[SEC-F27] principalToSubaccount uses no domain separator

[krpeacock]

Observation

IIUC the invoice subaccounts use ‘invoice-id’ as domain separator for hashing, whereas the principal subaccounts don’t use a domain separator.

Risk description

There could be risk of hash collisions if the domains are not properly separated (not investigated in further detail).

Recommendations

For domain separation to be effective, we recommend to also add a domain separator to principalToSubaccount.