Matasano Cryptopals Challenge 1: convert hex to base64

Cryptopals challenge 1

Using binary strings to convert from hex to binary felt kinda hacky. I'd like to hear if there are better ways to do that.

I used memcpy() and pointer to the end of the string instead of strcat() to avoid the Shlemiel the Painter problem.

EDIT: follow up question

```
#include
#include

char hex_2_base64(char _hex)
{
char *hex_2_bin[16] = { "0000", "0001", "0010", "0011", "0100", "0101", "0110", "0111", "1000", "1001", "1010", "1011", "1100", "1101", "1110", "1111" };
char *dec_2_base64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

//allocating memory for binary string
int bin_size = strlen(_hex) * 4;
while (bin_size % 6 != 0) //add space for zero padding
bin_size += 8;
char *bin = malloc(bin_size + 1);
memset(bin, 0, bin_size + 1);

//these are for strtol, its arguments need the zero terminator
char buf[2] = { 0 };
char b64buf[6 + 1] = { 0 };

//converting hex input to binary
char *bin_end = bin;
for (int i = 0; i < strlen(_hex); i++)
{
buf[0] = _hex[i];
memcpy(bin_end, hex_2_bin[strtol(buf, NULL, 16)], 4);
bin_end += 4;
}

//pad binary string w/ zeroes
while (strlen(bin) < bin_size)
strcat(bin, "00000000");

//allocating memory for b64 output
int b64size = (strlen(bin) / 6) + 1;
char *out = malloc(b64size);
memset(out, 0, b64size);

//walk through binary string, converting chunks of 6 bytes into base64 chars
char *bin_ptr = bin;
char *out_end = out;
int index_b64;
while (*bin_ptr)
{
strncpy(b64buf, bin_ptr, 6);
index_b64 = strtol(b64buf, NULL, 2);
if (index_b64 == 0)
buf[0] = '=';
else
buf[0] = dec_2_base64[index_b64];
memcpy(out_end, buf, 1);
out_end += 1;
bin_ptr += 6;
}

free(bin);
return out;
}

int main(void) {
char *out = NULL;

out = hex_2_base64("49276d206b696c6c696e6720796f757220627261696e206c696b65206120706f69736f6e6f7573206d757368726f6f6d");
assert(strcmp(out, "SSdtIGtpbG

Base 16 -> Base 2 -> Base 64: Cut out the middleman

You are currently converting each hex digit to 4 binary digits, then converting 6 binary digits into one Base64 digit. Instead of doing two conversions, how about if you took each three hex digits (= 12 binary digits) and converted that into two Base64 digits? You can do this up until the end of your hex string, where you might have 1 or 2 leftover hex digits. You can handle those leftover digits by padding with extra zero bits and outputting extra Base64 digits, including the special '=' digit.
Bug

Your code doesn't convert zeros correctly. The problem is here:

if (index_b64 == 0)
  buf[0] = '=';
else
  buf[0] = dec_2_base64[index_b64];

As you can see, you output '=' for each Base64 zero digit. However, the correct digit to output is 'A'. The special '=' digit is only used to indicate zero bits for padding. Therefore, if I try to convert the hex string "0000", your program returns "====" when the correct answer is "AAA=".

StackExchange Code Review Q#143361, answer score: 4