程序代写代做代考 SQL scheme python javascript dns database Java cache algorithm 2017 – cscodehelp代写

2017
CS 161 Computer Security
Final Exam
Print your name: , (last)
(first)
I am aware of the Berkeley Campus Code of Student Conduct and acknowledge that any academic misconduct on this exam will be reported to the Center for Student Conduct and may lead to a “F”-grade for the course.
Sign your name:
Print your class account login: cs161- and SID:
Your TA’s name:
Number of exam of person to your left:
Number of exam of person to your right:
You may consult two sheets of notes (each double-sided). You may not consult other notes, textbooks, etc. Calculators, computers, and other electronic devices are not permitted. Please write your answers in the spaces provided in the test.
You have 180 minutes. There are 9 questions, of varying credit (600 points total). The questions are of varying difficulty, so avoid spending too long on any one question. Parts of the exam will be graded automatically by scanning the bubbles you fill in, so please do your best to fill them in somewhat completely. Don’t worry—if something goes wrong with the scanning, you’ll have a chance to correct it during the regrade period.
If you have a question, raise your hand, and when an instructor motions to you, come to them to ask the question.
Question: 1 2 3 4 5 6 7 8 9 Total Points: 80 74 48 72 64 54 96 56 56 600 Score:
Do not turn this page until your instructor tells you to do so.
Page 1 of 21

Problem 1 True/False (80 points) For each of the following, FILL IN THE BUBBLE next to True if the statement is correct, or next to False if it is not. Each correct answer is worth 4 points. Incorrect answers are worth 0 points. Answers left blank are worth 1 point.
(a) Thanks to strong cryptography, a TLS connection to your bank is secure even if their web server’s TCP/IP implementation has a buffer overflow vulnerability.
True False
(b) Thanks to strong cryptography, a TLS connection to your bank is secure even if your home router’s TCP/IP implementation has a buffer overflow vulnerability.
True False
(c) To protect against Kaminsky blind spoofing attacks requires servers to implement a new version of the DNS protocol.
True False
(d) Using DNSSEC to resolve example.com guarantees authenticity and integrity on subsequent HTTP connections to example.com, but not confidentiality.
True False
(e) A properly configured firewall can prevent any DDoS attack from disrupting the ability of remote users to access your network.
True False
(f) Using a prepared statement to feed user input to an SQL query ensures that nothing the user enters will be treated as an SQL command.
True False
(g) VPN can enable you to safely connect to your company when using an untrusted public WiFi network.
True False
(h) When configuring a firewall, it’s safer to use a whitelisting approach than it is to use a blacklisting approach.
True False
(i) A malicious website can execute a successful clickjacking attack even if the victim website uses HTTPS and the user’s browser correctly implements the same origin policy.
True False
(j) A secure hash function will not produce any collisions.
True False
Final Exam
Page 2 of 21 CS 161 – SP 17

(k) Recall that secure-cookies are cookies which the browser will only transmit over HTTPS connection. Using HTTPS and secure-cookies is one way to prevent click- jacking attacks.
True False
(l) SupposeAlicehassignedupfortext-messagetwofactorauthenticationonbank.com. If bank.com randomly generates a long number (e.g., a 16-digit number) for its 2FA codes and an attacker doesn’t hijack Alice’s phone number, then Alice’s bank.com account is secure against phishing attacks.
True False
(m) For AES-CBC encryption, the IV does not need to be kept secret.
True False
(n) For AES-CTR encryption, the IV does not need to be kept secret.
True False
(o) If all messages are the same length and a message is never repeated, then it is secure to re-use the same one-time-pad for encryption.
True False
(p) To securely store user passwords, a server should use AES to encrypt each user’s password and only store the ciphertexts in its database.
True False
(q) If Website A loads a website from another domain (Website B) inside of an iframe, the same origin policy prevents Javascript from Website A from accessing any of the other website’s content in the iframe.
True False
(r) A certificate authority that issues a TLS certificate for example.com can also pas- sively decrypt TLS traffic to example.com.
True False
(s) Consider a worm that spreads by each infected instance uniform randomly selecting a 32-bit IP address. We would expect the worm to initially spread exponentially fast, but then slow down its spread during the later part of its propagation.
True False
(t) The Slammer worm spread extra-fast because each infected instance of the worm kept increasing its scanning speed.
True False
Final Exam
Page 3 of 21 CS 161 – SP 17

