Is there a cheap method to do zero-one masking?

[lifematrix]

Hello, everyone,

I am new to Homomorphic Encryption and try to use HElib to implement a complex neural convolutional network. Retrieving an
entry on the certain position of the cipher text vectoris a very common and useful operation. It can be implemented by multiplying a zero-one vector, but very expensive.

I read Shai Halevi's paper Algorithms in HElib which discussed zero-fill shift. The paper gave me the feeling that the shift operation can be achived with a lower cost.

I found HElib really provided zero-fill shift operation, so a masking can be achieved by two shifts and one rotation, i.e., suppose position is i (zero-based), shift left i, shift right n-1, and rotate right n-1-i. However, the experimental code I wrote showed the shift operation is more expensive both in noise budget (like multipication) and speed (like rotation).

So, is there a cheap method to do zero-one masking in HElib? Or do I have any misunderstanding?

Thank you very much,

Steven

[fionser]

zero-filling is noting but just multiplying zero-one vectors then rotation.

[lifematrix]

@fionser Thank you very much for your reply.
I could understand the approach you described. But multiplication will cost the noise budget. So I want to find a cheaper one.

I just read the source code of HELib. It really did mask vector multiplying with type zzX type in rotate function of EncryptedArray.cpp. I guess maybe it is cheaper rather than multiplying a plaintext mask vector that is encoded from a mask vector of float?

[tzukpolinsky]

i found that multiply ctxt in a zeroed ptxt where ptxt[i]=1 is the best way to create masking for all slots!=i

cause except in a very low security levels contexts or m init var is power of 2, you have a large number of slots, and the amounts of shifts needed generates much more noise than one multiplication, but thats base on trail and error so i dont have mathematics to backup this routine