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 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.
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 characters, 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 significant here (instead of only the first 8 bytes).
Programs using this function must be linked with -lcrypt.