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
[azAZ09./]. This string is used to perturb the algorithm in
one of 4096 different ways.
By taking the lowest 7 bit of each character 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
Warning: The key space consists of 2**56 equal 7.2e16 possi-
ble values. Exhaustive searches of this key space are pos-
sible 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
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.
SVID, X/OPEN, BSD 4.3
login(1), passwd(1), encrypt(3),