Computer Security

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Computer Security

2019/2020

P. Lafourcade M. Lombard-Platet

Final Exam 120 minutes

Date: 02.03.2018 TOTAL: 90 points

Notice: the number of points corresponds approximately to the number of minutes needed for solving an exercise.

Exercise 1 (Course Questions, 6 points)

1. (1.5 point) Give the name of three asymmetric encryption schemes.

2. (1.5 point) Give three differences between symmetric and asymmetric encryption schemes.

3. (1.5 point) What are the three properties of a cryptographic hash function?

4. (1.5 point) What is the aim of TOR?

Exercise 2 (Boneh-Lynn-Shacham Signature Scheme, 4 points)

The BLS signature scheme uses a bilinear pairing e : G×G → G and satisfying the following property:

e(g

^a

, g

^b

) = e(g, g

^b

)

^a

= e(g, g)

^ab

,

where g is a public generator of the group G of prime order r, and a, b ∈ J 0, r − 1 K . Alice has the secret key sk and the public key pk, where sk = x ∈ J 0, r − 1 K and pk = g

^x

. She signs the message m as follows:

σ = H(m)

^x

,

where H : {0, 1}

^∗

→ G is a public hash function, and sends the following signature (m, σ).

How do you verify the signature σ of the message m with the public key pk?

Exercise 3 (Pizza Gourmet Unlimited, 17 points)

The company Pizza Gourmet Unlimited is composed of subsidiaries in France. It has been decided to modernize the IT infrastructure to improve the security via a PKI architecture.

The parent company in Grenoble decides that each subsidiary has the responsibility of itself. Each subsidiary has two servers:

• Extern web server for clients orders;

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• Intern web server accessible via an Intranet shared between subsidiaries to sale raw material.

The aim of the parent company is to secure transactions between subsidiaries, to au- thenticate intern employees of the same subsidiary, and to secure pizza orders done from the web server.

To keep autonomy, each subsidiary has its own web server. Clients use it to order pizzas. For example, the website https://clermont.pizza-gourmet.com is managed by the subsidiary of Clermont and clients are authenticated during their order to receive some discounts.

The chosen solution to secure transactions is the use of PKIX with X.509 certificates.

Each subsidiary has a Certification Authority (CA) and each employee has a X.509 certifi- cate. The secret key of an employee is stored on a smart card where a password is asked to perform private operations. Each computer of the subsidiary has a smart card reader.

1. Technical choice

(a) (2 points) What are the components of a X.509 certificate?

(b) (1 point) What is the principle of the trust model in OpenPGP?

(c) (1 point) Justify the choice to use PKI with X.509 certificate in the case of Pizza Gourmet Unlimited.

2. Deployment

(a) (1 point) Instead of use smart cards, it would have been possible to store employ- ees private keys in files protected by passwords on a hard drive shared between subsidiaries. Which is the advantage given by using smart cards?

(b) (1 point) Which is the CA that should emit the certificate of the extern web server of the Nantes subsidiary?

(c) (1 point) Orders between subsidiaries are done by subsidiaries employees via a Web portal. The Web portal is available from the Intranet shared between subsidiaries. The employee chooses the subsidiary with whom he wants to make the transaction, and the portal redirect the employee to the intern web server of the desired subsidiary. Which is the entity that should emit the certificate of the Web portal?

3. A session of inter-subsidiaries order : the Clermont subsidiary will soon run out of mushrooms. The Lyon subsidiary is ready to sale it some mushrooms. The portal redirects a Clermont employee to the intern server of a Lyon employee such that he can order mushrooms.

(a) (2 points) How the Web browser of a Clermont employee can validate the server

certificate of Lyon subsidiary? Give details on each implicated certificate.

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(b) (2 points) How the Lyon server can validate the certificate of a Clermont em- ployee? Give details on each implicated certificate.

4. A client order : Bob is a client of Nantes subsidiary and orders his favorite pizza via the website https://nantes.pizza-grounet.com. His Web browser shows him a green lock, then Bob gives his login and associated password to get discount. He performs the order and waits for the pizza delivery.

(a) (2 points) Explain how the Bob’s Web browser validates the identity of Nantes subsidiary’s website.

(b) (2 points) An other solution is to deliver a X.509 certificate to Bob to order inline. Explain advantages and drawbacks of the X.509 certificate for Bob com- pared to the method of login/password.

(c) (2 points) What are the modifications that Pizza Gourmet Unlimited company has to perform on his website if the chosen solution is to deliver X.509 certificates to their clients?

Exercise 4 (SSI Presentations, 55 points)

1. (5 points) Give five different types of malwares with one example for each of them.

2. (5 points) Give an example of SQL injection and a countermeasure.

3. (a) (2 points) Give an example of a zero-knowledge proof.

(b) (1.5 point) Give an application of a zero-konwledge proof.

(c) (1.5 point) Give the three properties satisfied by a zero-knowledge proof.

4. (a) (1 point) On which operating system Active Directory is designed?

(b) (1 point) What does LDAP mean?

(c) (1.5 point) What is the aim of Active Directory?

(d) (1.5 point) What are the 3 main principals used in Active Directory?

5. (a) (1 point) When the KRACK attack was discovered?

(b) (4 points) Can KRACK be used to obtain wifi credentials that could be used to normally connect to an Access Point? Explain why.

6. (a) (1 point) When the Heartbleed attack was discovered?

(b) (2 points) Explain what is the Breach attack?

(c) (2 points) Explain what is the Heartbleed attacks?

7. (a) (2 points) Explain the differences and relationship between Trojan, DDOS and Botnet?

(b) (1 point) What is Zeus?

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(c) (2 points) Give and explain the two classic Botnet architectures.

8. (a) (2.5 points) What is a XSS attack? Give an example.

(b) (2.5 points) Give three types of XSS attacks.

9. (a) (1 point) When WannaCry was discovered?

(b) (1 point) What is the impact of WannaCry?

(c) (3 points) What are the two vulnerabilities that exploit WannaCry?

10. (a) (2 points) What is a TPM?

(b) (3 points) Give two uses of TPM.

11. (a) (1 point) What does IPSec mean?

(b) (1 point) What kind of encryption is used in IPSec?

(c) (1 point) Which protocol is used to establish keys?

(d) (2 points) What are the two main modes in IPsec?

Exercise 5 (Scytale, 8 points)

David has intercepted the following message:

SUTSTRUISREERYNAOCTAOSCIIOPDTIPCS

Knowing that it has been generated using a scytale, what is the message that he will read?

Hint: what is the size of the ciphertext?