Code responsible for allowing user to login once entering the correct password

[lee_can]

How can i find the responsible code which allow user to enter his password and to verify if the password is correct during login in process.

Best Regards

[Lars Noodén]

That would probably be in the package login (http://packages.ubuntu.com/saucy/login). You can check out the source package (deb-src) and look there.

[SeijiSensei]

Linux uses "pluggable authentication modules" to handle user authentication. Modules provide access to various backends like the password file or an LDAP server. The directory /etc/pam.d contains the configuration files for various applications. The file /etc/pam.d/login contains the directives that handle ordinary logins. It invokes the "common-*" files that supply the code to use the password file.

If you are looking for source code, you might want to start here: https://fedorahosted.org/linux-pam/

However if you're simply interested in the method for obtaining and verifying user credentials, then the login package Lars mentioned may be all you need.