need help with these questions

1. (10 points) Suppose that you use the following encryption scheme: each bit in the key R (a sequence of bits) is XORed with its corresponding bit in the plaintext M in order to get the ciphertext. This simple scheme can be written as C=M XOR R. You want to use this scheme to encrypt your messages, where the language used for the messages has only two words represented as binary: A=111 and B=000. For example, a message M could be 000111111000 but it can never be 010101.

You have two messages, M1 and M2. You generated two keys R1 and R2. Unfortunately, R1=R2. You apply the encryption scheme mentioned above and get two ciphertexts, C1 and C2.

C1 = 011101101001000111001

C2 = 011010010110000111110

Please identify a pair of plaintexts, M1 and M2, which yield C1 and C2. (5 points) And justify your solution. (5 points)

2. (18 points) Considering the following assignment of clearance/classification levels for both subjects and objects.

Clearance/Classification Level	Subject	Object
High	Administor	System Configuration
Medium	Regular User	User Configuration
Low	Guest	Temporal File

1. Fill out all cells for the access control matrix if a system uses Biba Model. If the subject has read access to the object, you can put r in that cell. If the subject has write access to the object, you can put w in that cell. For instance, if A can read and write the object B, you can put r,w into the cell corresponding to (A, B). (9 Points 1 point for each cell)

	System Configuration	User Configuration	Temporal File
Administor
Regular User
Guest

2. Fill out all cells for the access control matrix if the system uses Low-Water Mark policy after the administrator reads the Temporal File. (9 Points 1 point for each cell)

	System Configuration	User Configuration	Temporal File
Administor
Regular User
Guest

3. (10 points) Modern operating systems have incorporated a bit non_executable to label whether the content in a memory page (e.g., a memory block) can be executed. If stack frames are organized into these memory pages and labeled as non_executable.
   1. Will this stop the shellcode injected to the stack (e.g., the buffer allocated in the stack) from be executed? Please justify. (10 points for undergraduates; 5 points for graduates)
   2. Will this eradicate the possibility that an attacker executes unwanted code in the target system? (undergraduates do not need to answer this question. 5 points for graduates) Please justify. (hint: read attacks mentioned in this paper Anomaly Detection Using Call Stack Information )