The point is not whether an applicant can or cannot do it, the point is that the question is deliberately rephrasing to obscure a candidates understanding. If you ask your average candidate to do that in laymans terms or they were given 5 seconds to google this would be a trivial task and you would have a different outcome. All this selects for is the candidate to have a working definition of an ascii decimal string in their head, it represents no ability to problem solve whatsoever.

There is a difference between "handle this string to do a normal thing, but make it secure" and "do something stupid with this string, which goes against all good practice, oh and you can't use the obvious tools because reasons".

for example, if you are using strings to do addition, why the fuck aren't you allows to to atoi? also, why are you using strings to hold numbers?

everything to do with that question is something that you'd insta-reject in a diff/pr/mr.

Surely converting from string to int, validating it, catching errors and generally making it nice, is a much better test?

Its like going to an interview for a copywriter(someone who writes text) for safety sign company, and they ask you to make a riddle in English, but you can't use any words that are derived from Latin. Sure it shows an impressive command of both etymology and english, but its totally opposite to making clear, easy to understand text.