unRSA: solve the private key
Python 3, 77 bytes



def f(n,e):r=range(n);all(any(m-pow(m,e*d,n)for m in r)or print(d)for d in r)


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Direct translation of the requirement. any(...) becomes false when the smallest correct d is found, and print(d) returns None, making all(...) stop running.

76 bytes, if unlimited memory is allowed



def f(n,e):r=range(n);all(any(m**(e*d)%n-m for m in r)or print(d)for d in r)


Try it online!