char *crypt(const char *key, const char *salt);
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(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.
Programs using this function must be linked with -lcrypt.