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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LEARN ABOUT DEBIAN

gfs_pio_gets

GFS_PIO_GETS(3) 														   GFS_PIO_GETS(3)

NAME

       gfs_pio_gets - read one line

SYNOPSIS

       #include <gfarm/gfarm.h>

       char *gfs_pio_gets (GFS_File f, char * s, size_t size);

DESCRIPTION

       gfs_pio_gets()  reads one line from the file specified by the parameter gf to the buffer specified by the parameter s.  You need to specify
       the size of the buffer by the parameter size.

       Unlike gfs_pio_getline(3), this function doesn't remove newline character at the end of the buffer. But if whole contents of the line  can-
       not  be	stored to the buffer due to its length, or if the file reaches the end without newline, there will be no newline at the end of the
       buffer. In any case, one '' character is appended to the end of the buffer.

       If the file reaches its end, the length of the string in the buffer becomes 0.

RETURN VALUES

       NULL   The function terminated successfully.

       GFARM_ERR_NO_MEMORY
	      Insufficient memory was available.

       Others An error except the above occurred.  The reason is shown by its pointed strings.

EXAMPLES

   EXAMPLE OF GFS_PIO_GETS FUNCTION
       #include <stdio.h>
       #include <stdlib.h>
       #include <gfarm/gfarm.h>

       int
       main(int argc, char **argv)
       {
	    char *e;
	    GFS_File gf;
	    char buffer[512];

	    e = gfarm_initialize(&argc, &argv);
	    if (e != NULL) {
		 fprintf(stderr, "gfarm_initialize: %s
", e);
		 return (EXIT_FAILURE);
	    }
	    if (argc <= 1) {
		 fprintf(stderr, "missing gfarm filename
");
		 return (EXIT_FAILURE);
	    }
	    e = gfs_pio_open(argv[1], GFARM_FILE_RDONLY, &gf);
	    if (e != NULL) {
		 fprintf(stderr, "%s: %s
", argv[1], e);
		 return (EXIT_FAILURE);
	    }
	    e = gfs_pio_set_view_global(gf, 0);
	    if (e != NULL) {
		 fprintf(stderr, "%s: gfs_pio_set_view_global: %s
",
		     argv[1], e);
		 return (EXIT_FAILURE);
	    }

	    while ((e = gfs_pio_gets(gf, buffer, sizeof buffer)) != NULL &&
		*buffer != '') {
		 printf("got: %s", buffer);
	    }
	    if (e != NULL) {
		 fprintf(stderr, "ERROR: %s
", e);
		 return (EXIT_FAILURE);
	    }
	    e = gfs_pio_close(gf);
	    if (e != NULL) {
		 fprintf(stderr, "gfs_pio_close: %s
", e);
		 return (EXIT_FAILURE);
	    }
	    e = gfarm_terminate();
	    if (e != NULL) {
		 fprintf(stderr, "gfarm_initialize: %s
", e);
		 return (EXIT_FAILURE);
	    }
	    return (EXIT_SUCCESS);
       }

SEE ALSO

       gfs_pio_open(3), gfs_pio_getline(3), gfs_pio_readline(3), gfs_pio_readdelim(3)

Gfarm								    13 May 2004 						   GFS_PIO_GETS(3)