Problem 2 Multiple Choice (74 points)
(a) (6 points) Suppose an attacker steals the private key of a website that uses TLS, and remains undetected. What can the attacker do using the private key? LL that apply.
Decrypt recorded past TLS sessions that used RSA key exchange.
responses? LL that apply.
Confidentiality Authentication Integrity Availability None of these
(c) (8 points) “Mixing program control and user data” is a class of vulnerabilities where a program/application accidentally treats user input as code and executes it. Which of the following attacks exploit this class of vulnerabilities? LL that apply.
Decrypt recorded past TLS sessions that used Diffie–Hellman key ex- change.
None of these.
(b) (6 points) DNSSEC provides which of the following security properties for DNS
Final Exam
Page 4 of 21
CS 161 – SP 17
Successfully perform a MITM attack on future TLS sessions.
Buffer overflows CSRF
SQL Injection None of these
Stored XSS Reflected XSS Clickjacking
(d) (6 points) To verify that she is visiting the correct website, Alice is told to make sure to check that the URL in the browser’s address bar is the URL she actually wants to visit. Which of the following statements are true? LL of the following statements that apply.
Of relevance for this situation is the principle of Least Privilege
Of relevance for this situation is the principle of Consider Human Factors
This will help Alice defend herself against CSRF attacks
This will help Alice defend herself against some DNS spoofing attacks
This will help Alice defend herself against some phishing attacks
None of these

(e) (6 points) Alice is trying to visit maps.google.com and neither her machine nor her local resolver have any entries in their DNS caches. In the following, assume that google.com subdomains use HTTPS and are on the predefined HSTS (HTTP Strict Transport Security) list in Alice’s browser. You do not need to worry about attacks on availability, nor attacks based on stealing private keys, malware infections, or obtaining a fraudulent google.com certificate. LL that apply.
Final Exam
Page 5 of 21
CS 161 – SP 17
For DNSSEC to work securely, the root and .com zones will need to sign their NS and glue/additional records.
For DNSSEC to work securely, the root and .com zones will need to en- crypt their NS and glue/additional records.
Because google.com subdomains are on the predefined HSTS list, Alice’s visit to maps.google.com is secure against DNS spoofing attacks.
(f) (8 points) Gandalf is surfing the web and
Assume that neither his machine nor his local resolver have any entries in their DNS caches, and that berkeley.edu is the authoritative name server for all berkeley.edu subdomains. Assuming global deployment and use of DNSSEC, and that DNS zones use Key Signing Keys (KSKs) and Zone Signing Keys (ZSKs), which of the following are True? LL that apply.
Gandalf ’s machine can use the berkeley.edu KSK to encrypt the query it sends to the berkeley.edu DNS server.
berkeley.edu’s ZSK will be signed by the root’s KSK.
berkeley.edu’s ZSK will be signed by berkeley.edu’s KSK.
Gandalf ’s machine will receive a final A record for gondor. berkeley.edu that is signed with berkeley.edu’s ZSK.
Gandalf ’s machine will receive a fi- nal A record for gondor.berkeley. edu that is encrypted with a pub- lic key that Gandalf provides in his DNS query.
The final A record for gondor. berkeley.edu will have object secu- rity.
If zones correctly implement DNSSEC, then Gandalf is secure against a MITM attacker who at- tempts to modify content retrieved from the gondor.berkeley.edu web site.
Because google.com subdomains are on the predefined HSTS list, Alice’s visit to maps.google.com is secure against MITM attacks.
Because google.com subdomains are on the predefined HSTS list, Alice’s visit to maps.google.com is secure against ssl-strip attacks.
None of these apply.
visits the URL http://gondor.berkeley.edu.

