Homework 2

Due Date: February 6, 1998 at 11:59PM
Late Date: February 9, 1998 at 11:59PM (20% penalty)

Points: 150

  1. (10 points) Consider the following 4 ciphertexts: XXXXX, VWXYZ, RETIC, JZQAT. Which of these ciphertexts could result from enciphering five letter words of English, using:
    1. Any transposition cipher (in which letters are mixed but not changed)?
    2. A substitution cipher, where each letter is replaced with some other letter, but the letters are not necessarily shifted as in the Cæsar cipher (thus A could be replaced with K, B with W, etc.)?
  2. (20 points) 4.4 BSD UNIX has extended the password hashing function by allowing a 4 character (24 bit) salt. DEC Ultrix has extended the password hashing function by allowing passwords of up to 16 characters; the algorithm hashes the first set of 8 characters using the conventional algorithm, hashes the second set of 8 characters using the conventional algorithm with the same salt, and concatenates the result. The salt used is the 2 character (12 bit) salt. When does the first method require more trials to guess a particular password? When does the second?
  3. (20 points) Consider the RSA encryption scheme with public key n = 35 and e = 7. Encipher the plaintext M = 10. Break the cipher by finding p, q, and d.

  4. (100 points) This problem asks you to explore file permissions and types in the UNIX operating system. You are to write a command called access that takes as arguments a user ID and a path name. It determines how the named user can access the object with the given path name. For example,
    access bishop /home/bishop/xyz
    bishop can read, write /home/bishop/xyz
    access bishop /home/cs153r/handin/hw1/knowles
    bishop cannot access /home/cs153r/handin/hw1/knowles

    If the option -l is given, the same information is to be printed for each element of the path:
    access -l bishop /home/cs153/public_html/homepage.html
    bishop can list, search /
    bishop can list, search /home
    bishop can search /home/cs153
    bishop can list, search /home/cs153/public_html
    bishop can read /home/cs153/public_html/homepage.html
    What follows are some suggestions that may help you.
    1. Read, write, and execute permission on directories are really list, create/delete file, and search permission. Without search permission, a process cannot access any files in the directory (even if read and create/delete files permissions are given).
    2. For symbolic links, list the permissions on the symbolic link, and on the next line the permissions of the referent. Use lstat(2) and stat(2) to go this.
    3. If the named user cannot access a component of a path, it cannot access any of the following components.

    You can also see this document in its native format, in Postscript, in PDF, or in ASCII text.
    Send email to cs153@csif.cs.ucdavis.edu.

    Department of Computer Science
    University of California at Davis
    Davis, CA 95616-8562

    Page last modified on 1/28/98