crypt(3) — Linux manual page


crypt(3)                Library Functions Manual                crypt(3)

NAME         top

       crypt, crypt_r - password hashing

LIBRARY         top

       Password hashing library (libcrypt, -lcrypt)

SYNOPSIS         top

       #include <unistd.h>

       char *crypt(const char *key, const char *salt);

       #include <crypt.h>

       char *crypt_r(const char *key, const char *salt,
                     struct crypt_data *restrict data);

   Feature Test Macro Requirements for glibc (see

           Since glibc 2.28:
           glibc 2.27 and earlier:


DESCRIPTION         top

       crypt() is the password hashing 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 hashed 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() 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

       crypt_r() is a reentrant version of crypt().  The structure
       pointed to by data is used to store result data and bookkeeping
       information.  Other than allocating it, the only thing that the
       caller should do with this structure is to set data->initialized
       to zero before the first call to crypt_r().

RETURN VALUE         top

       On success, a pointer to the hashed password is returned.  On
       error, NULL is returned.

ERRORS         top

       EINVAL salt has the wrong format.

       ENOSYS The crypt() function was not implemented, probably because
              of U.S.A. export restrictions.

       EPERM  /proc/sys/crypto/fips_enabled has a nonzero value, and an
              attempt was made to use a weak hashing type, such as DES.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       │ Interface              Attribute     Value                │
       │ crypt()                │ Thread safety │ MT-Unsafe race:crypt │
       │ crypt_r()              │ Thread safety │ MT-Safe              │

STANDARDS         top



HISTORY         top

              POSIX.1-2001, SVr4, 4.3BSD.

   Availability in glibc
       The crypt(), encrypt(3), and setkey(3) functions are part of the
       POSIX.1-2008 XSI Options Group for Encryption and are optional.
       If the interfaces are not available, then the symbolic constant
       _XOPEN_CRYPT is either not defined, or it is defined to -1 and
       availability can be checked at run time with sysconf(3).  This
       may be the case if the downstream distribution has switched from
       glibc crypt to libxcrypt.  When recompiling applications in such
       distributions, the programmer must detect if _XOPEN_CRYPT is not
       available and include <crypt.h> for the function prototypes;
       otherwise libxcrypt is an ABI-compatible drop-in replacement.

NOTES         top

   Features in glibc
       The glibc version of this function supports additional hashing

       If salt is a character string starting with the characters "$id$"
       followed by a string optionally terminated by "$", then the
       result has the form:


       id identifies the hashing method used instead of DES and this
       then determines how the rest of the password string is
       interpreted.  The following values of id are supported:
              ID   Method
              1    MD5
              2a   Blowfish (not in mainline glibc; added in some Linux
              5    SHA-256 (since glibc 2.7)
              6    SHA-512 (since glibc 2.7)

       Thus, $5$salt$hashed and $6$salt$hashed contain the password
       hashed with, respectively, functions based on SHA-256 and

       "salt" stands for the up to 16 characters following "$id$" in the
       salt.  The "hashed" part of the password string is the actual
       computed password.  The size of this string is fixed:
              MD5       22 characters
              SHA-256   43 characters
              SHA-512   86 characters

       The characters in "salt" and "hashed" are drawn from the set
       [a-zA-Z0-9./].  In the MD5 and SHA implementations the entire key
       is significant (instead of only the first 8 bytes in DES).

       Since glibc 2.7, the SHA-256 and SHA-512 implementations support
       a user-supplied number of hashing rounds, defaulting to 5000.  If
       the "$id$" characters in the salt are followed by "rounds=xxx$",
       where xxx is an integer, then the result has the form


       where yyy is the number of hashing rounds actually used.  The
       number of rounds actually used is 1000 if xxx is less than 1000,
       999999999 if xxx is greater than 999999999, and is equal to xxx

SEE ALSO         top

       login(1), passwd(1), encrypt(3), getpass(3), passwd(5)

Linux man-pages (unreleased)     (date)                         crypt(3)

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