Cryptographically secure generators, also known as CSPRNGs, should satisfy an additional properties over other generators: given the first k bits of an algorithm's output sequence, it should not be possible using polynomial-time algorithms to predict the next bit with probability significantly greater than 50%.
Some generators may satisfy an additional property, however this is not required by this trait: if the CSPRNG's state is revealed, it should not be computationally-feasible to reconstruct output prior to this. Some other generators allow backwards-computation and are consided reversible.
Note that this trait is provided for guidance only and cannot guarantee suitability for cryptographic applications. In general it should only be implemented for well-reviewed code implementing well-regarded algorithms.
Note also that use of a
CryptoRng does not protect against other
weaknesses such as seeding from a weak entropy source or leaking state.