How to output plain ascii from base64 decode? - c

Question

Encoded normal plain text bmVv decoded to neo without any issues but this
AAAAgMom/1a/v0lblO2Ubrt60J2gcuXSljGFQXgcyZWveWLEwo6prwgi3iJIZdodyhKZQrNWp5nKJ3srRXcUW+F1BD3baEVGcmEgqaLZUNBjm057pKRI16kB0YppeGx5qIQ5QjKzsR8ETQbKLNWgRY0QRNVz34kMJR3P/LgHax/6rmf5AAAAAwEAAQ==
encoded string is not decoding. Just getting output buffer NULL/0.
This string decoded buffer should be single character. If I do that then I will get unsigned decode ASCII, but that ASCII code just viewable and writeable, can't use encoder to encode again that ASCII code because I am still getting buffer length of 0. It is because I used printf with %c to output buffer. It is not string to output using %s.
I need to output ASCII buffer as string. How is it possible to output ASCII buffer as string without using %c with printf/fwrite/scanf/putchar?
I expecting binary ASCII code as string to use again for encoding.
Expecting:
��&�V��I[��n�zН�r�Җ1�Axɕ�yb��"�"He���B�V���'{+Ew[�u=�hEFra ���P�c�N{��Hשъixly��9B2��M�,ՠE�D�s߉
%���k��g�
Here is base64 decode full code that I tried:
#include <stdio.h>
#include <stdlib.h>
char base64_map[64] = {
'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', '+', '/'
};
char *base64_decode(char *str)
{
int counts = 0;
char buffer[4];
char *plain = malloc(strlen(str) * 3 / 4 + 1);
int i = 0, p = 0;
for (i = 0; str[i] != '\0'; i++) {
int k;
for (k = 0; k < 64 && base64_map[k] != str[i]; k++);
buffer[counts++] = k;
if (counts == 4) {
plain[p++] = (buffer[0] << 2) + (buffer[1] >> 4);
if (buffer[2] != 64) {
plain[p++] = (buffer[1] << 4) + (buffer[2] >> 2);
}
if (buffer[3] != 64) {
plain[p++] = (buffer[2] << 6) + buffer[3];
}
counts = 0;
}
}
plain[p] = '\0';
return plain;
}
int main()
{
char *str = "AAAAgMom/1a/v0lblO2Ubrt60J2gcuXSljGFQXgcyZWveWLEwo6prwgi3iJIZdodyhKZQrNWp5nKJ3srRXcUW+F1BD3baEVGcmEgqaLZUNBjm057pKRI16kB0YppeGx5qIQ5QjKzsR8ETQbKLNWgRY0QRNVz34kMJR3P/LgHax/6rmf5AAAAAwEAAQ==";
printf("%s", base64_decode(str));
return 0;
}

Answer 1

If you're decoding binary data, as here, you can't print it using printf, because printf will stop at the first null byte.
To print binary data, you could have your base64_decode function return an explicit length, and then print that many characters. Perhaps something like this:
int main()
{
char *str = "…";
int len;
char *out = base64_decode(str, &len);
for(int i = 0; i < len; i++) putchar(out[i]);
return 0;
}
To make that work you would rewrite base64_decode's signature to
char *base64_decode(char *str, int *lenp)
and add the line
if(lenp != NULL) *lenp = p;
at the end, right before the return plain; statement.
Rather than the loop calling putchar, you could also use fwrite:
char *out = base64_decode(str, &len);
fwrite(out, 1, len, stdout);
And depending on what you're trying to do with the output, it would be easier to see (though perhaps not easier to process) if you output a hexadecimal representation of the bytes, rather than the bytes themselves:
for(int i = 0; i < len; i++)
printf("%02x", (unsigned char)out[i]);