From the definition of one one function :
The function f is injective(synonym of one-one) if and only if for all a and b in A, if f(a) = f(b), then a = b; that is, f(a) = f(b) implies a = b. Equivalently, if a ≠ b, then f(a) ≠ f(b).
So if we represent them in logic form , we get S1 is immediate logical translation of an statement..And since implication represented by S1 is true , so is its contrapositive which is represented by S2 ..
So S1 and S2 are valid logical propositions regarding one one functions..