FeldmanVerifier::verify() panics if given a malformed Share

[jgallagher]

Hi! I was testing vsss-rs with arbitrary Shares, and expected this to pass (i.e., passing an empty share should not verify):

    let sk = SecretKey::random(&mut osrng);
    let nzs = sk.to_secret_scalar();
    let (_shares, verifier): (_, FeldmanVerifier<Scalar, ProjectivePoint>) = Feldman { t: 3, n: 5 }
        .split_secret(*nzs, None, &mut osrng)
        .unwrap();

    assert!(!verifier.verify(&Share(vec![])));

but it panics with an out of bounds slice access:

thread 'main' panicked at 'range start index 1 out of range for slice of length 0', /home/john/.cargo/registry/src/github.com-1ecc6299db9ec823/vsss-rs-2.0.0-pre2/src/standard/share.rs:60:10
stack backtrace:
   0: rust_begin_unwind
             at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/std/src/panicking.rs:498:5
   1: core::panicking::panic_fmt
             at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/core/src/panicking.rs:116:14
   2: core::slice::index::slice_start_index_len_fail
             at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/core/src/slice/index.rs:35:5
   3: <core::ops::range::RangeFrom<usize> as core::slice::index::SliceIndex<[T]>>::index
             at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/core/src/slice/index.rs:329:13
   4: core::slice::index::<impl core::ops::index::Index<I> for [T]>::index
             at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/core/src/slice/index.rs:15:9
   5: <alloc::vec::Vec<T,A> as core::ops::index::Index<I>>::index
             at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/alloc/src/vec/mod.rs:2520:9
   6: vsss_rs::standard::share::Share::value
             at /home/john/.cargo/registry/src/github.com-1ecc6299db9ec823/vsss-rs-2.0.0-pre2/src/standard/share.rs:60:10
   7: vsss_rs::standard::verifier::feldman::FeldmanVerifier<F,G>::verify
             at /home/john/.cargo/registry/src/github.com-1ecc6299db9ec823/vsss-rs-2.0.0-pre2/src/standard/verifier/feldman.rs:86:37
   8: vsss_rs_panic::main
             at ./src/main.rs:15:14
   9: core::ops::function::FnOnce::call_once
             at /rustc/9d1b2106e23b1abd32fce1f17267604a5102f57a/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

It looks like Share's implementation assumes the vec it wraps is at least 1 byte long in all three of these methods. I could add a check that the vec isn't empty before creating the share, but it doesn't seem ideal that one can create a Share (using either the pub vec like I did above, or via the TryFrom<&[u8]> that always succeeds) that will then panic if used.

[jgallagher]

I don't think this is directly relevant to the issue at hand so I omitted it from the main writeup, but I had a hard time finding a combination of crate versions that would compile. The above example was with

p256 = "0.9"
rand = "0.8"
vsss-rs = { version = "2.0.0-pre2", default-features = false, features = ["std"] }

[mikelodder7]

@jgallagher Can you check and see if this is still an issue?