Differential cryptanalysis tutorial of Howard M. Heys I am currently working on some research about differential cryptanalysis. There is a lot of literature about that topic in the internet. One of the most known paper is the tutorial on linear and differential cryptanalysis by Howard M. Heys. May be some of you know that paper, if not have a look here:

https://www.engr.mun.ca/~howard/PAPERS/ldc_tutorial.pdf

In the meanwhile I got a good idea about the attack and how it can be practically done. In the most tutorials on the net the process works a little bit different as by Heys. My focus here is the extraction of the key bits. This part is described in section 4.4 in the Heys paper. Heys starts the extraction at the end of the cipher, he wants to recover the last subkey. He is doing it by trying to decrypt the last round (How can the other keys be obtained? Heys does not tell about). As far as I currently understand the differential cryptoanalysis, it is not possible to encrypt single rounds (until now I read the paper of Heys). Because: All I have are some plaintext / ciphertext pair which I could obtain from the user of the cipher. The user encrypts with the key which I want to recover by doing the attack. So it should not be possible to obtain single outputs of a specific round or decrypt a single round by the user. (Me as an attacker should not be able to interfere the decryption and get input / outputs of a round while the user uses the algorithm)
Is my guess here wrong? How could the partial decryption could be done? I am referring to page 26 "A partial decryption..." and so on. Or what is Heys here doing?

Thanks for your attention and help.
There is no assumption of obtaining any outputs of an internal round at all.

Having done the analysis, the attacker submits enough plaintext pairs with fixed difference $\Delta P$ so that he can be sure with high probability that the most common ciphertext difference is $\Delta C$.

Note that in his example, the 2nd and 4th Sboxes are active, so the target key bits are those at the output of Sboxes $S_{4,2}$ and $S_{4,4}$. 

Now he can do a loop where he tries all the $2^8$ possible key bit hypotheses
$$(\widehat{K_{5,5}},\widehat{K_{5,6}},\widehat{K_{5,7}},\widehat{K_{5,8}},\widehat{K_{5,13}},\widehat{K_{5,14}},\widehat{K_{5,15}},\widehat{K_{5,16})},$$
and under each hypothesis (since he knows the ciphertexts) he can determine the hypothetical differentials (correct subject to the key guess being correct) at the output of the Sboxes $S_{4,2}$ and $S_{4,4}$ which by inverting the Sboxes can be converted to hypothetical input differences of the form $\Delta U_{4,5},\ldots,\Delta U_{4,16}$.

Whichever key combination gives the most likely differential $$\Delta U_4=[0000~0110~0000~0110]$$ is declared the most likely key guess for the last round.
So he would keep a count corresponding to each hypothesized round 5 key as the loop runs and check which value gave the maximum count.

Then the rounds can be peeled off one by one as in the comments.