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unexpected behaviour of get_representative_least_absolute_residue() #68
Description
Describe the bug
According to the documentation, get_representative_least_absolute_residue should do the following:
Returns the Z value of the Zq element with the representatives close to 0.
The output value is in the range of [-modulus/2, modulus/2]. For even moduli, the positive representative is chosen for the element modulus / 2. Use Zq::get_representative_least_nonnegative_residue if they should be in the range [0, modulus).
So for a value >= modulus / 2 it should output value - modulus. In reality it outputs modulus - value.
To Reproduce
use qfall_math::{integer::Z, integer_mod_q::Zq};
let x = Zq::from((256, 257));
let actual = x.get_representative_least_absolute_residue();
// 256 ≡ -1 (mod 257).
assert_eq!(actual, Z::from(-1));Expected behavior
actual should be -1.
- Version of qFALL-tools: 0.11
Additional context
Solution
Activity
jnsiemer commentedon Oct 20, 2026
Dear Jonas,
Thanks for making us aware of this issue.
Fortunately, I encountered this issue myself whilst implementing qfall/schemes#10 (ML-DSA / Dilithium) and fixed it in qfall/math#531 (as it was an issue of
qfall-math).I took this issue as reason to specify the doc-comments further and include additional tests for this edge case in qfall/math#538.
The new release of
qfall-mathandqfall-tools(andqfall-schemes) in version0.2resolves the described issue.Cheers!