Dave Peterson wrote...
This type of security in excel is not very robust. *If you really want to stop
all others from seeing that data, then don't put it in excel or don't share the
workbook with others.
....

Expanding on this, *ANY* workbook Smith can open, Smith can access the
values of *ALL* cells in *ALL* worksheets even without trying to crack
passwords. If Smith has his own worksheet named Smith - Private, and
if he knew that Jones had a comparable worksheet, then Smith could
reasonably conclude the workbook contained a hidden worksheet named
Jones - Private. All Smith would need to do find values in the Jones -
Private worksheet would be entering formulas like

='Jones - Private'!G17

Excel will happily return the value.

The only robust security Excel provides is workbook-open passwords,
which prevent users from OPENING files unless they provide correct
password. If there were multiple people who'd need access to their own
data in the current all-user workbook, the only robust approach in
Excel is creating separate copies of the workbook, one for each user
with that user's data, then saving these files with a DIFFERENT
workbook-open passwords. If you need shared data, use another workbook
to hold it which all users could access.