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crypt - password and data encryption
char *crypt(const char *key, const char *salt);
Link with -lcrypt.
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
The DES algorithm itself has a few quirks which make the use of the
crypt() interface a very poor choice for anything other than password
authentication. If you are planning on using the crypt() 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.
SVr4, 4.3BSD, POSIX.1-2001
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 "$1$<salt>$<encoded>", where "<salt>" stands for the up to 8
characters following "$1$" in the salt, and "<encoded>" is a further 22
characters. The characters in "<salt>" and "<encoded>" are drawn from
the set [a–zA–Z0–9./]. The entire key is significant here (instead of
only the first 8 bytes).
login(1), passwd(1), encrypt(3), getpass(3), passwd(5),
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