#define _XOPEN_SOURCE /* See feature_test_macros(7) */
#include <unistd.h> char *crypt(const char *key, const char *salt); #define _GNU_SOURCE /* See feature_test_macros(7) */
#include <crypt.h> char *crypt_r(const char *key, const char *salt, struct crypt_data *data);
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.
By taking the lowest 7 bits of each of the first eight characters of the key, a 56-bit key is obtained. This 56-bit key is used to encrypt repeatedly a constant string (usually a string consisting of all zeros). The returned value points to the encrypted password, a series of 13 printable ASCII characters (the first two characters represent the salt itself). The return value points to static data whose content is overwritten by each call.
Warning: The key space consists of 2**56 equal 7.2e16 possible values. Exhaustive searches of this key space are possible using massively parallel computers. Software, such as crack(1), is available which will search the portion of this key space that is generally used by humans for passwords. Hence, password selection should, at minimum, avoid common words and names. The use of a passwd(1) program that checks for crackable passwords during the selection process is recommended.
The DES algorithm itself has a few quirks which make the use of the crypt() interface a very poor choice for anything other than password authentication. If you are planning on using the crypt() interface for a cryptography project, don't do it: get a good book on encryption and one of the widely available DES libraries.
crypt_r() is a reentrant version of crypt(). The structure pointed to by data is used to store result data and bookkeeping information. Other than allocating it, the only thing that the caller should do with this structure is to set data->initialized to zero before the first call to crypt_r().
If salt is a character string starting with the characters "$id$" followed by a string terminated by "$":
|ID | Method|
|1 | MD5|
|2a | Blowfish (not in mainline glibc; added in some|
| Linux distributions)
|5 | SHA-256 (since glibc 2.7)|
|6 | SHA-512 (since glibc 2.7)|
So $5$salt$encrypted is an SHA-256 encoded password and $6$salt$encrypted is an SHA-512 encoded one.
"salt" stands for the up to 16 characters following "$id$" in the salt. The encrypted part of the password string is the actual computed password. The size of this string is fixed:
|MD5 | 22 characters|
|SHA-256 | 43 characters|
|SHA-512 | 86 characters|