More on Passwords

• Traversing the password file.

  •        #include <sys/types.h>
           #include <pwd.h>
           struct passwd *getpwent(void);
           void setpwent(void);
           void endpwent(void);
    	

  • getpwent
    • returns a pointer to the first entry in the password file.
    • Each further call returns the next entry
    • Or NULL at the end.
      • setpwent: rewinds to the beginning of the password file.
      • endpwent: closes the opened password file
    • There are equivalent getgrent, setgrent, endgrent.
    • There are equivalent writing commands

      • #include <stdio.h>
        #include <sys/types.h>
        #include <pwd.h>
        
        int putpwent(const struct passwd *p, FILE *stream);
        	

      • Writes the password structure to a file.
      • -1 on fail.
    • And for the shadow password file

      • #include <shadow.h>
        
        struct spwd *getspnam(const char *name);
        
        struct spwd *getspent(void);
        void setspent(void);
        void endspent(void);
        
                   struct spwd {
                       char *sp_namp;     /* Login name */
                       char *sp_pwdp;     /* Encrypted password */
                       long  sp_lstchg;   /* Date of last change
                                             (measured in days since
                                             1970-01-01 00:00:00 +0000 (UTC)) */
                       long  sp_min;      /* Min # of days between changes */
                       long  sp_max;      /* Max # of days between changes */
                       long  sp_warn;     /* # of days before password expires
                                             to warn user to change it */
                       long  sp_inact;    /* # of days after password expires
                                             until account is disabled */
                       long  sp_expire;   /* Date when account expires
                                             (measured in days since
                                             1970-01-01 00:00:00 +0000 (UTC)) */
                       unsigned long sp_flag;  /* Reserved */
                   };
        	

      • Fields
        • man 5 shadow
        • Login name: must exist on the system (ie in /etc/passwd or equivalent)
        • encrypted password: more on this later
          • Empty: no password is required to log in.
          • But some applications may deny access in this case.
          • Starts with ! means no log in, the account is locked.
        • Date of last password change. Int, seconds since Jan 1, 1970
          • Blank means no password aging is in place
          • 0 means the user must reset their password next time they long in.
        • minimum password age: int, minimum number of days before the user is allowed to change their password again.
        • maximum password age: the number of days after which the user must change their password.
        • warning period: number days to warn the user about a password about to expire.
        • inactivity period: The number of days after the password has expired, but the password is still accepted.
        • The date the password expires.
          • No log in, no password change, account gone.
    • Passwords
      • The general scheme for passwords is a one way encryption algorithm.
        • A plain text password is run through an algorithm.
        • An encrypted password is output.
        • There is no reversal algorithm possible.
      • To validate a password
        • Get the plain text password from the user.
        • encrypt it using the encryption algorithm
        • get the stored encrypted password from the file.
        • Compare the two, if they match, the plain text password is good.
      • getpass

        • #include <unistd.h>
          
          char *getpass( const char *prompt);
          
          	

        • This function is obsolete. Do not use it.
        • But other than raw terminal manipulation, there is no replacement as far as I can tell.
        • This prints the prompt.
        • Turns off echo
        • Reads and stores the next line.
        • Turns echo back on.
        • Returns a pointer to the static null terminated string
        • Or NULL on failure.
        • Note, you should wipe this data out as soon as you are finished with it to prevent the data being available in a core dump.
      • Crypt:

        • #define _XOPEN_SOURCE       /* See feature_test_macros(7) */
          #include <unistd.h>
          
          char *crypt(const char *key, const char *salt);
                     

        • This is the standard DES algorithm
        • It returns a 13 character string
        • It takes a password and a salt.
        • The salt is to perturb the algorithm.
          • A two character salt means that a single password can be encrypted 4096 different ways.
          • This was intended to stop someone from encrypting and storing all possible 8 character or less passwords.
        • The salt is the first two characters of the encrypted password.
      • Crypt is expanded in glibc2 (man crypt)
        • You can add an ID to the salt
        • General Form : $id$salt$encrypted
          • $1$ means you are using MD5
          • $2$ or $2a$ means you are using blowfish
          • $5$ means you are using SHA-256
          • $6$ means you are using SHA-512