CHAPTER 3: CLASSIC CRYPTOGRAPHY

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CHAPTER 3 CLASSIC CRYPTOGRAPHY Motivation
Information in any form, written, typed, or
electronic is subject to unauthorized disclosure,
modification, and/or misuse since it is human
readable. Need Methods for providing
secret/private communication to protect
information in any state (during processing,
while stored, or in transit). This also
implies the need to be able to recover and read
secret communications.
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CRYPTOLOGY Secret communication
means Cryptology from the Greek Crypto
meaning secret or hidden, and ology meaning
theory, or science Two major divisions
Cryptography Cryptanalysis
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Cryptography Cryptography - communications in
the presence of adversaries Means to turn
ordinary text (plaintext) into unreadable
ciphertext. Only unreadable as long as an
adversary cannot invert (recover) the original
plaintext. Used to communicate
secrets. CODEMAKING!
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Cryptanalysis Cryptanalysis - recovering
plaintext from ciphertext Methods that recover
plaintext from ciphertext and/or methods to forge
ciphertext so it appears to be authentic.
CODEBREAKING!
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Conventional Cryptology
X
Cryptanalyst
K
Source
Destination
Now is the time for all.. . ... country
Now is the time for all.. . ... country
X
Encryption Algorithm
Decryption Algorithm
X
Y
Ciphertext Message
Plaintext Message
Plaintext Message
K
K
K
Key Source
Secure Distribution Channel
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Cryptography - Assumptions 1. Adversary has
access to the ciphertext. 2. Adversary knows the
encryption algorithm. 3. Secret key is conveyed
over a secure channel and is unavailable to the
attacker. 4. The ciphertext is completely
randomized as it is encrypted. 5. The key is
composed of random characters and is long enough
to defeat guessing.
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Access to Ciphertext/Algorithm Access to
ciphertext knowledge of algorithm are based on
many years of real experience - cant be
avoided. If the secret key is not protected all
is lost since the adversary has the same
information as the legitimate receiver. The key
can be used to invert the ciphertext and recover
the original message.
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Random Ciphertext Randomizing the ciphertext
during encryption is necessary to removed any
structural relationship between the plaintext and
the ciphertext. Structure is a characteristic of
any language, but is the enemy of encryption.
Structure allows an adversary an opening to
break the cipher.
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Key Length and Randomness Keys must be long
enough so they cannot be guessed. Given automated
(computer) guessing, length is getting longer
over time. Randomizing the key is also essential
since non-random keys can be predictable and they
reduce the key space (i.e., not all keys
generated are equally probable).
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Methods for Information Hiding Steganography -
literally meaning covered writing and depends on
hiding the existence of a secret message from an
adversary. Cryptography - uses an algorithm and
key to transform a message into an unreadable
form that can only be inverted by using the same
key and running the algorithm backwards. It is
also possible (as usual) to combine methods.
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Steganography - Classical Hides the message
using a secret algorithm. Knowing the algorithm
typically breaks the secret. Examples Wax
covered tablets, hidden tattoos Microdots,
hidden in a letter Invisible ink, revealed by
chemicals or light Selected characters (e.g.,
first letter of each word, or letters that have
been perforated by a pin). Primary problem is
algorithmic secrecy.
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Steganography - Injection Secret message is
embedded (hidden) in another message. Example
Put message in areas that are usually ignored
when displayed such as hidden fields in html
pages. Use options field in TCP/IP header to
carry message w/o options flag set. Then use
special software to read the options field in
multiple packets to recover the message.
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Steganography - Substitution Secret message
replaces existing information. Use the least
significant bit of every pixel in a complex
graphics image. At 2048 x 2048 x 24 bits (x,y,
and color), using 1 bit of the 24 affords a 4.19
Mbit or 524 kByte message space in each image and
is not readily noticed (i.e., the image appears
unchanged). Many other possibilities!
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Steganography - Tools S-tools Puts message in
least significant bits (lsb) of .bmp image
files. ftp//idea.sec.dsi.unimi.it/pub/security/
crypt/code/s-tools4.zip http//members.tripod.com/
steganography/stego/software.html MP3 stego
hides messages in mpeg files. S-Mail hides
messages in .exe and .dll files. Invisible
secrets in banner ads on web sites. Stash
hides messages in several image types.
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Detecting Use of Steganography Normal bmp
files have few duplicate colors. A bmp with an
embedded message has many. So.Search for
duplicates or near duplicates Use file size
signatures in well-known .exe, .dll files and
compare to the suspect files. Reported as being
used by Al Qaida (also has been denied)! Truth
is unclear but there is high interest.
