Trapdoor One-Way Function
A trapdoor one-way function is a one-way function with an additional requirement. Informally, a one-way function might be described as a function for which evaluation in one direction is straightforward, while computation in the reverse direction is far more difficult. Such a function becomes a trapdoor one-way function when we add the requirement that computation in the reverse direction becomes straightforward when some additional (trapdoor) information is revealed .
While there are alternative descriptions  we might describe a trapdoor one-way function as a function f with domain X and range (codomain) Y where f (x) is ‘easy’ to compute for all \(x \in X\)
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