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Full Discussion: how to check for valid password
Top Forums Shell Programming and Scripting how to check for valid password Post 302580444 by Corona688 on Thursday 8th of December 2011 01:53:08 PM
Old 12-08-2011
From man 5 shadow

Code:
...

       encrypted password
           Refer to crypt(3) for details on how this string is interpreted.

           If the password field contains some string that is not a valid
           result of crypt(3), for instance ! or *, the user will not be able
           to use a unix password to log in (but the user may log in the
           system by other means).

           This field may be empty, in which case no passwords are required to
           authenticate as the specified login name. However, some
           applications which read the /etc/shadow file may decide not to
           permit any access at all if the password field is empty.

           A password field which starts with a exclamation mark means that
           the password is locked. The remaining characters on the line
           represent the password field before the password was locked.

...

From man 3 crypt:

Code:
...
   Glibc Notes
       The glibc2 version of  this  function  supports  additional  encryption
       algorithms.

       If  salt is a character string starting with the characters "$id$" fol-
       lowed by a string terminated by "$":

              $id$salt$encrypted

       then instead of using the DES machine,  id  identifies  the  encryption
       method  used  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)

       So   $5$salt$encrypted   is   an   SHA-256   encoded    password    and
       $6$salt$encrypted is an SHA-512 encoded one.

...

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

       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).

...

So don't look for !*, look for anything except [0-9a-zA-Z./$] in that field
 

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LEARN ABOUT DEBIAN

lsh-krb-checkpw

LSH-KRB-CHECKPW(8)						    lsh manuals 						LSH-KRB-CHECKPW(8)

NAME

       lsh-krb-checkpw - program to check a Kerberos username/password combination

SYNOPSIS

       lsh-krb-checkpw username-to-check

DESCRIPTION

       This manual page documents briefly the lsh-krb-checkpw command.	This manual page was written for the Debian distribution because the orig-
       inal program does not have a manual page.  Instead, it has documentation in the GNU Info format; see below.

       lsh-krb-checkpw is a program that checks if a username and password combination is valid for login  by  doing  a  kerberos  lookup.  It	is
       designed to be used as a password helper program for lshd (8), (eg.  --password-helper=/usr/sbin/lsh-krb-checkpw.

       lsh-krb-checkpw	takes  one  required argument, which is the username, and reads the password from stdin, then returns 0 if the password is
       valid, or 1 otherwise. Note that the password must be supplied exactly, ie. there must be no newline after the  password,  so  if  invoking
       from a shell, just type "<your-password><CTRL-D>".

SEE ALSO

       lsh(1), lshd(8).   The programs are documented fully by Lsh, available via the Info system.

AUTHOR

       This manual page was written by Timshel Knoll <timshel@debian.org>, for the Debian GNU/Linux system (but may be used by others).

lsh-krb-checkpw 						    Nov 15 2005 						LSH-KRB-CHECKPW(8)
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