"has to be" or "is to be"

Solution 1:

"Is to be used" is grammatically acceptable but stylistically weak. Better to stick to one of these:

• "has to do": it is required (Sometimes people use this hyperbolically: "I have to go to the movies, all my friends are."). In a technical setting like you allude to, this often means something that fundamental to the project blows up if you don't: CEO goes to jail, client withdraws funding, people get fired, etc. It is usually too strong to use it for an alternative implementation that is merely a mistake.
• "should do": there are other valid alternatives, but it has been decided that this is the best one
• "will do": this usually is reserved for a future certainty: "It will be February soon," "It will be night in 4 hours," "The sun will rise tomorrow," etc.

Solution 2:

If the emphasis is on the necessity of using a proper XYZ algorithm, then use has to be used. An alternative is must be used. (Also consider if instead of in case.)