Decode Base64 Text Data
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Basics

There are many different alphanumeric systems in everyday use. For example: Base10, otherwise known as the decimal system, uses 10 different digits (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0) to convery information. Similary the British and American alphabets can be thought of as Base26 as they convey information using 26 different letters (A, B, C, D etc.) if you ignore punctuation marks etc. The full ASCII system uses 256 different characters (8 bit), either represented by the character or its corresponding number (0 to 255).

Base64 refers to a series of methods for numerically translating a string from using a combination of (in the case of ASCII text) 256 characters (8 bit) to only using 64 different characters (6 bit). The term Base64 originates from a specific MIME content transfer encoding.

Different methods of Base64 coding use different characters for the 64 different digits. The MIME Base64 format (on which this website is based) uses the following conversion table (from Wikipedia):

Value Char   Value Char   Value Char   Value Char
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 /

Worked example conversion: Decoding Base64 text to ASCII

Using the table below as an example, if we convert "Y2Fy" from Base64 we get "car":

Base64-encoded Y 2 F y
Index 24 54 5 50
Bit pattern 0 1 1 0 0 0 1 1 0 1 1 0 0 0 0 1 0 1 1 1 0 0 1 0
ASCII 99 97 114
Text content c a r

In this example, "Y2Fy" was converted to its corresponding numbers from the comparison table above: 24 54 5 50. These numbers are broken down into their binary equivalent consisting of groups of 6 1's and 0's. These groups of 6 are then regrouped into groups of 8. The 4 groups of 6 bits are now 3 groups of 8 bits which can be converted back into their ASCII equivalent.