better remove #[derive(Default)] in bitflags data structure

[longlongyang]

When debugging the issue #63 , I found that, the match pattern doesn't check the default value 0, the value 0 means responder doesn't support mutual authentication feature.
But when creating object and setting bits on the bitflags object, default value 0 is used, it may result in inconsistency.

I would recommend to remove all #[derive(Default)] in bitflags data structure, and add default item to the bitflags data structure explicitly, then It may minimize the gap programmer make mistakes.

[longlongyang]

Example:
consider

bitflags! {
    #[derive(Default)]
    pub struct SpdmKeyExchangeMutAuthAttributes: u8 {
        const MUT_AUTH_REQ = 0b00000001;
        const MUT_AUTH_REQ_WITH_ENCAP_REQUEST = 0b00000010;
        const MUT_AUTH_REQ_WITH_GET_DIGESTS = 0b00000100;
    }
}

the match pattern

match mut_auth_requested {
            SpdmKeyExchangeMutAuthAttributes::MUT_AUTH_REQ => Ok(()),
            SpdmKeyExchangeMutAuthAttributes::MUT_AUTH_REQ_WITH_ENCAP_REQUEST
            | SpdmKeyExchangeMutAuthAttributes::MUT_AUTH_REQ_WITH_GET_DIGESTS => {
                self.get_encapsulated_request_response(session_id, mut_auth_requested)
                    .await
            }
            _ => Err(SPDM_STATUS_INVALID_MSG_FIELD),
        }

in this case, when responder doesn't support mutual authentication, the value of mut_auth_requested of requester will be SpdmKeyExchangeMutAuthAttributes::empty() which is 0,
By pattern match, requester will run into Err(SPDM_STATUS_INVALID_MSG_FIELD). This is not a correct behavior, requester should just ignore the mutual authentication and proceed instead of running into error and break the start session.

My suggestion,

change definition of SpdmKeyExchangeMutAuthAttributes to:

bitflags! {
    pub struct SpdmKeyExchangeMutAuthAttributes: u8 {
        const MUT_AUTH_DISABLED = 0b00000000;
        const MUT_AUTH_REQ = 0b00000001;
        const MUT_AUTH_REQ_WITH_ENCAP_REQUEST = 0b00000010;
        const MUT_AUTH_REQ_WITH_GET_DIGESTS = 0b00000100;
    }
}

impl Default for SpdmKeyExchangeMutAuthAttributes {
    fn default(&self) -> Self {
        self::MUT_AUTH_DISABLED
    }
}

the match pattern

match mut_auth_requested {
            SpdmKeyExchangeMutAuthAttributes::MUT_AUTH_REQ => Ok(()),
            SpdmKeyExchangeMutAuthAttributes::MUT_AUTH_REQ_WITH_ENCAP_REQUEST
            | SpdmKeyExchangeMutAuthAttributes::MUT_AUTH_REQ_WITH_GET_DIGESTS => {
                self.get_encapsulated_request_response(session_id, mut_auth_requested)
                    .await
            },
           SpdmKeyExchangeMutAuthAttributes::MUT_AUTH_DISABLED => Ok(()),
        }

the benefit is that, there are only 4 explicit kinds of value in matching SpdmKeyExchangeMutAuthAttributes, compiler will force programmer to implement logic for all of them.

[jyao1]

looks good to me