ENGR/CS 101 CS Session
Lecture 2
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Starting with the next class, we will be using
Visual Studio C#. If you would like to have a
copy for your own computer:
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Lecture 2
Can download the C# Express Edition, link on the
CS session webpage. Contains only the C#
compiler.
All students in EECS classes are enrolled in the
UE MS Alliance program. Email with instructions
should have been sent. Full Visual Studio 2010 is
available (beta VS 2012 and Windows 8, too).
ENGR/CS 101 Computer Science Session
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Outline
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Software life cycle
Problem: How to send a secret message?
Codes and ciphers
Substitution ciphers
Lecture 2
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Software Life Cycle
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Specification of the problem/task
Analysis and design of a solution
Implementation (coding) of the solution
Testing and debugging
Maintenance and evolution of the system
Obsolescence
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Problem:
How to send a secret message?
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Steganography ("concealed writing"):
science of sending concealed messages.
Includes physical concealment like invisible
ink, microdots...
Cryptography ("hidden writing"): how to
obscure message so it cannot be read even if
intercepted. Use codes and ciphers.
Lecture 2
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Codes & Ciphers
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Code: whole words or phrases replaced by a
word, letter, or a number. Like an alien
language; uses translation code book.
Cipher: individual letters are replaced by
other letters or symbols.
Plaintext: message in normal language
Ciphertext: message in secret form
Lecture 2
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Ciphers
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Transposition cipher: rearrange letters of
message.
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Scytale: strip of writing material wrapped around a
dowel; write message across dowel.
Block: arrange message into a block, rewrite
vertical lines
Substitution cipher: replace letters with other
letters
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Cipher = Algorithm + Key
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Algorithm: a series of well-defined steps that
can be followed as a procedure.
Key: auxiliary information used by an
algorithm. Different keys produce different
ciphers using the same algorithm.
Lecture 2
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Caesar Shift Cipher
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Algorithm: substitute a letter with the letter n
places to the right
Key: letter to shift 'A' to that is n places to the
right. E.g. A -> I is shifting 8 places to the right:
plain
A
B
C
D
E
F
G
H
I
J
K
L
M
cipher
I
J
K
L
M
N
O
P
Q
R
S
T
U
plain
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
cipher
V
W
X
Y
Z
A
B
C
D
E
F
G
H
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In-class Exercise
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Practice enciphering and deciphering using
Caesar shift cipher.
Turn in worksheet at the end of class.
Lecture 2
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What if the key is unknown?
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How many possible keys are there for the
Caesar Shift Cipher?
How easy would it be to find the key?
Lecture 2
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Polyalphabetic Ciphers
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Make cipher harder to break by using multiple
substitution alphabets
Vigenere cipher: key is a "word" rather than
just a single letter. Algorithm is to use the
key letters to change the Caesar cipher shift
key for each letter of plaintext.
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Vigenere Cipher Example
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For example, if the
key word is "LION"
and the plaintext
message is "GO
ACES", the ciphertext
would be "RW
OPPA", formed as
shown to the right.
Lecture 2
plain
key
cipher
G
L
R
O
I
W
A
O
O
C
N
P
E
L
P
S
I
A
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Polyalphabetic Ciphers
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Suppose we allow any letter to be substituted
by any other letter? E.g. a cryptoquip puzzle.
The key would a substitution table mapping
each letter to another letter.
How many possible keys are there for this
cipher?
Lecture 2
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400 Million Billion Billion
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Just how big is this number?
6.5 billion people on Earth; 31 million
seconds in a year. If everyone on Earth
checked one key per second, ...
We can conclude that checking every
possible key is not a feasible way of trying to
decipher an arbitrary substitution cipher.
Lecture 2
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A Better Way to Decipher
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English letter
frequency: E, T,
A, O, ...
One/two letter
words: "I", "a",
"to", "of", ...
Common words:
"the", "and", ...
Repeated letters
Context
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Enigma Machine
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Random substitution cipher
represented using a code
wheel. Originally 3 code
wheels, later 5 wheels.
Instead of always starting with
same letter on wheel as A,
just encipher the current letter
of plaintext with the next letter
on wheel.
Lecture 2
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