Cannot use large params

[crockeea]

I'm trying to use parameters larger than natively permitted by SEAL with CKKS. Specifically, I'm trying to use a much larger ring dimension so that I can use more moduli.

I've set SEAL_POLY_MOD_DEGREE_MAX to 262144 and SEAL_COEFF_MOD_COUNT_MAX to 122 in defines.h. I'm creating the CKKSContext using context = SEALContext::Create(*params, true, sec_level_type::none);, but I"m still getting "encryption parameters are not set correctly" from line 26 in ckks.cpp. I did more investigation and traced the problem to context creation, specifically parameter validation in context.cpp. Near the end of validate, it tries to create a base_converter, which seems to be failing in the block below the comment // generate Bsk U {mtilde} small ntt tables which is used in Evaluator.

How might I resolve this problem?

[crockeea]

I'm using the 3.4.5 branch.

[kimlaine]

Thanks for reporting this Eric. If you look at native/src/seal/utils/globals.cpp, you'll see several hard-coded primes towards the end of the file in the namespace internal_mods. These primes are not of the appropriate form for your poly_modulus_degree, and you'll need to regenerate them to make it work.

Specifically, you'll need all of the primes in internal_mods to be 61 bits in size and congruent to 1 mod 2*poly_modulus_degree. They all need to be distinct too. To be clear, this includes m_sk, gamma, and the 128 primes in aux_small_mods. To create these primes compute 2^61 - 2*poly_modulus_degree*A + 1, and then increment A until you find a prime.

There might still be an issue related to some lazy reductions possibly failing with so many coeff_modulus primes, but I'm not sure without digging deep which I can't do right now.

We'll try to make this work correctly and automatically in an upcoming patch (3.4.6) or minor release (3.5.0) at the latest.

[crockeea]

Thanks! That got me through this particular problem.