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 |
/ |

Figure 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:

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

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

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

Each of these ...

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