The RSA and Rabin encryption functions $E_N ( \cdot )$ are respectively defined by raising $x \in Z_N $ to the power e (where e is relatively prime to $\varphi (N)$) and squaring modulo N (i.e., $E_N (x) = x^e (\bmod N)$, $E_N (x) = x^2 (\bmod N)$, respectively). We prove that for both functions, the following problems are computationally equivalent (each is probabilistic polynomial-time reducible to the other): (1) Given $E_N (x)$, find x. (2) Given $E_N (x)$, guess the least-significant bit of x with success probability $\tfrac{1}{2} + {1 {{\operatorname{poly}}(n)}}$ (where n is the length of the modulus N). This equivalence implies that an adversary, given the RSA/Rabin ciphertext, cannot have a non-negligible advantage (over a random coin flip) in guessing the least-significant bit of the plaintext, unless he can invert RSA/factor N. The proof techniques also yield the simultaneous security of the $\log n$ least-significant bits. Our results improve the efficiency of pseudorandom number generation and probabilistic encryption schemes based on the intractability of factoring.

We prove that RSA least significant bit is 1/2 + (1/[logcN]) secure, for any constant c (where N is the RSA modulus). This means that an adversary, given the ciphertext, cannot guess the least sigiiilicatnt bit of the plaintext with probability better than 1/2 + (1/[logcN]), unless he can break RSA.