Convert 20 Byte Hex (char string) to 10 Byte Binary Char String in C

Question

I have the following string stored. 16 Bytes for 1-F and 4 nullBytes at the end.

e.g. 1234567890ABCDEF0000
unsigned char input[] = {0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x30, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x00, 0x00, 0x00, 0x00};

How do I get the 10 Byte Binary of this?

EDIT:

Im trying to use the SHA1 function of the openssl crypto library properly. I have the task to read a "salt" and a "password" from the command line.

Then add them together such that I have "salt" + "|" + "password".

If no salt is passed, the salt is just "\\0\\0\\0\\0\\0\\0\\0\\0\\0\\0" which is 10 bytes right? but if a salt is passed it could be "1234567890ABCDEF"

I then have to fill this up to the right with null Bytes, so that i have 10 bytes in total But the "1234567890ABCDEF" is already 16 Bytes so i have to convert it. I dont know, I'm really struggling with the memory part in c

Answer 1

The easiest could be to:

Create a 0-initialized array of 10 bytes:

unsigned char salt[10] = { 0 };

then read in the hexdigits bytewise with sscanf() :

sscanf(input, "%02hhx%02hhx%02hhx%02hhx%02hhx%02hhx%02hhx%02hhx%02hhx%02hhx",
              &salt[0], &salt[1], &salt[2], &salt[3], &salt[4],
              &salt[5], &salt[6], &salt[7], &salt[8], &salt[9]);

This will convert as many bytes as needed; if only 6 hexdigits are given as salt, the first three bytes are filled and the rest remains 0.

This should do what you expect.

Answer 2

Hey I didn't get much from your example, but what you describe as bellow + the constrains could be solved like this. See snippet.

If no salt is passed, the salt is just "\\0\\0\\0\\0\\0\\0\\0\\0\\0\\0" which is 10 bytes right? but if a salt is passed it could be "1234567890ABCDEF"

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

#define SALT_MAX_BYTES 10

int main(int argc, char *argv[]) {
    // Init the whole array with 0-s, which is the same value as '\0'
    char salt[SALT_MAX_BYTES] = {0};
    // Here get the input, now assuming ./a.out [salt]
    if (argc > 1) // The executable name is always passed
    {
        printf("Input: %s\n", argv[1]);
        // Assuming ASCII...
        // Assuming you want to use the ASCII value representation of input "42"
        // and not the number 42 ... 
        strncpy(salt, argv[1], SALT_MAX_BYTES);
        // Note: from here on you must strictly handle salt as length terminated.
        // => input may have more then SALT_MAX_BYTES
    }
    else
    {
        puts("Usage: ...");
        return -1;
    }

    // Left aligned output, showing nothing for \0 bytes...
    printf("Entered salt is : <%-*.*s>\n", SALT_MAX_BYTES, SALT_MAX_BYTES, salt);
    return 0;
}