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Cryptography - Formally 3 finite sets
Plaintext space P, Ciphertext space C, and
Key space K. 2 functions Encryption e ? E
and Decryption d ? D (e is a specific
encryption in the space E). (d is a specific
decryption in the space D).
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Cryptography - Encryption For each k ? K there
is an encryption rule e ? E such that f(ek)
P ? C or C ek(P) A key k is used to encrypt
(e) operating on a plaintext P and producing
ciphertext C.
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Cryptography - Decryption for each k ? K there
is a decryption rule d ? D such that dk C ?
P or P dk(C) Key k used to decrypt (d) a
ciphertext C, producing a plaintext P. ek dk
are inverses of each other for all p ? P and k ?
K such that dk(ek(p)) p for every plaintext
element p ? P.
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Cryptography - Long History 1900 BC -
Non-standard Egyption hierogliphs 1700 BC - Clay
of Phaistos still unrecovered 600 BC - Book of
Jeremiah encoded 60 BC - Caesar used
encryption 790 AD - First writing (by
reference) 1200 AD - Roger Bacon describes
methods 1518 AD - First printed book on
cryptography 1861 AD - 1st U. S. patent issued
1927 AD - Used during prohibition by
criminals 1942 AD Wartime use
(Germany/Japan/US) 1976 AD - Public key
Cryptography invented
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Classical Cryptography -Algorithms Substitution
- Plaintext symbols are replaced with ciphertext
symbols using a substitution algorithm. (e.g., if
AT, T X, then AT TX). Transposition -
Plaintext symbols are permuted (re-arranged)
using a permutation algorithm. (e.g., if position
1position 2, position 2 position 1, then AT
TA) Product - Uses alternate steps of
substitution and transposition.
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Substitution Ciphers -Monoalphebetic Each symbol
of the plaintext alphabet is mapped into a
single ciphertext symbol. Atbash used by ancient
Hebrews. Julius Caesar cipher used by Romans.
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Monoalphabetic Substitution Ciphers Keyspace
set of all permutations on 0, 1, 2, .,25. For
a given key ? and algorithm Ek(P) C E?(x1x2,
.xn) ?(x1)?(x2) .. ?(xn), and D?(y1y2..yn)
?-1(y1) ? -1(y2).. ? -1(yn) Caesar C Ek(P)
(P k) mod 26 Where C Ciphertext symbol, P
Plaintext symbol k 0 lt k lt 26, E (Pk)mod
26
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Caesar Cipher Symbolkey relationship is defined
numerically A B C D E .. I .. L . W
X Y Z 0 1 2 3 4 8 11. 22 23
24 25 Suppose K 11, P wewill Algorithm is (P
k)mod 26 (e.g., 22 11 33/26 Q 1 R
7) Text 22 4 22 8 11 11 Add 11 7 15 7
19 22 22 Cipher H P H T W W X
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Monoalphabetic Ciphers Graphically, the Caesar
cipher, for k 3 is
Z
A
B
Y
C
X
D
W
E
V
F
U
G
T
H
S
I
J
R
K
Q
L
P
M
O
N
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Monoalphabetic Ciphers Decryption P Dk(C)
(C - k) mod 26 Easily broken. Since k is the
key, there are only 26 possible keys and each one
could be tried. Example for k 3. P we will
attack at dawn through the left flank C zh zloo
dwwdfn dw gdzq wkurxk wkh ohiw iodqn
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Brute Force Decryption Key try Message produced
1 gy yknn cvvcem cv feyp vtqwj vjg..
2 xf xjmm buubdl bu edxo uispvi uif.. 3 we
will attack at dawn through the.. 4
5 . 25 ai ampp exxego ex hear
xlvvsyl xli.. To break could use the frequency
of occurrence of letters. E is the most common
character in frequency of appearance in English.
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Frequency of Occurrence
Common libraries exist for single, double,
triple, etc. occurrences in a particular
language. Simplifies the guesswork.
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Early Substitution Ciphers - Atbash Used by the
Hebrews 500B.C. in the Bible (Jeremiah 25) .
Substitutes by position first letter for last,
second for next-to-last (A-Z, B-Y, etc.).
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Early Substitution Ciphers - Polybius Polybius
Checkerboard 205-123 B.C. substitutes numbers
for letters.
R 42 T 44
Encrypted Polybius 35 34 31 54 12 24 45 43
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Substitution Ciphers Homophonic - Each symbol is
mapped into one of several possible ciphertext
symbols (or reverse) (Playfair). Invented by
Charles Wheatstone in 1854. During this period,
making and breaking ciphers was a public sport
often done by taking ads in the newspaper. Sort
of challenge/response.
