CS 423 NETWORK SECURITY: MIDTERM ACTUAL 2025/2026 UPDATE

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CS 423 NETWORK SECURITY: MIDTERM ACTUAL 2025/2026 QUESTIONS AND 100% CORRECT ANSWERS What is the level of security provided by double DES? - Answer -2^56 (due to meet-in-the-middle attack) What is a digital certificate and what is it used for? - Answer -It is a digital document, signed by a trusted third party (certificate authority) establishing the binding between a subject's identity and the subject's public key. It is used to establish the authenticity of users' public keys A pixel map (consisting of a matrix corresponding to the pixel values) of image in Figure (a) was encrypted with Triple-DES in two different modes, ECB and CBC, to obtain the images in Figures (b) and (c) "not" necessarily in that order. Which mode does Figure (b) correspond to? Figure (c)? Explain why. - Answer -Figure B: CBC; Figure C: ECB; ECB encrypt the same planter blocks to same ciperhtext blocks every time. Hence, it preserves the patterns of plaintext into cipher text (e.g., the same pixel values in the plaintext image get mapped to the same pixel values in the cipher text image). In contrast, CBC is randomized tend due to the use of an IV (and chaining), the same plaintext gets encrypted to entirely different cipher text each time. A pixel map (consisting of a matrix corresponding to the pixel values) of image in Figure (a) was encrypted with Triple-DES in two different modes, ECB and CBC, to obtain the images in Figures (b) and (c) "not" necessarily in that order. Which image, B or C, represents a "good" encryption? Explain why. - Answer -Image B represents a good encryption because there ar no decipherable patterns in the encrypted image. In case of image C, the pattern of pixels in original image is carried over to the encrypted image due to lack of randomization. Hence, an adversary can deduce the Need assistance on Online classes, Exams & Assignments? Reach out for instant help!! Full Course Assistance, Plagiarism-free Essay Writing, Research Paper, Dissertation, Discussion Posts, etc…. Confidential & Secure services. Tutors are available for all subjects! Email now at: tutorjean01@gmail.com original pixel pattern if he observes the same pattern in the encrypted image. Thus, image B is secure (against CPA) whereas image C is not. Does a MAC scheme provide non-reputation? Explain your answer. - Answer -No, Since the MAC is generated using a symmetric key (shared between the sender and the verifier), either can create it and keep on blaming that the other created it True or False: PGP uses RSA to encrypt the email messages. - Answer -False, RSA is used to agree upon a symmetric key and symmetric key algorithm is used to encrypt the message. True or False: PGP uses RSA to sign the email messages. - Answer -True In which order, does PGP perform message encryption, signature, and compression operations? - Answer -Signature, compression, encryption Alice's RSA public key is N=15 and e = 3. Show how an attacker (who knows N and e), can learn Alice's private key d. Show all steps. It is not necessary to calculate the value of d, just show the steps needed. - Answer -N = 15, e = 3 So public key would be the pair (e, N) i.e. (3, 15) The private key 'd' is the inverse of 'e' in Z_phi(n) so we first need to calculate phi(n). The attacker knows that N is a product of two primes so if he is able to factorize N as p*q where p and q are primes, he can get phi(n). Factoring N=15, we get p=3, q = 5 (both are prime). So phi(n) = (p-1)(q-1) = (3-1)(5-1) = 2 * 4 = 8. Now the attacker just needs to find the inverse of 'e' in Z_phi(n) i.e. Z_8 = {0,1,2,3,4,5,6,7}. For 'd' to be inverse of e in Z_8, ed = 1(mod 8) or 3d = 1 (mod 8). d = 3 satisfies the above equation because 3*3 = 9 = 1(mod 8). So Alice's private key is 3. Describe how a DNS Reflection Attack works (a form of DoS attack). - Answer -The attacker sends a large number of queries to DNS servers spoofing the IP address of victim machine. All the DNS server responses will then be sent/reflected back to the victim's machine, probably Need assistance on Online classes, Exams & Assignments? Reach out for instant help!! Full Course Assistance, Plagiarism-free Essay Writing, Research Paper, Dissertation, Discussion Posts, etc…. Confidential & Secure services. Tutors are available for all subjects! Email now at: tutorjean01@gmail.com