Due: Wednesday, February 5, 1997 at 11:59PM
A computer security expert claims that one of the measures necessary to
obtain computer security is the separation of programmers (who write
the programs) and operators (who run the programs once they leave the
development phase). Why does this improve security?
Modify Anderson's formula to take into account that not all passwords
will be the same length. Assume instead that a password is at least one
The Morris-Thompson scheme with the n-bit random numbers was designed
to make it difficult for an intruder to discover a large number of
passwords by hashing common strings in advance. Does the scheme also
offer protection against an attacker who is trying to guess one
particular user's password on his or her machine?
Many public key cryptosystems can provide both authenticity and
confidentiality. But we usually think of classical (secret key)
cryptosystems as providing only confidentiality. How can a classical
cryptosystem provide authentication of a user instead of
confidentiality of a message?
Write a program to change passwords.The program should prompt the user
for his or her current password, validate it, and then request the new
password. The program must check that the password is reasonably
well-chosen; at a minimum, the password must meet the following
If the password meets these criteria, it is hashed and the hash is put
into the password file. The password file you are to use is ~cs153/hw2/passwd;
do not use the system password file!
It cannot be the user's login name or the user's login name reversed;
It cannot be the user's real name (this is stored in the gecos field of
the password entry; all names there are of the form firstname
it cannot be an English word (use the spelling checker for this one!)
it must at least 6 characters long, with at least one non-alphanumeric
Hints: You need to get this program working only on the
DECStations. Use putpwent(3) to write out the password file. For part
c, you will need to spawn a subprocess to run spell(1), the spelling
Prove that a classical cryptosystem cannot provide both confidentiality
of a message and authentication of a user at the same time unless a
trusted third party is involved.
Think of some other criteria that a good password should meet, and add
them to your program.
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Department of Computer Science
University of California at Davis
Davis, CA 95616-8562
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