class Person(nodes.GraphEnabled):

    @nodes.graphMethod(nodes.Stored)
    def Name(self):
       return None

    @nodes.graphMethod(nodes.Stored)
    def PublicKey(self):
        return None

    @nodes.graphMethod(nodes.Settable)
    def PrivateKey(self):
        return None

class EncryptedMessage(nodes.GraphEnabled):

    @nodes.graphMethod(nodes.Stored)
    def From(self):
        return None

    @nodes.graphMethod(nodes.Stored)
    def To(self):
        return None

    def setBody(self, value):
        return [nodes.NodeChange(self._BodyStored, encrypt(value, self.To().PublicKey()))]

    @nodes.graphMethod(delegate=setBody)
    def Body(self):
        if not self.To().PrivateKey():
            raise RuntimeError("No private key has been set for %s!" % self.To().Name())
        return decrypt(self._BodyStored(), self.To().PrivateKey())

    @nodes.graphMethod(nodes.Stored)
    def _BodyStored(self):
        return ''

alice = Person(Name='Alice', PublicKey=...)
bob = Person(Name='Bob', PublicKey=...)

message = EncryptedMessage(From=alice, To=bob)
message.Body = "The president is on the line."

print message._BodyStored()    # Encrypted.
print message.Body()           # <error>  (No private key has been set for Alice!)

alice.PrivateKey = ...

print message.Body()           # "The president is on the line.