Secure email

There are various issues concerning secure email.....

How to relate SPKI with PGP keys: how could you implement a secure
email framework similar to PGP using SPKI, focusing on identities and
trust management/relations in PGP?

what are the components (e.g. certificates) of SPKI that should be used
in order to achieve the desired functionality, and what they correspond
to in the PGP setting ?

thank you for reading......
cheers!!!!