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Massachusetts
Institute
of
Technology
Handout
13
6.857:
Network
and
Computer
Security
October
9,
2003
Professor
Ronald
L.
Rivest
Quiz
1
1.
This
quiz
is
intended
to
provide
a
fair
measure
of
your
understanding
of
the
course
material
to
date
(Homeworks
1–3
and
Lectures
1–10).
2.
Do
not
open
this
quiz
booklet
until
the
quiz
begins.
Read
all
the
instructions
first.
3.
Do
not
discuss
any
aspect
of
this
quiz
with
anyone
(except
6.857
staff
)
until
noon
on
Friday,
October
10,
2003.
4.
When
the
quiz
begins,
write
your
name
on
every
page
of
this
quiz
booklet.
5.
This
quiz
booklet
contains
12
pages,
including
this
one.
An
extra
sheet
of
scratch
paper
is
attached.
6.
This
quiz
is
openbook,
opennotes.
No
calculators
or
programmable
devices
(including
laptop
computers)
are
permitted.
7.
Write
your
solutions
in
the
space
provided.
If
you
need
more
space,
write
on
the
back
of
the
sheet
containing
the
problem.
Do
not
put
part
of
the
answer
to
one
problem
on
the
back
of
the
sheet
for
another
problem;
pages
may
be
separated
for
grading.
8.
Partial
credit
will
be
given. You
will
be
graded
not
only
on
the
correctness
of
your
answer,
but
also
on
the
clarity
with
which
you
express
it.
Be
neat.
9.
Good
luck!
Problem
Points
Grade
Initials
24
9
36
12
10
10
10
Total
111
Your
Name:
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Handout
13:
Quiz
1
Name:
Problem
Q11.
Short
Answer
[24
points]
(a)
At
a
recent
Red
Sox
game,
you
observed
the
catcher
making
a
variety
of
signals
to
the
pitcher
with
his
hands
before
every
pitch.
Following
the
signals,
you
observe
the
type
of
pitch:
e.g.,
fast
ball,
curve
ball,
slider,
knuckleball,
beanball,
etc.
By
the
end
of
the
game,
you
are
able
to
predict
every
pitch
after
seeing
the
catcher’s
signals.
What
sort
of
cryptographic
attack
have
you
successfully
executed?
Solution:
This
is
a
knownplaintext
attack.
Half
credit
was
given
for
the
answer
“passive
attack.”
(b)
What
is
11
−
1
(mod
29)?
Show
your
work.
Solution:
11
−
1
(mod
29)
=
8,
because
8
·
11
=
88
=
29
+
1.
We
can
get
this
3
·
result
by
using
Euclid’s
extended
algorithm.
Half
credit
was
given
for
the
answer
11
−
1
=
11
28
(mod
29),
based
on
Fermat’s
Little
Theorem.
(c)
You
are
watching
an
encrypted
conversation
between
Alice
and
Bob.
You
notice
that
the
prefixes
of
many
of
the
ciphertexts
agree
for
several
hundred
bytes.
In
addition,
these
identical
prefixes
are
always
a
multiple
of
16
bytes
long.
However,
you
never
observe
two
identical
chunks
of
ciphertext
of
any
significant
length
following
the
identical
prefixes.
Conjecture
what
cipher
is
being
used,
what
mode
of
operation
is
being
used,
and
what
Alice
and
Bob
are
doing
wrong.
Solution:
The
answer
we
had
in
mind
was
AES
(or
DES)
under
CBC
mode,
(incor
rectly)
using
the
same
IV
for
every
message.
We
also
gave
full
credit
for
an
answer
such
as
AES
or
DES
in
ECB
mode,
with
some
explanation
(e.g.,
all
messages
have
long,
common
headers.)
(d)
Next,
you
start
spying
on
a
different
encrypted
conversation
between
Alyssa
and
Ben.
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 Spring '03
 Rivest
 Cryptography, Computer Security, Ben, MD5

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