ICOM 5018 EXAM I - Spring 2005
Open books and notes. Only the text, slide printouts and your own notes may be used.
In the interests of originality
and creativity please turn off all electronic communication devices including celulares, laptops, pocket computing devices and telepathic
capability if you have it.
relate to symmetric (private-key) cryptography.
a. The RC4 algorithm is described on page 194ff. It is used in IEEE 802.11, which is the usual standard for wireless networks (PRTC uses it in its DSL modems). The keyword to be used is a 10-digit decimal number printed on the bottom of the modem.
number is truly random, what is the expected time to discovery by brute-force analysis. Assume the
data rate is 11 Mbits/sec and that the device is
Average detection time = Average attempts * bits_in_detectable_sample/bitrate
Assume bits_in_detectable_sample=50 bits about 6 characters – I got 18000 sec – 5 hours
What is the
propagation length for a single-bit reception error? Explain how you obtain this result. (Read carefully).
One bit – the single-bit error is xor’ed with the stream, and only that single-bit is in error
RC5, described on
page 188ff, is not a Feistel block cipher, but it is
invertible. Each round uses three types
of operations. Please answer the
operations are as described. If shifting
is replaced by Galois field multiplication, what do you use for an inverse of
You have to derive the inverse of the side input in the Galois field – this takes a modified
Why is addition
mod 2w rather than addition mod a prime used?
Addition mod 2w covers the entire space, addition mod a prime doesn’t, because a 2w bit prime is smaller.
connection to the right input of the first XOR on the left of the encryption
round is moved just after the right hand XOR instead of before it. How does this make the cipher no longer invertible.
It creates a feedback loop.
the connection is moved to just ahead of the addition of S(1). Is this now invertible? Explain.
Yes, no feedback loop is created, so you can just reverse the direction of signal flow and replace each operation by its inverse.
2. The following apply to
public-key cryptography. Please answer
the following, briefly, but avoiding the dreaded RADQ.
Text problem 9.7
3. On the following page is the description of a seminar
to be given tomorrow. Since it relates
quite closely to the material of this course I chose to ask about some of the
implications of this proposal.
implies that at least a public-key, with its associated private key is stored
in the smart card. It also implies that
the smart card actually computes the signature of the memo. What information does Bob need at the
receiving end to foil a person-in-the middle attack that suppresses this
message and substitutes another?
corrupted software in the conference computer can substitute a different
message to the smart card for signature computation and transmission. Can you devise a protocol that works by
splitting the message in two or more parts, and mixing some of the verification
info. (This is
an email transaction, so you can’t use a challenge/response method or anything
that depends on a reply).
You can’t use a feedback message, but you can use clocks set some time before, and a significant delay between messages. Then the idea is to transmit something based on the encrypted version of the message yet to be sent inside the first segment. Then Mallory doesn’t know what information to fake.
mentions the use of artificial intelligence techniques to achieve data
integrity in this protocol. What would
be a possible nature and benefit of this method?
The idea is to use something the computer can’t detect readily, but that a human user can. An example is for Bob to know anything legitimate contains 7 Spanglish spellings or other odd grammar, such as list, etc. (this is incorrect in English, typical in Spanish); the misspellings might include profesor, analisis, assisting at a meeting, or buying textbooks at the library.
The description below is
taken from the announcement of a seminar Enabling
Humans to Sign by Dr. Andre L. M. dos Santos of Georgia Tech to be given