Network Security Assignment V Solution

A: Understanding Questions

As we near the completion of this course, we now focus more on conceptual understanding questions instead of factual questions. The rst set of questions in this assignment explores this critical aspect of computer security practice. When you become a security programmer/expert/czar, you will be expected to employ your understanding of previously unseen communication scenarios. This set of questions aims at introducing you to such challenges in a controlled setting.

Question 1: Certi cates Infrastructures [2+2+3 points]

    • Public key infrastructures employ certi cate revocation mechanism using certi cate revocation lists (CRLs). In presence of CRLs, why do we include expiration dates in certi cates? Envision two advantages of including expiration dates in certi cates.

    • Every CA must reissue its CRL periodically. It is true even when no new certi cates have been revoked after the previous issue CRL has been published. Why? Suggest a possible attack if CRLs are not issued periodically.

    • In certi cate transparency (CT), explain how the use of a Merkle hash tree makes it possible for a CT log to prove the following three things e ciently.

The later version of a log includes everything in the earlier version, in the same order. All new entries come after the entries in the older version.

A particular certi cate has been included in the log.

To demonstrate e ciency, you should compare the Merkle hash tree mechanism with other trivial solutions.
Question 2: Nymity Level [4 pt]

In the lecture, we discussed the (ano)nymity levels as unlinkable anonymity, linkable anonymity, pseudonymity, and veriminity. Suggest the nymity levels for the monetary transactions with each of the following medium:

Cash

Gift Certi cates,

Loyalty card + cash Credit card

Question 3: TLS [6 pt]

For each of the following threats, explain in detail what mechanism is used in SSL/TLS to provide protection, and how it is used. Do not make any assumptions about the speci c encryption/key exchange/signature scheme used by SSL/TLS, as it is supposed to be compatible with multiple schemes.

a A man-in-the-middle adversary records all of the server’s messages in an SSL handshake. Later, he

impersonates the server by replaying these messages to a client.    [2 pt]

    • A network attacker poisons DNS cache on the client’s recursive resolver and tricks the client into visiting an attacker-controlled IP address instead of an actual IP address of the domain that the victim client

was hoping to reach.    [2 pt]

c A network attacker modi es the client’s \Hello" message in transit and tricks the server into thinking

that the client supports only relatively weak cryptographic algorithms.    [2 pt]

Question 4: Veri ed By Visa [3+2 pt]

Many websites invoke third-party payment services, such as PayPal or ‘veri ed by Visa’. These services reduce the risk of exposure of client’s credentials such as credit-card number, by having the seller’s site open a new ‘pop-up’ window, at the payment provider’s site, say PayPal; and then having the users enter their credentials at PayPal’s site.

    • Assume that a user is purchasing at the attacker’s site. Explain how that site may be able to trick the user into providing their credentials to the attacker. Assume typical user, and exploit typical human vulnerabilities. Present the most e ective attack you can.

b Propose and explain two things that may help to reduce the chance of such an attack signi cantly.

Question 5: Understanding Privacy Risks [10 pts]

Imagine you were in year 2020. All of us are concerned about the spread of COVID-19. To prevent the spread of the disease, your local public health department would want to have information about the contacts of all infected individuals. One way of achieving this is the following: All residents have a health app installed on their phones. When two users A and B come in close contact with one another, the phone apps exchange some information through the use of Bluetooth. Thus, if A is infected, the information on A’s phone can be turned over to the health department and used to contact B and notify B of possible
exposure to the disease. While having access to such information is useful, the residents are also worried about being surveilled by the public health department, other residents, and/or the app developers. As a security expert, you have been given the task of designing a system that helps the public health department as well as the residents while solving privacy concerns. In this context, answer the following questions:

    • A rst suggestion is to share the following information between the phone apps: when in close contact, each phone app shares the following tuple (name, phone number) of the phone owner. If resident A is infected, the health department retrieves all the contact tuples on A’s phone and publicly announces them on a website. Does this satisfy the correctness requirement from the perspective of the public

health department? What are the integrity and privacy concerns with such a system design against an

active/passive adversary?    [4 pts]

b A second suggestion is to instead share the tuple (r; H(rjjname); H(rjjphone number)) where H is a cryptographic hash function, r is a random 258-bit salt, and jj is concatenation operation. Does this satisfy the correctness requirement? What are the integrity and privacy concerns with such a system

design against an active/passive adversary?    [4 pts]

c Suppose I own a restaurant in your locality and I install a phone with this app installed at the entrance of my restaurant. What information can I learn about my customers just based on the information

collected by my phone for each of the above two approaches?    [2 pts]

B: Exploratory Questions

Another important aspect of security practice is to read and understand technical material such as blogs, tech-report, and research papers, and employ those in your work. This section explores this essential brach.

Question 7: Cryptocurrencies Only Needs Consistent Broadcast [5+3 pt]

In the lecture, we studied that basic two-party (payer-payee) payment mechanism doesn’t require consensus mechanism such as atomic broadcast. We described that consistent broadcast can be su cient for the task. One of the rst paper to discuss that was Fastpay (https://arxiv.org/abs/2003.11506) from Novi/Facebook. Read the FastPay paper (reading only rst fours pages until the end of Section 4.1 will be su cient) and answer the following two questions:

    • FastPay protocol requires 3f + 1 equally-trusted authorities, assuming a xed (but unknown) subset of at most f authorities among those are malicious/active adversaries. This amount to roughly 67% honest (non-malicious) authority servers.

If you recall from the lecture, something like Bitcoin only assumes 51% honesty level. This amounts to 2f + 1 equally-trusted total authorities for f malicious authorities.

Can you give an example attack when Fastpay reduces its replication factor from 3f + 1 to 2f + 1?

    • Can you compare the privacy of transactions over Fastpay with the Bitcoin privacy that we discussed in the lecture?
Question 7: Cryptocurrencies Makes Software Updating Harder [4+2+4 pt]

In the lecture, we discussed Bitcoin and Ethereum cryptocurrency/blockchain systems, and the nancial service it o ers to its users. During the last ve years, several hundred other cryptocurrencies and blockchain systems have emerged. Similar to other secure networking protocols, blockchains are also prone to attacks. Read the article ‘Responsible Vulnerability Disclosure in Cryptocurrencies’ at https: //dci.mit.edu/s/3372115.pdf and answer the following questions:

    • Using two real-world examples explain why vulnerability disclosures are particularly challenging for cryptocurrencies.

b Explain why the authors do not recommend cryptocurrencies to present bug bounties in their own coin.

    • With an example, explain how obscurity and lies are employed during cryptocurrency vulnerability disclosure and patching.

[Bonus Question 8]: Simplifying Tor session key establishment [6 pt]

In the lecture, we have studied the ntor protocol (slide 23) to perform 1W-AKE and thereby get a shared session key between a client and a server. In this approach, the server needs to perform two exponentiations to get the session key: in the last step, the server needs to compute H(gxy; gbx).

In this exercise, we will look at a variant of this protocol. Assume that we modify the 1W-AKE protocol presented in the lecture such that the session key is calculated as gxy gxb = gx (y+b). In this way, the server could calculate z = (y + b) and then do only one exponentiation (gx)z .
Is this modi ed version secure? If not, provide an attack.