admin extra option - very hard and importenat-Please help

ok guys look

my man asked me to builld something 4 him

he wanted extra pass in admin panel that will be changed every few mins.

4 ex i called the exstra pass ep2

so to login u need to enter

user / pass / ep2

ep2 is global pass that in the db in table ep2

now im looking 4 the place to find where the user / pass being checked
cuse i want to add the ep2 to the check

but i cant find where the pass being checked

any 1 please can help me ?