Operating Systems computer science homework help

1. What is a covert channel? What is the basic requirement for a covert channel to exist?

2. In a full access-control matrix, the rows are for domains and the columns are for ob- jects. What happens if some object is needed in two domains?

3. Two different protection mechanisms that we have discussed are capabilities and ac- cess-control lists. For each of the following protection problems, tell which of these mechanisms can be used. (a) Ken wants his files readable by everyone except his office mate. (b) Mitch and Steve want to share some secret files. (c) Linda wants some of her files to be public.

4. Represent the ownerships and permissions shown in this UNIX directory listing as a protection matrix. (Note: asw is a member of two groups: users and devel; gmw is a member only of users.) Treat each of the two users and two groups as a domain, so that the matrix has four rows (one per domain) and four columns (one per file).

–rw–r– –r– – 2 gmw Users 908 May 26 16:45

–rwxr–xr – x 1 asw Devel 432 May 13 12:35

–rw–rw– – – – 1 asw Users 50094 May 30 17:51

–rw–r– – – – – 1 asw Devel 13124 May 31 14:30

5. Break the following monoalphabetic cipher. The plaintext, consisting of letters only, is a well-known excerpt from a poem by Lewis Carroll.

kfd ktbd fzm eubd kfd pzyiom mztx ku kzyg ur bzha kfthcm ur mfudm zhx mftnm zhx mdzythc pzq ur ezsszcdm zhx gthcm zhx pfa kfd mdz tm sutythc fuk zhx pfdkfdi ntcm fzld pthcm sok pztk z stk kfd uamkdim eitdx sdruid pd fzld uoi efzk rui mubd ur om zid uok ur sidzkf zhx zyy ur om zid rzk hu foiia mztx kfd ezindhkdi kfda kfzhgdx ftb boef rui kfzk

6. Name three characteristics that a good biometric indicator must have in order to be useful as a login authenticator.

7. Authentication mechanisms are divided into three categories: Something the user knows, something the user has, and something the user is. Imagine an authentication system that uses a combination of these three categories. For example, it first asks the user to enter a login and password, then insert a plastic card (with magnetic strip) and enter a PIN, and finally provide fingerprints. Can you think of two drawbacks of this design?

8. Describe how stack canaries work and how they can be circumvented by the attackers.

9. Why are rookits extremely difficult or almost impossible to detect as opposed to viruses and worms?

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