TCP/IP Tutorial and Technical Overview - IBM Redbooks

characters that are known to be mail-safe. As noted in the previous section, there
are only 73 safe characters, so the maximum byte length usable is 6 bits, which
can be represented by 64 unique characters (thus the name Base64). Because
the input and output are both byte streams, the encoding has to be done in
groups of 24 bits (that is 3 input bytes and 4 output bytes). The process can be
seen as shown in Figure 15-8.
Figure 15-8 MIME: Base64 encoding - How 3 input bytes are converted to 4 output bytes
in the Base64 encoding scheme
The translate table used is called the Base64 alphabet, as shown in Table 15-4.
Table 15-4 The Base64 alphabet
Base64
value
ASCII
char
Base64
value
ASCII
char
Base64
value
ASCII
char
Base64
value
ASCII
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
Chapter 15. Mail applications 585