See http://en.wikipedia.org/wiki/Base64 for more detail.
Suppose we have this string, encoded in UTF-8, and we want to convert into an
ASCII-only form for storage or transmission.
abç ✂
Note that the special character is U+2702 BLACK SCISSORS.
Here's the UTF-8 version of this string:
[ a ] [ b ] [ ç ] [space ] [ ✂ ]
01100001 01100010 11000011 10100111 00100000 11100010 10011100 10000010
0x61 0x62 0xC3 0xA7 0x20 0xE2 0x9C 0x82
Now smoosh all the bits together:
0110000101100010110000111010011100100000111000101001110010000010
Break them into 6-bit chunks:
011000 010110 001011 000011 101001 110010 000011 100010 100111 001000 0010
If the last group isn't 6 bits, pad it with zeros:
011000 010110 001011 000011 101001 110010 000011 100010 100111 001000 001000
And for our convenience in the next step, let's convert those into decimal integers:
24 22 11 3 41 50 3 34 39 8 8
Use the six-bit integers as indexes into this chart:
0 A 16 Q 32 g 48 w
1 B 17 R 33 h 49 x
2 C 18 S 34 i 50 y
3 D 19 T 35 j 51 z
4 E 20 U 36 k 52 0
5 F 21 V 37 l 53 1
6 G 22 W 38 m 54 2
7 H 23 X 39 n 55 3
8 I 24 Y 40 o 56 4
9 J 25 Z 41 p 57 5
10 K 26 a 42 q 58 6
11 L 27 b 43 r 59 7
12 M 28 c 44 s 60 8
13 N 29 d 45 t 61 9
14 O 30 e 46 u 62 +
15 P 31 f 47 v 63 /
Which in this case yields:
YWLDpyDinII
Finally, if the number of characters in the result is not a multiple of 4,
pad it out with enough equals-signs to make it so:
YWLDpyDinII=
And that, friends, is the base64 encoding of the byte sequence we started with&emdash;that is,
the UTF-8 encoding of the string
abç ✂
Here's a little bit of Python code that will compute the same result:
import base64
originalString = 'ab\xC3\xA7 \xE2\x9C\x82'
print 'The original string: %s' % originalString
encodedString = base64.b64encode(originalString)
print 'The base64-encoded version of the string: %s' % encodedString
decodedString = base64.b64decode(encodedString)
print 'The re-decoded version of the string: %s' % decodedString