Should check whether a long fork is valid before trust it

[yangby-cryptape]

Issue

Let $N_{last}$ denote the constant LAST_N_BLOCKS.

Let $n{k}$ denote the block number of block $B{k}$.

Suppose there are follow steps:

• Start a light client C.

• C connected to a CKB node N.

• C gets last state of block $B_{1}$ and its proof from N.

  And C saves $N_{last}$ last-N headers in its storage.

• The last state from N changes to a fork chain which last state is block $B_{2}$.

  And the last common ancestor for block $B{1}$ and block $B{2}$ is $B_{fork}$.

When $n{1} \gt n{fork} + N{last}$, the client couldn't have the header of $B{fork}$.

Then we couldn't know whether the fork chain is valid, because the fork chain is not base on any known block.

So, a malicious node could send an invalid long fork chain to the light client easily, and it will cause the light client panics.

Solution

The light client has to do a full chain sampling for the long fork chain, before panics.