bug: CBC mode test fails randomly on out of bounds

[Autoparallel]

Describe the bug
Apparently we can, due to rng, get an out of bounds error in the CBC encryption mode. See: https://github.com/pluto/ronkathon/actions/runs/13179737670/attempts/1

To Reproduce
N/A?

[mahmudsudo]

I would suggest restricting the values/length of the generated numbers

[Autoparallel]

Do you know that these numbers being hit are sometimes too big/small?

[mahmudsudo]

// remove PKCS#7 padding by checking the last byte and removing all intermediate bytes
    let last_byte = plaintext[plaintext.len() - 1];
    if plaintext[plaintext.len() - last_byte as usize] == last_byte {
      plaintext.truncate(plaintext.len() - last_byte as usize);
    }

this is where the failure stems from, it seems last_byte is zero

[Autoparallel]

Ah 1/256 chance then. No wonder we haven't seen this but still likely enough to fail.

[Autoparallel]

@mahmudsudo good find there by the way. Marked this as "good first issue" since this shouldn't be too hard to fix.

[mahmudsudo]

On second thought , a little check on the value of last_byte might help