Hi,

I've been reading up on using a salt value when creating a password to make it more secure, what I can't get my head round is how do you remember this salt value?

I'm guessing that when a user logs in to be able to compare the password entered with the one in the database you would need to again add the salt value to the entered password.

Am I missing something really obvious?

Thanks in Advance

You don't; you just test a lot of salts. That's why salts are small, to make that tolerable.

The point is to add a lot more computational work to anyone trying to brute-force a hash. They can't just compare a list of known hashes to a shadow file.

At least for passwords made with crypt() (see 'man 3 crypt'), the salt is the first two characters of the generated hash - this makes duplicates look different, while allowing easy computation when entering the password.

Here's a test program I wrote a while ago demonstrating basic use of crypt(), but still find useful - if you run it you'll notice the first two characters of the output are the two-byte salt (compile with '-lcrypt'):

#define _GNU_SOURCE

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

#include <sys/time.h>
#include <unistd.h>

char *random_salt()
{
    // Failure is fine (assume garbage on stack will do at a push).
    struct timeval tv;
    if (gettimeofday(&tv, NULL) != 0) {
        fprintf(stderr, "Warning: Could not gettimeofday: %m.\n");
        fprintf(stderr, "Just using garbage on stack as randomness.\n");
    }
    srand(tv.tv_sec + tv.tv_usec);

    const char *salt_chars =
        "abcdefghijklmnopqrstuvwxyz"
        "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        "0123456789" "./";

    static char salt[3] = "\0\0\0";

    salt[0] = salt_chars[rand() % strlen(salt_chars)];
    salt[1] = salt_chars[rand() % strlen(salt_chars)];

    return salt;
}

int main(int argc, char *argv[])
{
    if (argc != 2 && argc != 3) {
        fprintf(stderr, "Usage: crypt PASSPHRASE [SALT]\n");
        fprintf(stderr, "(If no SALT is given, a random one is chosen)\n");
        return 1;
    }

    if (argc == 3 && strlen(argv[2]) != 2) {
        fprintf(stderr, "Error: salt must be 2 bytes long\n");
        return 1;
    }

    char *salt = (argc == 3) ? argv[2] : random_salt();

    char *pass = crypt(argv[1], salt);
    if (pass) {
        printf("%s\n", pass);
        return 0;
    } else {
        fprintf(stderr, "Error: %m\n");
        return 1;
    }
}