CRYPT(3) Library functions CRYPT(3)
crypt - password and data encryption
char *crypt(const char *key, const char *salt);
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.
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
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(3) interface a
very poor choice for anything other than password authentication. If you are planning on
using the crypt(3) interface for a cryptography project, don't do it: get a good book on
encryption and one of the widely available DES libraries.
A pointer to the encrypted password is returned. On error, NULL is returned.
ENOSYS The crypt function was not implemented, probably because of U.S.A. export restric-
The glibc2 version of this function has the following additional features. If salt is a
character string starting with the three characters "$1$" followed by at most eight char-
acters, and optionally terminated by "$", then instead of using the DES machine, the glibc
crypt function uses an MD5-based algorithm, and outputs up to 34 bytes, namely
"$1$<string>$", where "<string>" stands for the up to 8 characters following "$1$" in the
salt, followed by 22 bytes chosen from the set [a-zA-Z0-9./]. The entire key is signifi-
cant here (instead of only the first 8 bytes).
Programs using this function must be linked with -lcrypt.
SVID, X/OPEN, BSD 4.3, POSIX 1003.1-2001
login(1), passwd(1), encrypt(3), getpass(3), passwd(5)