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Playfair Cipher Multiple letter encryption
mapping two letters into a two cipher letters.
Masks the symbol frequency better than simpler
ciphers. Used by British in the Boer War, WWI,
and to some extent in WWII. Maps letters into a
5 x 5 matrix (Z is omitted) and follows three
rules. The matrix is populated and both ends
know the mapping.
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Playfair Mapping is a spiral starting at
lower-right corner.
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• Playfair Rules
• Arrange plaintext into pairs. If a double letter
• (e.g., tt) Insert an X. If an odd number, insert
  an
• X pad at the end.
• If pair is in same row, cipher pair is two
  letters to the right wrapped to left column (IG
  HF XB QL).
• 2. If pair is in same column, cipher pair is
  below, wrap to top (FQ SP UN VH FS SR).
• 3. If pair is at corners of a rectangle of
  letters, 1st encrypts to corner of same row, 2nd
  to corner in its row (EK IC UR SV AI ME).

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Playfair Example Plain ME RX RI LY WE RO LX LA
LO NG
Cipher AI YQ KF XK BH YP WQ BM XM OH
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Polygram Ciphers Symbol groups in plaintext are
substituted for symbol groups in ciphertext
(Hill). Invented by Lester Hill -1929. Multiple
letter substitution like Playfair, but
substitutions are designed to further mask
statistics (flatten) in the original text. By
this time, ciphers are getting much better (more
difficult to break).
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Polyalphabetic Ciphers Each symbol is mapped
into a cipher symbol as in the monoalphabetic
case, but the substitution changes for every
symbol encrypted Thus, it creates multiple
(i.e., poly) substitutions A major example is
the Vigenère cipher.
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Polyalphabetic Ciphers The encrypting alphabet
is changed as symbols are encrypted. The key may
be in the form of a numeric matrix or a text
passphrase. For each input symbol the
corresponding symbol in the matrix or passphrase
is used to determine the shift used to determine
the cipher character. Vigenère cipher function
f(a) (a ki)mod n See Stallings, pages 40-43.
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Vigenère Autokey Ciphers A priming key is used
to initiate encryption. The key may be a single
letter, word, or a group of words. For each
symbol in the plaintext, the corresponding symbol
in the column of the tableau is used to locate
the letter in the row labeled by the key to
determine the cipher character. For a priming key
K Plaintext ALL THE FINE YOUNG
CANNIBALS Key KAL LTH EFIN EYOUN
GCANNIBALS Cipher KLW EAL
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Transposition Ciphers Rearrange plaintext to
form the ciphertext with no substitution.
Instead, symbols are transposed. Classically
done using a geometric figure as a template
(e.g., rail fence, 2-D rectangle, 3-D cube,
etc.). Rail fence Text meet me for the drop
at noon tomorrow m e m f r h d o a n o t m r o
(easy to see) e t e o t e
r p t o n o o r w
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Product Ciphers Combines substitution/transpositi
on - German ADFGX cipher used in WW1 (2 step
process). 1 Transpose one plaintext character
into a limited set of 2-character symbols (the
inner matrix can be changed) A D F G
X A n b x r u D q
o k d v F a h s g
f G m z c l t X e
i p j w
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ADFGX Cipher Step 1 M forced to retreat ten km
to abbeville few casualties A D F G
X A n b x r u D q
o k d v F a h s g
f G m z c l t X e
i p j w forced becomes f FX o
DD r AG c GF e XA d DG FXDDAGGFXADG
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Product Ciphers ADFGX (contd.) Step 2
transpose using a sequence of numbers between 1
20 arranged in scrambled order (order changed
as often as needed). Key ) 8 9 14 7 19 13
16 1 15 6 3 10 17 2 20 5 11 18 4 12 F X D
D A G G F X A D G G X D D A G
X A G X A G X A F A G X G X X A
A A D F G A G X D D F A A D A
D X A D X X D G G G G X A F X X
A X X G F F A F F A X F A G
G G X X D X A F F For forced to retreat
ten km to abbeville few casualties
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Product Ciphers - ADFGX (contd) Output is taken
a column at a time from the transpose matrix in
numeric order (i.e., 1,2, etc) and blocked in
five character groups. Message on the previous
slide (forced to retreat.. ) FADXF XAXFD
GFXFG GGDAD XAXDF DGDXD FGGXG XXXAX GXAAA
DGFAA GGGAA AADAD FXXGA GGFAX FGXDF GFGAA
XFXXD AXA Not very strong. A Frenchman broke it
in 3.5 months. Later the code was changed - took
24 hours to break.
