b64.c

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

static char encoding_table[] = {'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', '+', '/'};
static char *decoding_table = NULL;
static int mod_table[] = {0, 2, 1};

void build_decoding_table() {
	decoding_table = malloc(256);
	for (int i = 0; i < 64; i++) decoding_table[(unsigned char) encoding_table[i]] = i;
}

char *base64_encode(const unsigned char *data, size_t input_length, size_t *output_length) {
	*output_length = 4 * ((input_length + 2) / 3);
	char *encoded_data = malloc(*output_length+1);
	if (encoded_data == NULL) return NULL;
	for (int i = 0, j = 0; i < input_length;) {
		uint32_t octet_a = i < input_length ? (unsigned char)data[i++] : 0;
		uint32_t octet_b = i < input_length ? (unsigned char)data[i++] : 0;
		uint32_t octet_c = i < input_length ? (unsigned char)data[i++] : 0;
		uint32_t triple = (octet_a << 0x10) + (octet_b << 0x08) + octet_c;
		encoded_data[j++] = encoding_table[(triple >> 3 * 6) & 0x3F];
		encoded_data[j++] = encoding_table[(triple >> 2 * 6) & 0x3F];
		encoded_data[j++] = encoding_table[(triple >> 1 * 6) & 0x3F];
		encoded_data[j++] = encoding_table[(triple >> 0 * 6) & 0x3F];
	}
	for (int i = 0; i < mod_table[input_length % 3]; i++) encoded_data[*output_length - 1 - i] = '=';

	encoded_data[*output_length]='\0';
	return encoded_data;
}

char *base64_decode(const char *data, size_t input_length, size_t *output_length) {
	if (decoding_table == NULL) build_decoding_table();
	if (input_length % 4 != 0) return NULL;
	*output_length = input_length / 4 * 3;
	if (data[input_length - 1] == '=') (*output_length)--;
	if (data[input_length - 2] == '=') (*output_length)--;
	unsigned char *decoded_data = malloc(*output_length+1);
	if (decoded_data == NULL) return NULL;
	
	for (int i = 0, j = 0; i < input_length;) {
		uint32_t sextet_a = data[i] == '=' ? 0 & i++ : decoding_table[data[i++]];
		uint32_t sextet_b = data[i] == '=' ? 0 & i++ : decoding_table[data[i++]];
		uint32_t sextet_c = data[i] == '=' ? 0 & i++ : decoding_table[data[i++]];
		uint32_t sextet_d = data[i] == '=' ? 0 & i++ : decoding_table[data[i++]];
		uint32_t triple = (sextet_a << 3 * 6) + (sextet_b << 2 * 6) + (sextet_c << 1 * 6) + (sextet_d << 0 * 6);
		if (j < *output_length) decoded_data[j++] = (triple >> 2 * 8) & 0xFF;
		if (j < *output_length) decoded_data[j++] = (triple >> 1 * 8) & 0xFF;
		if (j < *output_length) decoded_data[j++] = (triple >> 0 * 8) & 0xFF;
	}
	decoded_data[*output_length]='\0';
	return decoded_data;
}


char* b64c(char* str) {
	size_t rawtext_size=strlen(str);
	size_t output_length=0;
	return base64_encode(str, rawtext_size, &output_length);
}

char* b64d(char* str) {
	size_t encoded_size=strlen(str);
	size_t output_length=0;
	return base64_decode(str, encoded_size, &output_length);
}