How to avoid impersonation when using DescendOrigin XCM

Question

I'm using Transact to execute an extrinsic from one parachain to another. This extrinsic requires the origin to be a signed account which I'm verifying with ensure_signed.

The way I'm getting a signed account from an xcm origin is through the OriginConverter type defined on xcm_executor::Config in the runtime. I had to implement my own struct to retrieve the account id sent down and converted into a signed accountId:

pub struct SignedAccountId32FromXcm<Origin>(PhantomData<(Origin)>);
impl< Origin: OriginTrait> ConvertOrigin<Origin>
for SignedAccountId32FromXcm<Origin>
    where
        Origin::AccountId: From<[u8; 32]>,
{
    fn convert_origin(
        origin: impl Into<MultiLocation>,
        kind: OriginKind,
    ) -> Result<Origin, MultiLocation> {
        let origin = origin.into();
        log::trace!(
            target: "xcm::origin_conversion",
            "SignedAccountId32AsNative origin: {:?}, kind: {:?}",
            origin, kind,
        );
        match (kind, origin) {
            (
                OriginKind::Xcm,
                MultiLocation { parents: 1, interior: X2(Junction::Parachain(para_id), Junction::AccountId32 { id, network }) },
            ) =>
                Ok(Origin::signed(id.into())),
            (_, origin) => Err(origin),
        }
    }
}

The way I see it, there is no actual check that the person who sent the xcm is the owner of the accountId. There is no signature on the message or anything. I am just creating a Origin::signed(id) from the id extracted.

So how would I go about ensuring that an origin sent with DescendOrigin is signed, or trusted to be from the person who own the private key of that account?

Answer 1

So how would I go about ensuring that an origin sent with DescendOrigin is signed, or trusted to be from the person who own the private key of that account

Note the ConvertOrigin you mentioned is executed on receiver chain, it can trust any DescendOrigin instruction. And the security should be guaranteed by the sender chain.

For example:

ParaA send DescendOrigin(Alice) to ParaB, ParaA need to make sure Alice has signed, then can add DescendOrigin(Alice) to the message, and then send it to ParaB. ParaB only needs to execute the DescendOrigin instruction.

In addition, please note that the highest authority the sender chain has on the receiver chain is the authority of its sovereign account. So if the sender does not do enough security, the biggest loss is the loss of the sender's sovereign account. Therefore, it is better that the signature security is guaranteed by the sender chain.