base64 Encoding

Sending binary data over a protocol that expects text can sometimes cause problems. One solution to this problem is to use base64 encoding.  It converts binary data to text.  This code snippet is an example of how to do that.

#include <string.h>
#include <stdint.h>

//This is a helper function to convert a six-bit value to base64
char base64_encode_six(uint8_t six_bit_value){
	uint8_t x;
	char c;
	x = six_bit_value & ~0xC0; //remove top two bits (should be zero anyway)
	if( x < 26 ){
		c = 'A' + x;
	} else if ( x < 52 ){
		c = 'a' + (x - 26);
	} else if( x < 62 ){
		c = '0' + (x - 52);
	} else if( x == 62 ){
		c = '+';
	} else if (x == 63 ){
		c = '/';
	} else {
		printf("ERROR IN BASE 64\n");
		c = 'A';
	}
	return c;
}

//This is the function for encoding in base64
void base64_encode(char * dest, const char * src){
	int bits;
	int i;
	int j;
	int k;
	int len;
	uint8_t six_bits[4];
	len = strlen(src);

	k = 0;
	//We need to encode three bytes at a time in to four encoded bytes
	for(i=0; i < len; i+=3){
		//First the thress bytes are broken down into six-bit sections
		six_bits[0] = (src[i] >> 2) & 0x3F; 
		six_bits[1] = ((src[i] << 4) & 0x30) + ((src[i+1]>>4) & 0x0F);
		six_bits[2] = ((src[i+1] << 2) & 0x3C) + ((src[i+2]>>6) & 0x03);
		six_bits[3] = src[i+2] & 0x3F;
		//now we use the helper function to convert from six-bits to base64
		for(j=0; j < 4; j++){
			dest[k+j] = base64_encode_six(six_bits[j]);
		}
		k+=4;
	}

	//at the end, we add = if the input is not divisible by 3
	if( (len % 3) == 1 ){
		//two equals at end
		dest[k-2] = '=';
		dest[k-1] = '=';
	} else if ( (len %3 ) == 2 ){
		dest[k-1] = '=';
	}

	//finally, zero terminate the output string
	dest[k] = 0;
}