The process of decision "a" has multiple variations.
The following is a preferred embodiment which provides
optional steps for additional security and efficiency.

/--------------\
| Start of "a" |
\--------------/
    |
    |
    v
   / \
  /   \
 /     \      /--------------------------------\
/  (4)  \--->| (5) Return "No" to decision "a" |
\ "a.1" / N   \--------------------------------/
 \     /
  \   /
   \ /
    |
    | Y
    |
    v
/----------------------------------\
| (6) Return "Yes" to decision "a" |
\----------------------------------/

"a.1" == Does the provided digest of the content to be
         validated exist as a key within a map of cached
         content?