splitflap

// https://raw.githubusercontent.com/zhicheng/base64/master/base64.c
/* This is a public domain base64 implementation written by WEI Zhicheng. */

#include <stdio.h>

#include "base64url.h"

/* BASE 64 encode table */
static const char base64en[] = {
	'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
	'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P',
	'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X',
	'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f',
	'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',
	'o', 'p', 'q', 'r', 's', 't', 'u', 'v',
	'w', 'x', 'y', 'z', '0', '1', '2', '3',
	'4', '5', '6', '7', '8', '9', '-', '_',
};

#define BASE64_PAD	'='


#define BASE64DE_FIRST	'+'
#define BASE64DE_LAST	'z'
/* ASCII order for BASE 64 decode, -1 in unused character */
static const signed char base64de[] = {
	/* '+', ',', '-', '.', '/', '0', '1', '2', */ 
	    62,  -1,  -1,  -1,  63,  52,  53,  54,

	/* '3', '4', '5', '6', '7', '8', '9', ':', */
	    55,  56,  57,  58,  59,  60,  61,  -1,

	/* ';', '<', '=', '>', '?', '@', 'A', 'B', */
	    -1,  -1,  -1,  -1,  -1,  -1,   0,   1, 

	/* 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', */
	     2,   3,   4,   5,   6,   7,   8,   9,

	/* 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', */ 
	    10,  11,  12,  13,  14,  15,  16,  17,

	/* 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', */
	    18,  19,  20,  21,  22,  23,  24,  25,

	/* '[', '\', ']', '^', '_', '`', 'a', 'b', */ 
	    -1,  -1,  -1,  -1,  -1,  -1,  26,  27,

	/* 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', */ 
	    28,  29,  30,  31,  32,  33,  34,  35,

	/* 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', */
	    36,  37,  38,  39,  40,  41,  42,  43,

	/* 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', */
	    44,  45,  46,  47,  48,  49,  50,  51,
};

int base64url_encode(const unsigned char *in, unsigned int inlen, char *out)
{
	unsigned int i, j;

	for (i = j = 0; i < inlen; i++) {
		int s = i % 3; 			/* from 6/gcd(6, 8) */

		switch (s) {
		case 0:
			out[j++] = base64en[(in[i] >> 2) & 0x3F];
			continue;
		case 1:
			out[j++] = base64en[((in[i-1] & 0x3) << 4) + ((in[i] >> 4) & 0xF)];
			continue;
		case 2:
			out[j++] = base64en[((in[i-1] & 0xF) << 2) + ((in[i] >> 6) & 0x3)];
			out[j++] = base64en[in[i] & 0x3F];
		}
	}

	/* move back */
	i -= 1;

	/* check the last and add padding */
    
	if ((i % 3) == 0) {
		out[j++] = base64en[(in[i] & 0x3) << 4];
		//out[j++] = BASE64_PAD;
		//out[j++] = BASE64_PAD;
	} else if ((i % 3) == 1) {
		out[j++] = base64en[(in[i] & 0xF) << 2];
		//out[j++] = BASE64_PAD;
	}

    out[j++] = 0;

	return BASE64_OK;
}

int base64url_decode(const char *in, unsigned int inlen, unsigned char *out)
{
	unsigned int i, j;

	for (i = j = 0; i < inlen; i++) {
		int c;
		int s = i % 4; 			/* from 8/gcd(6, 8) */

		if (in[i] == '=')
			return BASE64_OK;

		if (in[i] < BASE64DE_FIRST || in[i] > BASE64DE_LAST ||
		    (c = base64de[in[i] - BASE64DE_FIRST]) == -1)
			return BASE64_INVALID;

		switch (s) {
		case 0:
			out[j] = ((unsigned int)c << 2) & 0xFF;
			continue;
		case 1:
			out[j++] += ((unsigned int)c >> 4) & 0x3;

			/* if not last char with padding */
			if (i < (inlen - 3) || in[inlen - 2] != '=')
				out[j] = ((unsigned int)c & 0xF) << 4; 
			continue;
		case 2:
			out[j++] += ((unsigned int)c >> 2) & 0xF;

			/* if not last char with padding */
			if (i < (inlen - 2) || in[inlen - 1] != '=')
				out[j] =  ((unsigned int)c & 0x3) << 6;
			continue;
		case 3:
			out[j++] += (unsigned char)c;
		}
	}

	return BASE64_OK;
}