attestation protocol is not cancel safe

i took a shortcut implementing the attestation protocol and used [read_exact](https://docs.rs/tokio/latest/tokio/io/trait.AsyncReadExt.html#method.read_exact). This function is not cancel safe. Probably worth the effort to just use the cancel safe [read](https://docs.rs/tokio/latest/tokio/io/trait.AsyncReadExt.html#method.read) function and do the extra work to handle `io::ErrorKind::Interrupted`.