A campus messaging app uses Diffie-Hellman key exchange for students to share notes. Here’s how it works:

1. Students publicly agree on:
   Prime and base .
2. Alice picks a private secret (e.g., , computes , and sends A to Bob.
3. Bob picks a private secret (e.g., , computes , and sends B to Alice.
4. Both compute a shared secret key to encrypt messages:
   Alice:
   Bob:

An attacker intercepts and , impersonates Bob to Alice, and sends fake exam answers (e.g., "The answer to Q3 is 42"). Alice trusts the message because she thinks it’s from Bob.

How does this scenario shift responsibility for security from the app developers to the users? 

Determine the shared secret. Show this from both Alice’s and Bob’s perspective, proving that the secret is the same for both. 

Apply the RSA algorithm (in Cryptool Online) to encrypt the following:  

What is the Diffie-Hellman key exchange used for?

 Using OpenSSL (via Cryptool online) select the ‘Hashes’ tab. Under input, enter your name and hash it using sha256. Now hash the same input using the ‘blake2b512’ hash function and again using the ‘md160’ hash function.

What do you notice about the hashes that are being computed for the same input? What impact might this have when these different hashes are used in real-life scenarios?

Using https://www.cryptool.org/en/cto/rsa-step-by-step/  solve the following question and ensure you upload up to 3 screenshots for your answer. (in your Part B assessmnet, a correct answer without screenshots will not get full marks).

Using p = 33461 and q = 65837, find a suitable e and encrypt the message 1987654321.

For the last step on the cryptool page, decrypt your ciphertext and confirm you get the plaintext. Enter your value of e and your ciphertext in the response box as well as screenshots of your cryptool page.

Using analogies rather than mathematics, prepare an introduction to cryptography that you could present to teenaged schoolchildren, explaining the difference between symmetric and public-key encryption, and how these are combined to create a secure channel.

Define "Computational security".

A system encrypts data by splitting it into 128-bit blocks and applying a complex mathematical transformation (including substitution and permutation) to each block independently.

What type of cipher is this?

Many sites alert users of suspicious logins with encrypted notifications.

Which goal is being maintained?