At an office
Identity
.
.
Secretary: That’s all I want to know. That’s all I need to know.
Man: You don’t know my nationality.
Secretary: Ha, ha, ha, oh, yes, I do. You’re Canadian.
Man: How did you know that?
Secretary: Because, you were born …
.
.
That’s all I need to know.
What does "need "mean in this sentence?
Does it mean “have to/must/be obliged to” or something else?
Thank you
It simply means that the secretary has all the information she requires to do her job.
Thank you so much, Torsten 
want = desire
need = necessary
I need to eat food to live.
I want to eat ice cream because it tastes good.
Thank you so much, NearlyNapping
“Beauty is truth, truth beauty,—that is all
Ye know on earth, and all ye need to know.”
–John Keats. Ode on a Grecian Urn
“Need to know” is also a phrase used for controlling sensitive information.
“The D-Day plans were only told to people with a need to know.”
“The company’s secret formula was only distributed on a need to know basis.”
In both these cases, people were told the information only if it was necessary for them to do their job.
Thank you so much, Arinker