Composite Numbers That Give Valid RSA Key Pairs for Any Coprime p

RSA key pairs are normally generated from two large primes p and q. We consider what happens if they are generated from two integers s and r, where r is prime, but unbeknownst to the user, s is not. Under most circumstances, the correctness of encryption and decryption depends on the choice of the public and private exponents e and d. In some cases, specific ( s , r ) pairs can be found for which encryption and decryption will be correct for any ( e , d ) exponent pair. Certain s exist, however, for which encryption and decryption are correct for any odd prime r ∤ s . We give necessary and sufficient conditions for s with this property