Harry writes a letter to Sirius Black and sends it through his owl, Hedwig via owl post. This represents one user sending information to another.
I always knew that boy was up to no good.
Harry, ~~~~~~~~ ~~~~~~~~~ ~~~~~~~~~ Sirius Black
Caught in the act, Potter!
Sirius gets the letter and writes a reply, sending Hedwig back to Hogwarts so Harry can receive it. This symbolizes the receiver sending a response.
Awh...you poor thing!
Just outside Hogwarts, Professor Umbridge performs a spell that paralyzes Hedwig so she can gain possession of the letter. This represents the "man in the middle" intercepting the message.
Professor, Hedwig came back from her trip with a broken wing.
Someone intercepted her! They've been reading my letters!
London is awfully far away, Potter. I'm sure she just got overexerted.
Umbridge and Draco look through the letters they have intercepted, finding information about students, such as Harry's relationship with Sirius. This shows the cyber criminal obtaining personal information about their targets.
Hedwig is sent back to Harry, where he notices his owl's broken wing. This represents the target being forwarded the information that was intercepted.
Harry suspects that his letter has been read and reports it to Professor McGonagall. In his future letters to Sirius, he refers to him as Snuffles and remains vague about his information and uses secretive terms. This represents the victim encrypting their letters to combat attacks.