The crypt function in crypt.h

**guraknugen** · 07-14-2013

I'm a beginner (and I will probably be forever…), so I visited the FAQ section. Right now I'm studying the FAQ > Environment and user data page and I try to make the examples there work by modifying them to make sense.

I failed with the last one however.
Here's the example code from that page:

Code:

#include <sys/types.h> 
#include <pwd.h> 
#include <unistd.h> 
#include <stdio.h> 

struct passwd *p;
p = getpwnam( "username" );
if (p) 
{
   if( strcmp( crypt( "Password", p->pw_passwd ), p->pw_passwd ) == 0 ) 
      {
        puts( "match" );
      } 
   else 
   {
        puts( "no match" );
   }
}

Of course this doesn't work out of the box, so I modified it a bit:

Code:

#include <sys/types.h> 
#include <pwd.h> 
#include <unistd.h> 
#include <stdio.h>
#include <string.h>
#include <crypt.h>

int
main(int argc, char *argv[])
{
   if (argc==2) {
      struct passwd *p;
      p=getpwnam(getlogin());
      if(p) {
         if(strcmp(crypt(argv[1], p->pw_passwd), p->pw_passwd)==0)
            puts("Right!");
         else
            puts("Wrong!");
      }
      return 0;
   }
   else {
      puts("Syntax:");
      puts("Password <password>");
      return 1;
   }
}

Maybe I did an obvious mistake, I don't know. Maybe I just misunderstood the whole concept.

The problem is that whatever I try with as input, as long as there are exactly one parameter, the output is ”Wrong!”, even when the correct password is used.

So what obvious little thing did I miss?

Here's what the output from running it would look like, if the correct password is ”ThisIsTheCorrectPa55word”:

Code:

$ ./PasswordTest
Syntax:
PasswordTest <password>
$ ./PasswordTest aligraljrhgor
Wrong!
 $ ./PasswordTest ThisIsTheCorrectPa55word
Wrong!
$

Compiled with:

Code:

gcc PasswordTest.c -lcrypt -o PasswordTest

There were no errors when compiling.

gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3

**HelpfulPerson** · 07-14-2013

Originally Posted by guraknugen

I'm a beginner (and I will probably be forever…), so I visited the FAQ section. Right now I'm studying the FAQ > Environment and user data page and I try to make the examples there work by modifying them to make sense.

I failed with the last one however.
Here's the example code from that page:

Code:

#include <sys/types.h> 
#include <pwd.h> 
#include <unistd.h> 
#include <stdio.h> 

struct passwd *p;
p = getpwnam( "username" );
if (p) 
{
   if( strcmp( crypt( "Password", p->pw_passwd ), p->pw_passwd ) == 0 ) 
      {
        puts( "match" );
      } 
   else 
   {
        puts( "no match" );
   }
}

Of course this doesn't work out of the box, so I modified it a bit:

Code:

#include <sys/types.h> 
#include <pwd.h> 
#include <unistd.h> 
#include <stdio.h>
#include <string.h>
#include <crypt.h>

int
main(int argc, char *argv[])
{
   if (argc==2) {
      struct passwd *p;
      p=getpwnam(getlogin());
      if(p) {
         if(strcmp(crypt(argv[1], p->pw_passwd), p->pw_passwd)==0)
            puts("Right!");
         else
            puts("Wrong!");
      }
      return 0;
   }
   else {
      puts("Syntax:");
      puts("Password <password>");
      return 1;
   }
}

Maybe I did an obvious mistake, I don't know. Maybe I just misunderstood the whole concept.

The problem is that whatever I try with as input, as long as there are exactly one parameter, the output is ”Wrong!”, even when the correct password is used.

So what obvious little thing did I miss?

Here's what the output from running it would look like, if the correct password is ”ThisIsTheCorrectPa55word”:

Code:

$ ./PasswordTest
Syntax:
PasswordTest <password>
$ ./PasswordTest aligraljrhgor
Wrong!
 $ ./PasswordTest ThisIsTheCorrectPa55word
Wrong!
$

Compiled with:

Code:

gcc PasswordTest.c -lcrypt -o PasswordTest

There were no errors when compiling.

gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3

It seems to me you may have interpreted getlogin() wrong. I'm a Windows guy myself, but from reading the documentation here :

getlogin - Linux Command - Unix Command

It seems it retrieves your user password. Is that your user password that you gave us? Otherwise that may be your problem.

