Homework 04 and 05 ECE 443/518

Homework 04 and 05
ECE 443/518
1. (10 points) Let’s work on the garbled circuit between Alice and Bob who want to
compute f = NAND(a, b).
a) Suppose 0 and 1 on each wire is encrypted into a 5-bit number (0 to 31).
Alice chooses A0 = 7, A1 = 17, B0 = 19, B1 = 3, and O0 = 18, O1 = 6.
What are SA and SB?
b) For the encryption function ek1||k2
(x) = (k1+k2+x) mod 32, show how Alice
garbles the circuit. Suppose Alice chooses a = 1. What Alice should send to
Bob as her input?
c) Suppose Bob chooses b = 0. Show how Bob encrypts his input with Alice’s
help using OT. Assume Alice’s RSA public key to be (n = 35, e = 5).
d) Show how Bob computes with the garbled circuit and the encrypted inputs,
and then communicates with Alice to determine f.
e) Show that Bob cannot decide Alice’s choice of a (assuming OT only reveals
B0 but no additional information). As a hint, is it possible for Alice to choose
A0 = 17, A1 = 7 while sending Bob exactly the same garbled circuit and
inputs?
2. (5 points) (Chapter 2 Question 1, p35 in Introduction to Computer Security)
Consider a computer system with three users: Alice, Bob, and Cyndy. Alice owns
the file alicerc, and Bob and Cyndy can read it. Cyndy can read and write the
file bobrc, which Bob owns, but Alice can only read it. Only Cyndy can read and
write the file cyndyrc, which she owns. Assume that the owner of each of these
files can execute it.
a) Create the corresponding access control matrix.
b) Cyndy gives Alice permission to read cyndyrc, and Alice removes Bob’s
ability to read alicerc. Show the new access control matrix.
3. (5 points) (Chapter 5 Question 2, p71 in Introduction to Computer Security)
Given the security levels TOP SECRET, SECRET, CONFIDENTIAL, and UNCLASSIFIED (ordered from highest to lowest), and the categories A, B, and C,
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ECE 443/518 – Computer Cyber Security 2
specify what type of access (read, write, both, or neither) is allowed in each of
the following situations. Assume that discretionary access controls allow anyone
access unless otherwise specified.
a) Paul, cleared for (TOP SECRET, { A, C }), wants to access a document
classified (SECRET, { B, C }).
b) Anna, cleared for (CONFIDENTIAL, { C }), wants to access a document
classified (CONFIDENTIAL, { B }).
c) Jesse, cleared for (SECRET, { C }), wants to access a document classified
(CONFIDENTIAL, { C }).
d) Sammi, cleared for (TOP SECRET, { A, C }), wants to access a document
classified (CONFIDENTIAL, { A }).
e) Robin, who has no clearances (and so works at the UNCLASSIFIED level),
wants to access a document classified (CONFIDENTIAL, { B }).
4. (4 points) Suppose there are two COI classes {Bank of America, Citibank, Bank
of the West} and {Shell Oil, Union’76, Standard Oil, ARCO} in an investment
house using Chinese Wall Model. Alice would like to read 4 objects following the
sequence a, b, c, d. Assume CD(a)=Citibank, CD(b)=ARCO, CD(c)=Citibank,
CD(d)=Standard Oil. Show whether each read is granted and why.
5. (6 points) alice, bob and cyndy are three users on a Linux system. There are
three groups alice, bob and cyndy as well and each group has the user with the
same name as the member. In addition, assume the user bob is also the group
member of the group alice. Explain the following permissions by showing what
rights each user have.
a) A file F with alice as both owner and group owner, and permission ‘rw-rwr--’.
b) A file E with alice as owner and cyndy as group owner, and permission
‘rwxr-x---’.
c) A directory D with alice as owner and bob as group owner, and permission
‘rwxr-x--x’.