Constant CHALLENGE_LENGTH_IN_LIMBS 

pub const CHALLENGE_LENGTH_IN_LIMBS: usize = 2;

Number of 64-bit limbs used to represent a scalar challenge.

With 2 limbs, challenges are 128 bits. This is sufficient because:

Endomorphism decomposition: The challenge is converted to an effective scalar k = a·λ + b where both a and b are derived from the 128-bit input. Since λ is a cube root of unity in a ~255-bit scalar field, a 128-bit challenge provides enough entropy for both components.