digraph {
A1 [label="A's PDP-11"]
A2 [label="A's Osborne 2"]
AS [label="A's self-signing key"]
AU [label="A's user-signing key"]
BU [label="B's user-signing key"]
BS [label="B's self-signing key"]
B1 [label="B's Dynabook"]
B2 [label="B's VAX"]
AS -> A1
AS -> A2
AS -> AU
AU -> BS
BS -> BU
BU -> AS
BS -> B1
BS -> B2
}