2023 Mar 26

Got this error for this rust macro:

macro_rules! extend_secret_prefix_mac {
    ($mac:ident, $message:ident, $extension:ident, $digest_to_state:ident, $pad:ident, $hash_from_state:ident, $check:ident, $default:ident) => {{
        let mut new_mac = $default;
        let state = $digest_to_state(&$mac);
        for key_len in 0..=64 {
            let full_input_len = key_len + $message.len() as u64;
            let padding = $pad(full_input_len);
            let digest = $hash_from_state($extension, full_input_len + padding.len() as u64, &state);
            let final_message = [$message, &padding[..], $extension].concat();
            if $check(&digest, &final_message) {
                new_mac = digest.clone();
                break;
            }
        }
        new_mac
    }};
}

It gave me this error that digest and final_message didn’t live long enough to be borrowed. I thought this was odd because the function I modeled this after didn’t face the same problem. I learned that here it doesn’t know if $check will use these functions after the loop iteration is done, which I’m still sort of confused why that’s the case but it does explain the error. I asked GPT if I could tell the macro somehow that it doesn’t use these values and it said there wasn’t a way other than changing the $check function to take ownership of the value instead of references. Hmph. Feel like I’m using the macro wrong, like I’m treating it too much as a function that I want to pass functions into without making a generic interface for those functions heh…But it feels like I should be able to somewhat easily generalize this length extension attack to different hashes the way I’m intending but I just don’t know how.

Would be cool if there was a cryptopals equivalent specific for Bitcoin. Could call it bitcoinpals.
Do watchtowers claim revoked HTLCs? (not like claiming using the preimage, but like if the HTLC tx confirmed would a watchtower take that? (I guess the watchtower should’ve just revoked before that?)