In my work I am attempting to reduce some of the computational complexity needed with multiplication by using a version of bitshifting. I am wondering if this is possible? Despite reading through the documention and the code, how to actually interact with the FHE Ciphertext is not fully clear given …

Hi @cdailey , in OpenFHE, ciphertexts are generally opaque to the application (and user). You can really (should really) only interact with them via methods and function calls in the library. Can you elaborate on what it is you are trying to achieve? Are you trying to do bitshifting on the encrypte…

In BGV-like FHE schemes, a ciphertext is typically a vector of polynomials (in most cases, 2 polynomials). Each polynomial can be viewed as a vector of big integers. Depending on the FHE scheme used, multiplying a ciphertext by a constant is typically done by first iterating over the polynomials an…

So what i am attempting to accomplish is transciphering, basically using a lightweight cipher to encrypt ptxt, then encrypt that key and the resulting ctxt homorphically (using BFV), then homorphically decrypting the lightweight in order to be left with just a homorphically encrypted ctxt. To do thi…

What you are to do is doable in principle. But we still need more details to help you better. What encoding are you using with BFV? Are you homomorphically encrypting the light-cipher ctxt bit by bit or as an integer?

I was originally looking at using CoefPacked encoding (though if there is a better way then I am open to suggestions. The way I was homomorphically encrypting the LWC ctxt was by taking the ctxt vector into a HE plaintext using cc->MakeCoefPackedPlainedtext(ctxt). From there I was encrypting using …

@cdailey Some general notes related to your question: If the input is encoded as a vector of integers, then multiplication and addition are easy but bit shifting is hard. In this case, in BFV it typically requires bootstrapping (very expensive operation). If the input is in bits (for example, eac…

Thanks for the explanation. That definitely clears some things up. The complexity of the decryption circuit with BFV has been a forefront decision maker in the choices I have been making, which is why I was looking into the bit shifting. I will take a look at PASTA, RASTA, and FASTA along with re-e…

Left shifting is multiplying by a constant, but right shifting is dividing by a constant, which typically requires BFV bootstrapping. What specific bit shifting operations are you referring to?

The current lwc i was looking at had both left and right shifts.

Bit-Shifting with FHE Ciphertext

FHE Questions

ypolyakov September 16, 2022, 2:19am 7

@cdailey Please see a similar thread: Proxy Re-Encryption of Lightweight Cipher to BFV It discusses transciphering. We had a more detailed discussion there (after I answered your questions related to PRE).

Topic		Replies	Views
FHE multiplication FHE Questions	6	169	June 11, 2024
Homomorphic Rotations between first and second half of the vector Library Questions	6	326	March 21, 2024
Sign evaluation and addition Library Questions	1	251	June 16, 2023
I am encrypting a vector into ciphertext, and I want to exchange some values in the ciphertext for each other. What should I do? FHE Questions	3	253	July 24, 2023
Does Homomorphic Constant Multiplication Require Additional Circuit Depth? FHE Questions	8	346	April 10, 2025

Bit-Shifting with FHE Ciphertext

Related topics