(g) (6 points) A border firewall’s primary purpose is ( NE):
Final Exam
Page 6 of 21
CS 161 – SP 17
Block incoming VPN connections. Prevent CSRF attacks.
Detect buffer overflows.
Prevent XSS attacks.
Prevent a network intruder inside the network from spreading inter- nally.
None of these.
(h) (8 points) Which of the following attacks might allow an attacker to steal one of your browser cookies ( LL that apply):
Reflected XSS Stored XSS Clickjacking None of these
Buffer overflow TLS downgrade DDoS
(i) (6 points) Alice and Bob want to communicate over an insecure channel using one of the following schemes, where M is the message in plaintext. Which scheme should they use in order to avoid padding oracle attacks? Assume that (1) all of the algorithms are secure, and (2) MAC and Sign do not leak anything about M. LL that apply.
Enc(M ), MAC(M ) Enc(M || MAC(M)) None of these
Enc(M ), MAC(Enc(M )) Enc(M),Sign(M)

Final Exam
Page 7 of 21
CS 161 – SP 17
(j) (6 points) Let S be a publicly available trusted service that knows the public keys of all users. Alice communicates with S to obtain Bob’s public key using the following protocol:
1. A−→S:A,B
2. S−→A:[KB,B]K−1
S
In step 1, Alice sends along her identity A and asks S for Bob’s public key. In step 2, S responds by returning Bob’s public key KB along with his identity B, and signs the message.
Which of the following attacks is this protocol vulnerable to? LL that apply.
Mallory can tamper with S’s re- sponse so as to substitute her own public key KM instead of KB.
Mallory can tamper with S’s re- sponse so as to substitute an older key KB′ that Bob might have re- voked.
Since S’s response is not encrypted, Mallory can use KB to decrypt any messages Alice sends to Bob in the future.
None of these.
(k) (8 points) For the same situation as in the previous question, which of the following modifications to step 2 would defend against the attacks that the protocol in that question is vulnerable to? LL that apply.
S −→ A : [KB,A,B]K−1 S −→ A : [KB,B,A,N]K−1, where SS
S −→ A : [KB,B,T]K−1 where T is a timestamp S
S−→A:[KB,T]K−1 where is a nonce randomly selected by S S −→ A : [KB,B,N]K−1, where N
S
is a nonce randomly selected by S None of these
timestamp
S

Problem 3 Bypassing ASLR (48 points) Mallory is trying to perform a return-to-libc attack on a simple stack buffer overflow vulnerability. She wants to overwrite the return address of the vulnerable function with the address of the system function, and pass it an arbitrary command argument. But the system she wants to attack has ASLR enabled, so &system (the address of system) is different every time.
Wanting to explore this further, Mallory writes the simple program:
#include
void main() {
printf(“system is at 0x%x
”, &system);
}
She runs this five times, with ASLR enabled, and gets the following output:
system is at 0xbf9d7f14
system is at 0xbf9d7f99
system is at 0xbf9d7f88
system is at 0xbf9d7f36
system is at 0xbf9d7f08
(a) (16 points) She shouts “Eureka! It won’t work every time, but I can easily break this now!”. What did Mallory learn? How can she use it to successfully exploit the buffer overflow with a return-to-libc attack?
(b) (8 points) What is the probability that Mallory will succeed if she has 1 chance to perform her return-to-libc attack?
Final Exam Page 8 of 21 CS 161 – SP 17

