Ring-switching in CKKS

Library Questions
1

Hi OpenFHE team,

I am currently exploring CKKS implementations for privacy-preserving inference of large foundation models, especially Transformer/LLM inference. I have a question about whether OpenFHE currently supports, or could potentially support, a CKKS ring-degree switching / ring-packing primitive similar to the method used in recent FHE-based LLM systems.

The specific primitive I am interested in is not CKKS-to-FHEW/TFHE scheme switching. Instead, it is a CKKS-to-CKKS ring switching operation between different cyclotomic ring degrees.

Concretely, suppose we have a CKKS/RLWE ciphertext in a large ring:

R_N = Z_Q[X] / (X^N + 1)

and we want to switch it to smaller rings:

R_N' = Z_Q[Y] / (Y^{N'} + 1), where N' divides N and Y = X^{N/N'}.

The operation I have in mind is roughly:

  1. key-switch the ciphertext from a large-ring secret key sk_N to an embedded small-ring secret key sk_N'(X^{N/N'}) inside R_N;

  2. decompose the large-ring ciphertext polynomials by coefficient residue classes modulo k = N/N';

  3. reinterpret the decomposed polynomial components as multiple ciphertexts over the smaller ring R_N'.

In other words, one large-ring ciphertext would be converted into k = N/N' smaller-ring ciphertexts, each encrypting a block of coefficients from the original message. A reverse merge/repack operation would also be useful.

This type of primitive appears very useful for modern CKKS-based large model inference, because different operators may prefer different ciphertext formats and ring degrees. For example, nonlinear functions and bootstrapping may use a large ring with slot encoding, while some plaintext-ciphertext or ciphertext-ciphertext linear algebra routines could be much more efficient in smaller rings or coefficient-style layouts.

My questions are:

  1. Is there currently any way to implement this CKKS ring-degree switching / ring-packing operation using the public OpenFHE API?

  2. If it is not possible via the public API, are the required lower-level components already present internally in OpenFHE, such as:

    • generating key-switching material from a large-ring secret to an embedded small-ring secret,

    • manipulating DCRTPoly components across different ring dimensions,

    • constructing valid ciphertexts in a smaller CKKS CryptoContext from decomposed large-ring polynomial components,

    • preserving CKKS metadata such as scale, level, and modulus chain information?

  3. Would the OpenFHE team consider supporting this type of CKKS ring-degree switching primitive in the library?

I believe this would be very valuable for implementing efficient CKKS inference for large foundation models (e.g., LLM), in which the work from CryptoLab (https://arxiv.org/pdf/2601.18511) has already finished.

Thank you very much!