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Which is the encryption method used on /etc/shadow on GNU/Linux systems? I would like to write a small program for personal purpose that uses the same API, but at the moment I don't know where to start.

Thanks in advance

Benjamin Loison
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b3h3m0th
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4 Answers4

42

Use the crypt(3) function. On glibc, the method used depends on the salt, if it starts with:

  • $1$: it uses MD5.
  • $5$: it uses SHA-256.
  • $6$: it uses SHA-512.
  • $2a$: it uses blowfish, not supported everywhere.
  • Otherwise it uses DES.
ninjalj
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    The method used to "encrypt" the password plaintext does not depend on the salt! The string between the first pair of $s indicates the "encryption" method, the string between the second pair of $s is the actual salt for that "encryption" method. – fpmurphy Sep 30 '12 at 16:31
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    @fpmurphy: the second parameter to `crypt()` is called `salt`, and it includes an encryption algorithm identifier and the real salt. – ninjalj Sep 30 '12 at 16:48
  • Up 1 it was useful to find the what type of algorithm is used in my encryption. Thanks dude. – Q_SaD Mar 31 '14 at 13:06
6

Multiple encryption methods are available in glibc, see man 3 crypt, the Glibc Notes section: http://manpages.courier-mta.org/htmlman3/crypt.3.html

When verifying an existing password, just pass the encrypted form as salt; only the initial $id$salt part will be used. When creating new password, initialize id with whatever you need and put some random characters in salt.

Petr Baudis
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4

basic example with crypt() 

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

#define MAX_STR 256
#define MAX_SALT 12

int main(int argc, char *argv[]) {
    char password[MAX_STR];
    char salt[MAX_SALT];

    printf("salt: ");
    scanf("%s", salt);

    printf("password: ");
    scanf("%s", password);

    printf("Encrypt '%s' : '%s'\n", password, crypt(password, salt));

    return(EXIT_SUCCESS);
}

Compile program:

$ gcc -lcrypt test.c
b3h3m0th
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  • info about headers: http://stackoverflow.com/questions/6127921/is-the-crypt-function-declared-in-unistd-h-or-crypt-h – b3h3m0th Sep 30 '12 at 13:16
-1

I receive

undefined reference to `crypt'

so I think that you should compile with

$ gcc test.c -lcrypt

Georgi Dimov
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