Calculate MD5 in C - Display output as string

Question

So I am trying to create the MD5 hash of an arbitrary string in C using the openssl lib. This is what I go so far:

#include <stdio.h>
#include <string.h>
#include <openssl/md5.h>

void compute_md5(char *str, unsigned char digest[16]);

int main()
{
    unsigned char digest[16];
    compute_md5("hello world", digest);
    printf("%s", digest);
    return 0;
}

void compute_md5(char *str, unsigned char digest[16]) {
    MD5_CTX ctx;
    MD5_Init(&ctx);
    MD5_Update(&ctx, str, strlen(str));
    MD5_Final(digest, &ctx);
}

However the output is full of unprintable characters. How can I properly display it as a hex string?

Answer 1

You have it right, you just can't use printf("%s", digest); to print digest as a string. Note the unsigned char digest[16]; will be an array of unsigned char and will not be nul-terminated. You cannot print it as a string. Instead print each element as a hexadecimal number with 2 characters, e.g.

for (int i = 0; i < 16; i++)
    printf("%02x", digest[i]);
putchar ('\n');

Your complete example would then be:

#include <stdio.h>
#include <string.h>
#include <openssl/md5.h>

void compute_md5(char *str, unsigned char digest[16]);

int main()
{
    unsigned char digest[16];
    compute_md5("hello world", digest);
    for (int i = 0; i < 16; i++)
        printf("%02x", digest[i]);
    putchar ('\n');
    return 0;
}

void compute_md5(char *str, unsigned char digest[16]) {
    MD5_CTX ctx;
    MD5_Init(&ctx);
    MD5_Update(&ctx, str, strlen(str));
    MD5_Final(digest, &ctx);
}

Example Use/Output

$ ./bin/md5openssl
5eb63bbbe01eeed093cb22bb8f5acdc3

Creating A String From digest

If you need to create a string from digest that you can print with printf ("%s\n", buf); then you create a buffer and instead of writing the 2-char hex representation to stdout, use sprintf to write it to a buffer, nul-terminate the buffer and then print the string. You could do:

int main()
{
    unsigned char digest[16];
    char buf[sizeof digest * 2 + 1];
    compute_md5("hello world", digest);
    for (int i = 0, j = 0; i < 16; i++, j+=2)
        sprintf(buf+j, "%02x", digest[i]);
    buf[sizeof digest * 2] = 0;
    printf ("%s\n", buf);
    return 0;
}

(output is the same)

Let me know if that isn't what you are looking for.