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I am working on MPCs (multi party computation) in crypto, and now I am developing a implementation of GG 18.

In sign phase, algorithm needs MtA (Multiplicative to Additive) and uses a Paillier key pair for this.

Paillier uses modulus $N$ ($N=p_1 * p_2$†, prime numbers drawn at key generation). But we need to consider the order $q$ of the elliptic curve. spec256k1 for example, so the algorithm has some considerations.

Consider that Alice and Bob have $a$ and $b$ as their secrets. and they want to get $\alpha$ and $\beta$ so that $a*b = \alpha + \beta$, without revealing their secrets.

GG18 says that for modulus problem there are some considerations:

  • $a$ must be less than $q^3$.
  • $b$ must be less than $q^3$.
  • $\beta$ must be less than $q^5$.
  • $N$ must be greater than $q^8$.

In spec256k1, $q$ (115792089237316195423570985008687907852837564279074904382605163141518161494337) is very close to $2^{256}$. So $q^8$ is very close to $2 ^{2048}$.

If I generate random 1024-bit prime numbers $p_1$ and $p_2$ for Paillier key generation, I almost never can not satisfy this condition :

$N = p_1 * p_2 > q^8$

What can I do? I can use greater numbers for $p_1$ and $p_2$ (1025 bit for example. It gives me a 2050-bit $N$ that most of the time is greater than $q^ 8$)

Is there any other solution? I prefer use 1024 bit numbers for $p_1$ and $p_2$.


I use "$p_1$" and "$p_2$" instead of "$p$" and "$q$" for Paillier key generation to prevent confusion with "$q$" as order of elliptic curve.

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The difference between $2^{1024}$ and $q^4$ is over 898-bits, which leaves more than enough diversity for choosing prime numbers and protection from Fermat factoring. Simply choose a random $898$-bit number $r$, add it to $q^4$ and use this as a starting point for your prime search. Once you have found a prime number, pick another random 898-bit number and search again. Both primes that you find will be greater than $q^4$ and so their product will be greater than $q^8$.

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    $\begingroup$ Yes. This is safe against basic Fermat factoring, which cost is $(p_1-p_2)^2/(p_1+p_2)/k$ operations for some moderate $k$ (in the thousands, subject to a time/memory tradeoff). And also against known improvements e.g. this paper. We can't meet the FIPS 186-5 requirement that $|p_1-p_2|>2^{924}$, but I rescind my earlier comment that something based on Coppersmith's theorem might endanger what's proposed. SNFS won't be an issue either. $\endgroup$
    – fgrieu
    May 8, 2023 at 14:01

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