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Cipher Machines Jefferson Cylinder - 1790,
Wheatstone Disk 1817, Enigma- 1930s (Germany
WWII code). Rotor machines with multiple
cylindrical rotors, each with 26 input lines,
and 26 output lines. Each input line is
connected to an output line producing a simple
substitution cipher (e.g., a in, t out). For each
input character typed, the rotor advances. This
is a polyalphabetic cipher with a repeating cycle
of 26. Relatively easy to break.
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Cipher Machines Now make the output of each
stage, the input to the next stage up to n
stages. As each stage cycles through 26
positions, the next stage cycles by one
position. A 2-stage, 26 character system
presents 26 x 26 676 combinations before
repeating. Harder to break.
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Cipher Machines Adding stages adds complexity.
For example Number of Stages Repetition
Frequency (N)
(Characters) 1 261 26 2 262
676 3 263 17,576 4 264
456,976 5 265 11,881,376 Enigma had 3 and
4 rotor versions.
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The Enigma Machine Developed in 1923 as a
commercial product. German military noticed,
withdrew it from Market, and made further
improvements. Input is typed on a keyboard. Each
letter is sent to a series of rotors that
scramble the input and produce a different
character as output. Output indicator is a lamp.
The character is read by an operator and sent
out in Morse code.
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Reflector
Moving Rotors

C
Q
K
X
Scrambler
S
N
N
Y
Lamp Board
N
Q
Keyboard
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The Enigma Scrambler Unit 1st design used three
rotors/no reflector rotor. Input was on the left
and output on the right. To decrypt, input and
outputs were reversed. Reflector rotor was added
to avoid this problem. Encrypt or decrypt
without changing anything. Keyboard - 26
letters. Lamp Board - 26 indicator
lamps. Scrambler - 3 rotating wheels on common
shaft. Plugboard (not shown) - 5-13 plug (cable).

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The Rotors 26 positions, one per character.
Characters printed on a ring mounted on the
rotor rim. Could be independently set by the
operator (i.e., 26 x 26 x 26 possible initial
conditions ).
A B C D E F G H I
N O P Q R S T U V
C D E F G H I J K
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The Rotors The three rotors were fast, medium,
and slow in accordance with their speed of
advancement. Used because the initial setting
alone was not secure. The initial setting
implemented a variable Caesar shift cipher and
determining the shift for an initial setting
would be simple since the encrypted letter
frequency would reveal the plaintext.
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The Rotors To secure the method, each time a key
was pressed, the first rotor advanced one
position. This caused the encryption to vary
with each key stroke - a polyalphabetic
cipher. After the first rotor got to a certain
position, it caused the 2nd rotor to advance one
position. A notch/pin combination moved the next
rotor. The pin could be moved to vary when the
rotor advanced (from 1 to 26 positions).
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Rotor Advancement After the 2nd rotor advanced
26 positions, the 3rd rotor advanced one
position. The mechanics were such that the
advance of the 3rd rotor also caused an advance
of the middle rotor. Without this feature a
total of 263 17,576 characters were possible
before repeating. This feature caused the rotor
to skip a position for every step of the slow
rotor reducing the combination by 676 (26 x 26)
due to lost positions.
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Rotor External Ring Settings Ring settings on
each rotor could be changed by removing,
adjusting, and re-inserting the rotor. Altered
the position of the notch/pin so the advancement
character was altered. There were two final
complications 1., Rotor position order could be
changed to six (6) different rotor orderings.
(1,2,3), (1,3,2), (2,3,1), 2,1,3), (3,2,1), and
(3,1,2).
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Rotor External Ring Settings 2. The operator
could choose 3 rotors to use from 5 available
(1,2,3), (1,2,4), (1,2,5), (2,3,4), (2,3,5),
(3,4,5), (1,3,4), (1,3,5), (2,4,5),
(1,4,5). These had different notch pins. In all,
there are 17,576 x 6 x 10 initial positions
1,054,560 x 676 possible initial ring positions
712,882,560 states!
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First Problems with Enigma In 1928, the Germans
sent an enigma machine to their Warsaw legation
by ordinary freight an administrative error.
On discovering the error, they made urgent
inquiries of to Polish Customs Service tipped
off the Poles to importance of shipment. The
Poles sequestered the machine over a weekend for
a full examination and delivered it on Monday.