(c) (24 points) Suppose Mallory is able to control the input (i.e. argv[1]) to the following silly backup program, written by programmers from (assume headers necessary for this code to compile have been included):
// Protect our data by making 2 copies!
void double_copy(char *data) {
char buf1[16];
char buf2[16];
strcpy(buf2, data);
strcpy(buf1, data);
}
int main(int argc, char *argv[]) {
// recall: argv[0] is the name of the program
if (argc != 2)
return -1;
double_copy(argv[1]);
}
Give an input that will cause “sudo rm -rf /” to be run on the victim machine with probability equal to what you answered in the previous part.
Use the following assumptions about the victim system:
1. It is an IA-32 platform with 4-byte words (recall it’s also little endian).
2. The stack is aligned at word granularity.
3. Local variables of each function are placed on the stack in the order they appear in the source code.
4. ASLR is enabled for the stack segment.
5. argv[1] == 0x07070707 will always evaluate to true.
Hint: # is the shell comment character.
You can use x** (where the *s are replaced by hex digits) to represent a character in hexadecimal form. Fill in the answer below:
Final Exam Page 9 of 21 CS 161 – SP 17

Problem 4 Attacks on TLS (72 points)
Recall the TLS protocol, depicted in the figure below. We use the following notation:
{M}K denotes a message M encrypted using the key K. [M]−1 denotes a message M
along with a signature over M using the key K−1.
Client
Server
K
1. Client sends a 256-bit random number Rb and supported cipher suites C
2. Server sends a 256-bit random number Rs and chosen cipher suite Cser
3. Server sends certificate
4. DH: Server sends [g, p, ga mod p]K−1 server
5. Server signals end of handshake
6. DH: Client sends gb mod p
RSA: Client sends {P S}Kserver
Client and server derive cipher keys Cb,Cs andintegritykeysIb,Is fromRb,Rs,PS
7. Client sends MAC(dialog, Ib) 8. Server sends MAC(dialog, Is)
9. Client data takes the form {M1, MAC(M1, Ib)}Cb 10. Server data takes the form {M2, MAC(M2, Is)}Cs
Figure 1: TLS 1.2 Key Exchange
(a) (24 points) Suppose the client and server use RSA to exchange the premaster secret. Mallory intercepts the ClientKeyExchange message and replaces PS with a fake value PS′. Assume that Mallory can modify the messages after ClientKeyExchange as well, if required. Which of the following are true? LL that apply.
Final Exam
Page 10 of 21
CS 161 – SP 17
Mallory will be able to decrypt the application data sent by the client to the server.
Mallory will be able to decrypt the application data sent by the server to the client.
The server will detect the tampering when it receives ClientKeyExchange.
Mallory can avoid detection until the server receives Finished from the client, at which point she’ll be detected.
Mallory can avoid detection until the client receives Finished from the server, at which point she’ll be detected.
None of these
2 ServerHello
3 Certificate
4 ServerKeyExchange
5 ServerHelloDone
8 ChangeCipherSpec, Finished
10 Application Data
1 ClientHello
6 ClientKeyExchange
7 ChangeCipherSpec, Finished
9 Application Data

(b) Now suppose the client and server use Diffie-Hellman for exchanging the premaster secret. Mallory wants to decrypt the data sent by the server to the client by downgrading the cipher suites. She doesn’t care about the data sent by the client to the server. If the server always picks the strongest cipher suite and parameters available, specify whether Mallory’s attack will succeed in the following scenarios (Yes/No).
If yes, then list the handshake messages Mallory will need to necessarily modify. If not, explain why.
Assume that unless specified, all cryptographic algorithms supported by the client and server are secure.
i. (12 points) Suppose the client and server support 3DES in addition to AES. Mallory is aware of an attack on 3DES that allows her to learn any message encrypted using it. She therefore wishes to force the client and server to use 3DES instead of AES as the encryption algorithm.
ii. (12 points) Suppose the client and server support a weak variant of Diffie- Hellman (DHweak). Mallory is aware of an attack on DHweak that allows her to learn the exchanged secret. She therefore wishes to force the client and server to use DHweak instead of standard Diffie-Hellman.
(c) (24 points) Recall that ClientHello contains a nonce Rb, along with C, the cipher suites supported by the client. ServerHello contains a nonce Rs along with Cser, the cipher suite chosen by the server. Which of the following modifications to the TLS protocol would prevent Mallory from conducting any downgrade attacks on the cipher suites? LL that apply.
Final Exam
Page 11 of 21
CS 161 – SP 17
ServerKeyExchange includes [Rb]K−1 , [C]K−1
server server
ServerKeyExchange includes [Rb, C]K−1 server
ServerKeyExchange includes [C ] −1 Kserver
ServerKeyExchange includes [Cser ]K −1 server
ServerKeyExchange includes [C]K−1 , [Cser]K−1
server server
ServerKeyExchange includes [C || Cser] −1
ServerKeyExchange includes
[Rb || C || Cser]K−1 server
None of these
Kserver

