Interestingly it's not, but the thing is that you can't actually use quantum entanglement to send information from one particle to the other, so it does not violate the principles of special relativity.

So usually this is explained with two scientists, Alice and Bob, on far away planets. They're each in the possession of a particle that is entangled with the other, and in a superposition of state 1 and state 2. When Alice measures the state of her particle, it collapses into one of the states, say state 1. When Bob measures the state of his particle immediately after, before any particle travelling at light speed could get there, it will also be in state 1 (assuming they were entangled in such a way that the state will be the same).

Due to special relativity, for some observers it could actually have been Bob who measured the state of his particle first, before Alice did. In the end, it doesn't really matter. They both got the same information: "state 1", but since they can't control what state the particle will collapse to, no information can be exchanged between Alice and Bob.

In quantum encryption, it is that bit of shared information that Alice and Bob can use as a key to encrypt and decrypt messages, but those messages are still sent the old fashioned way, using light waves traveling at light speed.

If the mass vanishes, then the gravity would also vanish, at the same time.