Due Date: February 6, 1998 at 11:59PM
Late Date: February 9, 1998 at 11:59PM (20% penalty)
- (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:
- Any transposition cipher (in which letters are mixed but not changed)?
- 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.)?
- (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?
- (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.
- (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
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.
- 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).
- 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.
- 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 ASCII text.
Send email to
Department of Computer Science
University of California at Davis
Davis, CA 95616-8562
Page last modified on 1/28/98