Problem 5 Software vulnerability (64 points) Here is a fragment of Python source code for a fictitious email-based spell-checker service:
def process_incoming_email(msg):
return_addr = msg.get(“From”)
search_term = msg.get(“Subject”)
status = os.system(“fgrep ” + search_term + ” /usr/share/dict/words”)
if status == 0: # exit code 0 means success
response = “The word ” + search_term + ” is spelled right!”
else:
response = “Sorry, ” + search_term + ” is not a word.”
send_response(return_addr, response)
In this service, users submit a word to check as the Subject: header field of an email. For example:
To:
From:
Subject: phenommenon
The process incoming email function is responsible for checking the spelling of the word, generating an appropriate response message, and sending the response back to the original sender via email. The function works by extracting a search term from the Subject header field, then using the fgrep command to search for the term in /usr/share/dict/words, a file containing a list of English words. The fgrep command searches a file for a fixed text pattern; its syntax is ‘fgrep pattern filename ’.
os.system is a Python function that accepts a single string and executes the string using the command shell. It works the same as the C system() library routine that we discussed in lecture.
(a) (16 points) The process incoming email function has a vulnerability. What email Subject: could you send that would cause the server to pause for 10 seconds before replying?
Subject:
(b) (16 points) What email Subject: could you send that would tell you whether or not there is a user called dbadmin on the spellcheck server? The list of users is stored in the file /etc/passwd.
Subject:
(c) (32 points) State one way that you could fix the vulnerability? (If you name more than one, we will only grade the first.)
Final Exam Page 12 of 21 CS 161 – SP 17

Problem 6 Coffee Shop Worries (54 points) Alice and Bob just arrived at Brewed Awakening, the local coffee shop. Eve is already there, enjoying a cup of tea.
(a) (6 points) Alice wants to connect to Brewed Awakening’s WiFi network. Under which protocols would her connections be safe from sniffing attacks by other coffee shop visitors, such as Eve? Mark all that apply.
WEP WPA2 – Enterprise mode WPA2 – Personal mode None of these
(b) (24 points) Turns out that Brewed Awakening’s network has no encryption. Al- ice warns Bob that its not safe to use this connection, but Bob disagrees. Bob connects to the WiFi, and tests that he has Internet connectivity by going to https://kewlsocialnet.com. It loads without issues. Bob says the Alice: “See, no problem! That access was totally safe!”
If Bob is correct and the access to kewlsocialnet.com was safe, explain why he is correct. If he is not correct, provide a network attack against Bob.
Answer:
(c) (24 points) Now that he has tested his WiFi access, Bob then tells Alice: “I want to buy that last muffin at the counter. Let me check if I have enough money in my bank account.” Eve hears this and panics! She wants the last muffin too but is waiting for her friend Mallory to bring enough cash to buy it. She is now determined to somehow stop Bob from buying that last muffin by preventing him from checking his bank account. Through the corner of her eye, Eve sees Bob start to type https://bank.com in his browser URL bar . . .
Describe two network attacks Eve can do to prevent Bob from checking his bank account. For each attack, describe clearly in one or two sentences how Eve performs the attack.
Attack #1:
Attack #2:
Final Exam Page 13 of 21 CS 161 – SP 17

