Modulus switching for single ciphertext modulus in BFV

[helloboyxxx]

Context:

Let's say I want to use a 60-bit ciphertext modulus in BFV. After I finish all the homomorphic operations, I want to switch to a smaller modulus (30-bit). Now, let's say I created the context and seal::EncryptionParameters using the bit_sizes = {60, 60}. I noticed that SEAL only has mod_switch_scale_to_next in evaluator, which always tries to throw away one modulus on the chain.

Related issue:

#444 is related, but calculating round(ct/q) is only one useful special case for modulus switching, but not general enough. (q is the last small prime in the prime chain)

Question:

After the modulus switching, I only need decryption. Suppose I have added a function that calculates the general modulus switching round(ct*Q'/Q) for a Q' that has half the bit size. Then bit_sizes = {60, 60} should become {30, 60}. The problem is that I don't know how to modify the context accordingly. Is implementing a customized decryption function using the new smaller modulus the best choice for me?

Another approach is to use bit_sizes = {30, 30, 60} in the beginning. However, since RNS is using uint64_t to store all modulus, using two 30-bit modulus brings at least 2 times slow down for all calculations. One solution I can think of is to change the word size to 32 bit. This is mentioned before in #105 . It was considered not worth the big changes at that time. However, we now have Intel HEXL, additions and multiplications can be way faster than before. Example: _mm512_mul_epu32. But this approach is too heavy for me.

I would be grateful for any suggestions!