You could change this in your code :

Code:

p=getpwnam(getlogin());

to this :

Code:

p=getpwnam("ThisIsTheCorrectPa55word");

As I said, I do not know a lot about linux. That is my guess on what is wrong though.

**oogabooga** · 07-14-2013

Could it be that your system doesn't use crypt, but instead uses something like ecb_crypt(3): fast DES encryption - Linux man page?

**fnprintf** · 07-14-2013

What do you get by code below?

Code:

printf ("%s\n", p->pw_passwd);

man shadow.h

**fnprintf** · 07-14-2013

Maybe Ubuntu/Linaro doesn't use shadow?

Couldn't edit previous post, it's:
man shadow

**vart** · 07-14-2013

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

#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <crypt.h>

Link with -lcrypt.

DESCRIPTION
crypt() is the password encryption function. It is based on the Data
Encryption Standard algorithm with variations intended (among other
things) to discourage use of hardware implementations of a key search.

key is a user's typed password.

salt is a two-character string chosen from the set [a–zA–Z0–9./]. This
string is used to perturb the algorithm in one of 4096 different ways.

Does not looks like your salt is 2 char string as described in man page.

**guraknugen** · 07-15-2013

Originally Posted by HelpfulPerson

Is that your user password that you gave us?

Well, a fake one, yes. I replaced my real password with that text string, just to illustrate what I was doing, I think I mentioned that, but maybe I forgot. When I tested the program, I entered my real user password.

**guraknugen** · 07-15-2013

Originally Posted by HelpfulPerson

It seems to me you may have interpreted getlogin() wrong. I'm a Windows guy myself, but from reading the documentation here :

getlogin - Linux Command - Unix Command

It seems it retrieves your user password.

No, it retrieves the login name, in my case ”guraknugen” (I also tested this). If it was possible to retrieve the password, it wouldn't be very safe, would it…? As far as I understand, the password can't be retrieved with less than brutal force.

**guraknugen** · 07-15-2013

I saw another crypt example where they used a much longer salt than that, but I didn't test it, I think… Maybe you are right. Well, it certainly looks like it…

I'll take a look at it tomorrow, as I am a little bit too busy this evening, unfortunately.
Thank you, and thank all the others who replied! I'll take a closer look at all your replies and come back later.

**guraknugen** · 07-15-2013

Originally Posted by fnprintf

Maybe Ubuntu/Linaro doesn't use shadow?

Couldn't edit previous post, it's:
man shadow

At least the man page is installed. 96 lines all together.

**guraknugen** · 07-15-2013

Originally Posted by oogabooga

Could it be that your system doesn't use crypt, but instead uses something like ecb_crypt(3): fast DES encryption - Linux man page?

Maybe, I don't know… I'll take a look at it later. Not much time for it this evening, unfortunately… Thanks for replying!

**fnprintf** · 07-15-2013

Try testing it with test passwd entry first.

Add the following line to /etc/passwd:

Code:

testing:$1$5L1tFZUF$60ivH79nKUxC.q4DyhNIE1:2013:2013:DELETE AFTER TESTING:/dev/null:/sbin/nologin

Username: testing
Password: ThisIsTheCorrectPa55word

For more on passwd: man 5 passwd

Code:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/types.h>
#include <pwd.h>
#include <crypt.h>

int
main (int argc, char *argv[])
{
    struct passwd *pwd = NULL;
    char *username = "testing", *result = NULL;
    
    if (argc != 2)
    {
        printf ("Usage: %s <password>\n", argv[0]);
        return 0;
    }
    
    pwd = getpwnam (username);

    if (pwd)
    {
        result = crypt (argv[1], pwd->pw_passwd);
        
        if (!result)
            return 2;
        
        printf ("Result: %s\nPasswd: %s\n", result, pwd->pw_passwd);
        
        if (!strcmp (result, pwd->pw_passwd))
            printf ("Right!\n");
        else
            printf ("Wrong!\n");
    }
    else
    {
        printf ("'%s' not found.\n", username);
        return 1;
    }

    free (result);
    return 0;
}

Compile with -lcrypt.

https://www.gnu.org/software/libc/ma...ode/crypt.html

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