Extended private keys are defined in
BIP321 and are
used to recover funds in case of a loss, but recovering a wallet using just the
extended private keys is a tricky process and can sometimes fail to recover all
the funds as some metadata can be missing. The current implemenation also has a
weakness in which there is a limit to the incoming payment requests, handing
out more than
20 incoming payment requests could lead to destruction of funds.
To remedy this issue, an early draft of a new serialization/encoding format for extended public and private keys was proposed on the Bitcoin-dev channel.