Understanding Internet Security - Implementing SSL/TLS Using Cryptography and PKI - page 18

6 9 C 3 D 7

011010011100001111010111

0 11010 011100001111000111

26 28 15 7

ABC... G H I ... O P Q ... 3 4 5 6 ...

Figure 1-3: Base64 Encoding

As you see in Listing 1-14, Base64 encoding is pretty simple to implement
after you understand it; most of the complexity deals with non-aligned input:

Listing 1-14: “base64.c” base64_encode

static char *base64 =
“ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/”;
void base64_encode( const unsigned char *input, int len, unsigned char *output )
{
do
{
*output++ = base64[ ( input[ 0 ] & 0xFC ) >> 2 ];
if ( len == 1 )
{
*output++ = base64[ ( ( input[ 0 ] & 0x03 ) << 4 ) ];
*output++ = '=';
*output++ = '=';
break;
}

*output++ = base64[
( ( input[ 0 ] & 0x03 ) << 4 ) | ( ( input[ 1 ] & 0xF0 ) >> 4 ) ];

if ( len == 2 )
{
*output++ = base64[ ( ( input[ 1 ] & 0x0F ) << 2 ) ];
*output++ = '=';
break;
}

*output++ = base64[