Prof. Wesslen would like you to use two to four published sources such as books or journal articles. Theses and dissertations are not part of this set, but they may be used to find other published references.
Search the library catalogue for books by typing in keywords associated with your topic. If you are looking for RSA Encryption, for example, you can type in RSA Encryption, or you can think big and and look for Cryptography or Mathematical Cryptography:
Select books that may give you a broader overview. In the example shown below, I searched for RSA Encryption, but I kept scrolling through book results until I found a text on mathematical cryptography.
I clicked on the title of the book and was able to look at the full record for it. I looked at the Subjects, and I also looked at the Description. It helped me to get a sense that the book would be a good introduction to cryptography, including RSA Encryption.