[geekbeast]

tl;dr here be dragons.

The ability to evaluate statements such as x == 5 falls under the realm of functional encryption ( see Boneh, Sahai, and Waters for a good intro http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.383... ).

You have to be very careful when mixing functional schemes with FHE schemes as it is very easy to create a completely insecure system.

For example, if you could evaluate arbitrary circuits consider what happens if you can evaluate equals and bitwise AND, two simple circuits.

  ({x}&{0x1}) == {0x1}
  ({x}&{0x2}) == {0x2}
  ({x}&{0x4}) == {0x4}

Keep going for all the powers of two expressible in the system and you can quickly check if every bit is set. This makes proving security for functional schemes that allow malleability very difficult, since you have to prove that the reachable set of states with the built in malleability does not reveal sufficient information for the attacker to compromise the security of the system.

One more thing worth mentioning is that a direct comparison of ciphertexts doesn't work for semantically secure schemes. Anything that is IND-CPA, for example RSA-OAEP, will have the same plaintext encrypt to two different ciphertexts. Giving the untrusted server the ability to determine whether two ciphertexts will destroy the semantic security of the scheme, by definition.