings are about 33% larger than the original values.

Table E-1. Base-64 alphabet

0 A 8 I 16 Q 24 Y 32 g 40 o 48 w 56 4
1 B 9 J 17 R 25 Z 33 h 41 p 49 x 57 5
2 C 10 K 18 S 26 a 34 i 42 q 50 y 58 6
3 D 11 L 19 T 27 b 35 j 43 r 51 z 59 7
4 E 12 M 20 U 28 c 36 k 44 s 52 0 60 8
5 F 13 N 21 V 29 d 37 l 45 t 53 1 61 9
6 G 14 O 22 W 30 e 38 m 46 u 54 2 62 +
7 H 15 P 23 X 31 f 39 n 47 v 55 3 63 /

Screenshot E-1 shows a simple example of base-64 encoding. Here, the three-character input value "Ow!" is base 64-encoded, resulting in the four-character base 64-encoded value "T3ch". It works like this:

1.       The string "Ow!" is broken into 3 8-bit bytes (0x4F, 0x77, 0x21).

2.       The 3 bytes create the 24-bit binary value 010011110111011100100001.

3.       These bits are segmented into the 6-bit sequences 010011, 110111, 01110, 100001.

4.       Each of these 6-bit values represents a number from 0 to 63, corresponding to one of 64 characters in the base-64 alphabet. The resulting base 64-encoded string is the 4-character string "T3ch", which can then be sent across the wire as "safe" 8-bit characters, because only the most portable characters are used (letters, numbers, etc.).

Screenshot E-1. Base-64 encoding example

 

Hypertext Transfer Protocol (HTTP)