Name

The first argument to crypt is the data to hash (usually a password), in a NULL -terminated string. The second is the salt, in one of two forms:

Modular	If salt begins with the string "$digit$" then the Modular Crypt Format is used.
Traditional
	salt parameter is a two-character string chosen from the set [a-zA-Z0-9./].

        Modular crypt:

If salt begins with the string $digit$ then Modular Crypt Format is used. The digit identifies the algorithm used for encryption. Currently MD5 hash is implemented. So digit will be 1 and hence the second argument to this function will be a string beginning with "$1$" followed by at most 8 characters (actual salt to be used in the encryption), and optionally terminated by "$" If the optional "$" is included then the characters following the dollar sign are ignored. The output of this operation will be a string containing 34 characters in the format "$1$<string>$"

"<string>" consists of the actual salt (the at most 8 characters string following "$1$" in the salt), followed by 22 bytes of data from the set [a-zA-Z0-9./].

Other crypt formats may be easily added. An example salt would be:

$4$thesalt$rest

It is based on Data Encryption Standard algorithm (DES). salt is a two-character string chosen from the set [a-zA-Z0-9./]. In order to thwart the Dictionary Attack, the two-character salt is used to perturb the algorithm in 4096 ways.

The 56-bit key for the DES algorithm is obtained by taking the lowest 7 bits of each of the first eight characters of the key. The key thus obtained is used to encrypt a constant string (a string containing all zeroes).

The return value of this function is a pointer to a static buffer. So the function is not reentrant.

#include <stdlib.h>
#include <unistd.h>
void crypt_user()
{
        char *p = NULL,
             *q = NULL;
        /* Invoke crypt() to perform password hashing */
        p = crypt("password", "S1");    /* p contains the hash of "password"
                                                 * when "S1" is used as the key. DES
                                                 * encryption algoritm is used in this
                                                 * scenario
                                                 */
        q = crypt("password", "$1$Salt1");
                                                /* q contains the hash of "password"
                                                 * as computed by the MD5 hash algorithm
                                                 */
}

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