Hellman's Hints

[Norman Hardy]

I presume that Hellman meant to say 
            "K1 and K2, and thence UK"
in place of "K1 and K2, and thence K" at least it makes sense that way.
A later posting from Hellman (I think) emmended the description
of the transmitted message from
    E{ E[M; K], E[K; UK], serial number; SK}
to  E[M; K], E{ E[K; UK], serial number; SK}
 
If you know SK then you can compute (E[K; UK], serial number)
Then knowing UK (= K1+K2) you can compute K
from which you get M via E[M; K].