A question about Negative number multiplication

[siaaron045]

I am sorry to trouble you.
When i use FullRNS-HEAAN to do a negative number multiplication, the answer is wrong.
Can FullRNS-HEAAN do negative number multiplication?(negative number addition is right)
Thanks a million.

[KyoohyungHan]

Hi siaaron045

I tested as the following command.

// Encrypt mvec
Ciphertext cipher = scheme.encrypt(mvec, slots, L);

// Multiple negative value (= -0.1)
Ciphertext cmultCipher = scheme.multByConst(cipher, -0.1);
scheme.reScaleByAndEqual(cmultCipher, 1);

I checked the decrypted result. And It seems correct.
It would be better if you can attach some part of your code.

Best, Kyoohyung Han

[siaaron045]

I test your code and the answer is right,
but when ''cipher'' is a negative number's ciphertext or make multiplication of two negative ciphertext,the answer is wrong.
like this:
//Encrypt mvec=-5 Ciphertext cipher1 = scheme.encrypt(mvec, slots, L); Ciphertext cipher2=cipher1; Ciphertext cmultCipher = scheme.multByConst(cipher1, -1); //or Ciphertext cmultCipher = scheme.mult(cipher1, cipher2); scheme.reScaleByAndEqual(cmultCipher, 1);
both of the answers are wrong.
So the encrypted number can't be negative?

[KyoohyungHan]

Oh... when I put -1 instead of -5 it works.

This problem came from the negate function in Context.cpp.
I will fix this problem soon.

Thanks for the report.

[siaaron045]

Yes,but it works only when mvec=-1, i put -2,-3,-4 the answers are wrong.
And the function scheme.mult(cipher1, cipher2) does not work when make multiplication of two negative ciphertext expect -1.
Thanks.

[siaaron045]

The error still exist.
I hope you can take some time to fix it.
Thank you.

[pdroalves]

@KyoohyungHan Looks like you found the error source. Can you be more specific where is it? I think I can submit a patch.

[Cmiiii]

The error still exist. I hope you can take some time to fix it. Thank you.

Hi~ is this problem solved? I found the same error too,but i have no idea how to fix it?Where exactly does this problem occur?

[czh-rot]

This problem is not because of the negate function, but because of a problem with encode, where for copying simply adding the modulus leads to an encoding error. You can fix it by referring to the encoding functions of OPENFHE or Lattigo.