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BGV/BFV inverse of p mod Q #733
Description
Hi,
I have a (perhaps dumb) question. In BGV or BFV, the plaintext modulus is p or p^r and Q represents the ciphertext modulus. Is there any function to compute p^{-1} mod Q? I was looking at try_invert_uint_mod (https://github.com/microsoft/SEAL/blob/main/native/src/seal/util/uintarithmod.cpp#L15) but it only supports uint64 values, while Q can go more than 64-bit.
Thanks!
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kimlaine commentedon Nov 17, 2025
The function in
uintarithmod.cppsupports integers modulo a multi-word modulus. There are some overloads that are for single-word moduli, and in fact they are used (almost) always. Actually, I'm not sure if the multi-word function you mentioned is used at all, because everything is done in CRT representation.So, I'm not really sure what your question is. The function you pointed out takes
uint64_count(the word count) as a parameter.wahyudierwin commentedon Nov 17, 2025
Thanks for your response, Kim Laine (@kimlaine) . I just realized my question is ambiguous and confusing, my bad.
What I'm looking for is the
ctxt.divideByP()function similar to HElib (https://github.com/homenc/HElib/blob/master/src/Ctxt.cpp#L2415). Is there any such function in SEAL?