Problem 7 The Great Cannon (96 points) In 2015, Github experienced a DoS attack orchestrated by China using the so-called “Great Cannon” (GC). It worked as follows. (Some details of the attack have been simplified or modified for this problem.)
Many websites include a fetch for a script for analytics from Baidu, a large Inter- net service in China somewhat similar to Google. The script would be retrieved via http://hm.baidu.com/h.js. The GC operated in-path at the border between China and the rest of the Internet. Upon seeing a request for this script, the GC would prevent the original HTTP request from being forwarded, and would instead return a different script, which instructed clients to repeatedly load http://github.com/cn-nytimes.
You can assume that Baidu served its traffic using servers in China; Github did so from servers in the USA; and websites using the analytics script were hosted all over the world.
(a) (6 points) For which of the following layers would the GC need to guess or infer header values it could not directly determine in order to carry out the attack? LL that apply.
Physical Link Network
Transport Application None of these
(b) (6 points) Which layer was this attack meant to particularly stress regarding Github’s servers? Mark the BEST choice.
Physical Link Network
Transport Application None of these
(c) (4 points) Whose traffic contributed to the DDOS attack? Mark the BEST choice.
Web browsers inside China Both of these Web browsers outside China Neither of these
(d) (4 points) Which packets would the implementers of this attack need to inspect?
Mark the BEST choice.
Packets going into China Packets going out of China
Both of these Neither of these
Final Exam Page 14 of 21
CS 161 – SP 17

(e) (12 points) Why doesn’t the Same Origin Policy prevent this attack? (Limit your answer to no more than 2 sentences.)
(f) (12 points) For this and the next question, suppose that after the attack began, Github installed a NIPS to deal with this particular attack. Assume the NIPS is deployed on the Ethernet link connecting the github.com server to the public Internet. What kind of detection is MOST LIKELY to be effective under the circumstances? Mark the BEST choice and provide a short explanation.
Signature-based Anomaly-based Specification-based
Explanation:
Behavioral
Honeypots Vulnerability scanning
(g) (12 points) Suppose that the attack caused Github to receive 50 times as many bogus requests as legitimate requests, and that Github will consider a defense suc- cessful if it reduces the volume of flooding requests by at least a factor of 50, so the flooding is no larger than the volume of legitimate requests. Suppose further that Github found that their NIPS had a precision of 0.999 and a recall of 0.99 when detecting this attack. To what degree would this represent a successful defense? NE of the following and BRIEFLY explain (≤ 2 sentences) your answer.
Yes, the NIPS provided a successful defense.
Additional information is needed to tell whether the NIPS provided a successful defense.
Explanation:
No, the NIPS did not provide a suc- cessful defense.
Such a combination of precision and recall values is not possible under these circumstances.
Final Exam Page 15 of 21
CS 161 – SP 17

(h) (8 points) This attack occurred for sets of HTTP requests. Which of the following changes would have prevented the attack? Consider each choice in isolation (i.e., assess whether it prevents the attack assuming none of the other choices are in effect). LL that apply. For each choice, assume that the content that the site serves remains the same.
Final Exam
Page 16 of 21
CS 161 – SP 17
Every website that uses Baidu’s an- alytics switches to serve its content using HTTPS instead of HTTP.
Baidu’s analytics server redirects any incoming HTTP connection to a corresponding HTTPS URL.
None of these.
(i) (8 points) Which of the following techniques could Github have used to make the
attack ineffective? LL that apply. Blacklist any packets from Chinese
IP addresses
Use SYN cookies for all new connections
None of these
Baidu switches its analytics server over to only be accessible using an HTTPS URL.
Github’s server redirects any incom- ing HTTP connection to a corre- sponding HTTPS URL.
Move the affected Github server to a new IP address
Remove all use of Baidu analytics from Github web pages
Github switches its server over to only be accessible using HTTPS.

Final Exam
Page 17 of 21 CS 161 – SP 17
(j) The remainder of this problem concerns a Web security feature called Subresource Integrity (SRI). It works by adding an attribute to the script tag for externally loaded scripts: