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       crypt, crypt_r - password and data encryption


       #define _XOPEN_SOURCE       /* See feature_test_macros(7) */
       #include <unistd.h>

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

       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <crypt.h>

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

       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

       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 libraries.

       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().


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


       EINVAL salt has the wrong format.

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

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


       For an explanation of the terms used in this section, see attributes(7).

       │InterfaceAttributeValue                │
       │crypt()   │ Thread safety │ MT-Unsafe race:crypt │
       │crypt_r() │ Thread safety │ MT-Safe              │


       crypt(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.  crypt_r() is a GNU extension.


   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.

   Features in glibc
       The glibc version of this function supports additional encryption algorithms.

       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 encryption 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 distributions)
              5   | SHA-256 (since glibc 2.7)
              6   | SHA-512 (since glibc 2.7)

       Thus,  $5$salt$encrypted  and  $6$salt$encrypted  contain  the  password  encrypted  with,
       respectively, functions based on SHA-256 and SHA-512.

       "salt" stands for the up to 16 characters following "$id$" in the salt.   The  "encrypted"
       part  of  the password string is the actual computed password.  The size of this string is

       MD5     | 22 characters
       SHA-256 | 43 characters
       SHA-512 | 86 characters

       The characters in "salt" and "encrypted" 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 otherwise.


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


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                                            2018-04-30                                   CRYPT(3)