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### 32.3 Encrypting Passwords

— Function: char * crypt (const char *key, const char *salt)

The `crypt` function takes a password, key, as a string, and a salt character array which is described below, and returns a printable ASCII string which starts with another salt. It is believed that, given the output of the function, the best way to find a key that will produce that output is to guess values of key until the original value of key is found.

The salt parameter does two things. Firstly, it selects which algorithm is used, the MD5-based one or the DES-based one. Secondly, it makes life harder for someone trying to guess passwords against a file containing many passwords; without a salt, an intruder can make a guess, run `crypt` on it once, and compare the result with all the passwords. With a salt, the intruder must run `crypt` once for each different salt.

For the MD5-based algorithm, the salt should consist of the string `\$1\$`, followed by up to 8 characters, terminated by either another `\$` or the end of the string. The result of `crypt` will be the salt, followed by a `\$` if the salt didn't end with one, followed by 22 characters from the alphabet `./0-9A-Za-z`, up to 34 characters total. Every character in the key is significant.

For the DES-based algorithm, the salt should consist of two characters from the alphabet `./0-9A-Za-z`, and the result of `crypt` will be those two characters followed by 11 more from the same alphabet, 13 in total. Only the first 8 characters in the key are significant.

The MD5-based algorithm has no limit on the useful length of the password used, and is slightly more secure. It is therefore preferred over the DES-based algorithm.

When the user enters their password for the first time, the salt should be set to a new string which is reasonably random. To verify a password against the result of a previous call to `crypt`, pass the result of the previous call as the salt.

The following short program is an example of how to use `crypt` the first time a password is entered. Note that the salt generation is just barely acceptable; in particular, it is not unique between machines, and in many applications it would not be acceptable to let an attacker know what time the user's password was last set.

```     #include <stdio.h>
#include <time.h>
#include <unistd.h>
#include <crypt.h>

int
main(void)
{
unsigned long seed[2];
char salt[] = "\$1\$........";
const char *const seedchars =
"./0123456789ABCDEFGHIJKLMNOPQRST"
"UVWXYZabcdefghijklmnopqrstuvwxyz";
int i;

/* Generate a (not very) random seed.
You should do it better than this... */
seed[0] = time(NULL);
seed[1] = getpid() ^ (seed[0] >> 14 & 0x30000);

/* Turn it into printable characters from `seedchars'. */
for (i = 0; i < 8; i++)
salt[3+i] = seedchars[(seed[i/5] >> (i%5)*6) & 0x3f];

/* Read in the user's password and encrypt it. */

/* Print the results. */
return 0;
}
```

The next program shows how to verify a password. It prompts the user for a password and prints “Access granted.” if the user types `GNU libc manual`.

```     #include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <crypt.h>

int
main(void)
{
/* Hashed form of "GNU libc manual". */
const char *const pass = "\$1\$/iSaq7rB\$EoUw5jJPPvAPECNaaWzMK/";

char *result;
int ok;

/* Read in the user's password and encrypt it,
passing the expected password in as the salt. */
result = crypt(getpass("Password:"), pass);

/* Test the result. */
ok = strcmp (result, pass) == 0;

puts(ok ? "Access granted." : "Access denied.");
return ok ? 0 : 1;
}
```
— Function: char * crypt_r (const char *key, const char *salt, struct crypt_data * data)

The `crypt_r` function does the same thing as `crypt`, but takes an extra parameter which includes space for its result (among other things), so it can be reentrant. `data->initialized` must be cleared to zero before the first time `crypt_r` is called.

The `crypt_r` function is a GNU extension.

The `crypt` and `crypt_r` functions are prototyped in the header crypt.h.