[BUG]: Witness size to be a power of 2^n isn't checked in Spartan/crr1csproof.rs

[h-hafezi]

Program Information

In Spartan/crr1csproof.rs it's not checked the witness to be a power of two, in fact, it can be checked in

nexus-zkvm/spartan/src/crr1csproof.rs

Line 363 in 7f7789a

by adding the following lines:

    assert!(z.len().is_power_of_two(), "z must be a power of two");
    assert!(evals_ABC.len().is_power_of_two(), "eval_ABC must be a power of two");
    assert_eq!(z.len(), evals_ABC.len(), "vector z and eval_ABC should have equal length");

similarly, it can be checked in the conversion function in

https://github.com/nexus-xyz/nexus-zkvm/blob/main/nova/src/circuits/nova/pcd/compression/conversion.rs

when converting an r1cs instance to check witness size to be a power of two in

nexus-zkvm/nova/src/circuits/nova/pcd/compression/conversion.rs

Line 105 in f37401c

impl<G> TryFrom<RelaxedR1CSWitness<G>> for CRR1CSWitness<G::ScalarField>

Project Information

No response

Reproduction Steps

...

What is expected?

...

What is actually happening?

if this is not done you get a reading index of zero from an empty vector in the function

nexus-zkvm/spartan/src/crr1csproof.rs

Line 40 in 7f7789a

impl<F: PrimeField> SumcheckInstanceProof<F> {

System Information

No response

Any additional comments?

No response

[nexodon]

Thanks @h-hafezi for this thorough check and proposed fix! Definitely worth taking. We'll look.