Decoding a base64 encoded binary string in C

Question

I am trying to get my decoder code to work. I am using the example 64-bit encoded string from wikipedia, trying to reproduce the text they encoded.

#include <stdio.h>

//Convert raw binary character to the cb64 index
unsigned char get_cb64(unsigned char c){
if(c>='A' && c<='Z'){return c-'A';}
if(c>='a' && c<='z'){return c-'G';}
if(c>='0' && c<='9'){return c+4;}
if(c=='+'){return '>';}
if(c=='/'){return '?';}
else{return 0;}
}

void main(int argc, char** argv)
{
unsigned char* str = "TWFuIGlzIGRpc3Rpbmd1aXNoZWQsIG5vdCBvbmx5IGJ5IGhpcyByZWFzb24sIGJ1dCBieSB0aGlzIHNpbmd1bGFyIHBhc3Npb24gZnJvbSBvdGhlciBhbmltYWxzLCB3aGljaCBpcyBhIGx1c3Qgb2YgdGhlIG1pbmQsIHRoYXQgYnkgYSBwZXJzZXZlcmFuY2Ugb2YgZGVsaWdodCBpbiB0aGUgY29udGludWVkIGFuZCBpbmRlZmF0aWdhYmxlIGdlbmVyYXRpb24gb2Yga25vd2xlZGdlLCBleGNlZWRzIHRoZSBzaG9ydCB2ZWhlbWVuY2Ugb2YgYW55IGNhcm5hbCBwbGVhc3VyZS4=";

//convert each binary character to its cb64 index
int size = 360;
int num_bytes = 8;
unsigned char str_cb64[size + 1];
int cb64_idx;
int i;
for(i=0; i < size; i++){
    str_cb64[i]=get_cb64(str[i]);
}
str_cb64[size] = 0;

//convert blocks of 4 6 bit chars to 3 8 bit chars 
int end_size = size*6/8;
unsigned char ascii_out[end_size];
int out_idx = 0;
int in_idx = 0;
while(in_idx < end_size/4){
  ascii_out[out_idx]   = str_cb64[in_idx+0] << 2 | str_cb64[in_idx+1] >> 4;
  ascii_out[out_idx+1] = str_cb64[in_idx+1] << 4 | str_cb64[in_idx+2] >> 2;
  ascii_out[out_idx+2] = str_cb64[in_idx+2] << 6 | str_cb64[in_idx+3];
    out_idx += 3;
    in_idx += 4;
}   

for(i=0; i < end_size; i++){printf("%d\n",ascii_out[i]);}

}

To inspect, the code here prints the ascii value of each decoded character, which SHOULD be between 48 and 122, but there are values from (0, 255). I tested the conversion from the raw binary to the cb64 index, and that seems to work fine. The problem is in my shifting code. Any idea why it isn't working? I double checked the shifts and they look like they are coded correctly.

Thanks!

@piotr, why is the title misleading, if that is exactly what I am trying to do? If you aren't interested in helping, why do you bother checking posts on a forum meant for that purpose? — Rob, Jul 18 '11 at 15:58

score 0 · Answer 1 · answered Jul 15 '11 at 22:38

Your loop should be either while(in_idx < size) or while(out_idx < end_size). Right now you are comparing an input value to an output value, as well as dividing the output value even though you add one for each byte instead of each iteration. This will cause your loop to exit well before all of the data has been processed. Since ascii_out wasn't initialized, this could be the only problem if the beginning of the output is right, since the end will contain random data which happened to be in that space.

Decoding a base64 encoded binary string in C

1 Answers1