Using a salt value Post: 302712313

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Top Forums Programming Using a salt value Post 302712313 by JohnGraham on Tuesday 9th of October 2012 04:43:24 AM

10-09-2012

Registered User

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'):

Code:

#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;
    }
}

JohnGraham

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crypt

crypt(3C)						   Standard C Library Functions 						 crypt(3C)

NAME

       crypt - string encoding function

SYNOPSIS

       #include <crypt.h>

       char *crypt(const char *key, const char *salt);

   Standard conforming
       #include <unistd.h>

       char *crypt(const char *key, const char *salt);

DESCRIPTION

       The crypt() function encodes strings suitable for secure storage as passwords. It generates the password hash given the key and salt.

       The key argument is the plain text password to be encrypted.

       If  the	first  character of salt is "$", crypt() uses crypt.conf(4) to determine which shared module to load for the encryption algorithm.
       The algorithm name crypt() uses to search in crypt.conf is the string between the first and second "$", or between the first "$" and  first
       "," if a "," comes before the second "$".

       If the first character of salt is not "$", the algorithm described on crypt_unix(5) is used.

RETURN VALUES

       Upon successful completion, crypt() returns a pointer to the encoded string. Otherwise it returns a null pointer and sets errno to indicate
       the error.

       The return value points to static data that is overwritten by each call.

ERRORS

       The crypt() function will fail if:

       EINVAL	  An entry in crypt.conf is invalid.

       ELIBACC	  The required shared library was not found.

       ENOMEM	  There is insufficient memory to generate the hash.

       ENOSYS	  The functionality is not supported on this system.

USAGE

       The values returned by this function might not be portable among standard-conforming systems. See standards(5).

       Applications should not use crypt() to store or verify user passwords but should use the functions described on pam(3PAM) instead.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Standard			   |
       +-----------------------------+-----------------------------+
       |MT-Level		     |MT-Safe			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       passwd(1), crypt_genhash_impl(3C), crypt_gensalt(3C),  crypt_gensalt_impl(3C),  getpassphrase(3C),  pam(3PAM),  passwd(4),  policy.conf(4),
       attributes(5), crypt_unix(5), standards(5)

SunOS 5.11							    28 Sep 2004 							 crypt(3C)