crypt - password and data encryption
char *crypt(const char *key, const char *salt);
crypt provides acess to two algorithms for password encryption. One
it's 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 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 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
ENOSYS The crypt function was not implemented, probably because of
U.S.A. export restrictions.
If the salt starts with $1$ an MD5 based password hashing algorithm is
applied. The salt should consist off $1$ followed with eight characters.
Programs using this function must be linked with -lcrypt.
SVID, X/OPEN, BSD 4.3
login(1), passwd(1), encrypt(3), getpass(3), passwd(5)
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