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First Problems with Enigma This exposed the
secret algorithm to Poles. Design of modern
systems typically assumes knowledge of the
algorithm (a mechanical/ electrical one), the
design was secret and revealing information
about the algorithm clearly jump started the
effort to break the code. Not enough for a
complete break the daily settings (daily key)
was main secret .
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Enigma Weaknesses Each initial setting (key)
produced a different encryption and the
encryption of a particular letter varied with
each input character. However, it did have
weaknesses No character could encrypt to itself
due to the mechanical design. Reduced
encryption set. Set-up used a daily code book,
changed monthly. A stolen code book would reveal
keys. This is the Key Distribution Problem,
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Enigma Weaknesses One element was supposed to
be altered for each message sent (only the daily
initialization is the same at all sites). After
the 1st message the starting positions are
changed. This procedure, called the Indicator
System enabled a receiving operator to know how
to set his system to decode a specific message
from another site.
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Indicator Procedure The procedure called for the
sending operator to randomly select three rotor
positions. Then he would set the machine to the
daily settings from the code book and transmit
three chosen letters twice to indicate to the
receiver, the per message rotor settings. The
repetition was intended to reduce errors, but
was a weakness as it allowed the cryptographers
to work out the daily initial settings.
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Indicator Procedure Three letters were sent
twice with the machine in the initialized state.
This meant there would be two instances that
enciphered to the same ciphertext in the two
messages. These were called females and disclosed
the initial setup for the day. By observing
many females over the day, the per message
settings could be determined by manipulating a
stack of perforated sheets on a back-lit glass
table looking for Matches in any two pairs.
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Breaking the Code The Poles built an
electromechanical machine the bomba to
automate the process. The bomba searched for
rotor settings to obtain a match. Six machines
could be used, one for each possible rotor
order. This was passed on to the British at
Bletchley Park who had far more resources to
attack the problem. The Poles didnt have the
resources to deal with the four or five rotor
machines.
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Breaking Encrypted Messages Algorithmic attacks
Invert the cipher text without the key by
exploiting the algorithm. Key attacks Determine
the key structure and/ or do an exhaustive search
of the key space to recover the
key. Cryptanalysis Analyze ciphertext by
statistical and other means in order to recover
the plaintext (what the NSA codebreakers do)
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Cryptanalysis Methods (Codebreaking) 4
methods Ciphertext The encryption algorithm
and ciphertext to be decoded are known. Known
Plaintext Algorithm and ciphertext are known and
one, or more, plaintext/ciphertext pairs are
known.
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Cryptanalysis Methods (Codebreaking) Chosen
Plaintext Algorithm and ciphertext are known.
Analyst can choose a plaintext and get the
ciphertext from the chosen plaintext encrypted
with the secret key. Chosen Ciphertext
Algorithm and ciphertext are known. Analyst has a
chosen ciphertext along with the corresponding
plaintext decoded by the secret key.
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Ciphertext Only Most common case and also most
difficult to break. Can be broken by Brute
force on key search. Large key-spaces (i.e., long
keys) make this difficult or intractable. Use
statistical analysis on the ciphertext. Difficult
y is then based on how well encryption removes
the statistics of the underlying message.
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Know Plaintext If analyst knows something about
the contents being encrypted, like the language,
type of file (pdf, Excel, Java source listing,
C executable), then we know that all have
specific formats that tend to appear in specific
locations. Given the ciphertext and this
knowledge, the analyst will attempt to recover
the key from knowledge about part of the
message. Trial and error and is compute
intensive.
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Chosen Plaintext Analyst must get the sender to
encrypt plaintext selected by the analyst
(planted information). The analyst will choose a
plaintext to be encrypted. It should be carefully
selected to provide a full symbol set or specific
patterns of characters that may reveal the
structure of the key. Like black box
engineering If you know the input, output and
algorithm, you can determine the remaining
variable the encrypting key.
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Chosen Ciphertext Possible in theory not
practiced (we think). There is no obvious way to
select the ciphertext being produced by a target
system. Strong algorithms and keys will readily
withstand ciphertext only attacks. Strong
algorithms are also designed to withstand known
plaintext attacks.
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• Breaking a Cipher
• Consider an example of breaking a cipher using
• information from only the ciphertext.
• Ciphertext only attack
• Information used
• Repeating groups of characters.
• Frequency distribution of characters in the
• English language.

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Breaking the Vigenère Cipher WUBEFIQLZURMVOFEHMYM
WT IXCGTMPIFKRZUPMVOIRQMM WOZMPULMBNYVQQQMVMVJLE Y
MHFEFNZPSDLPPSDLPEVQM WCXYMDAVQEEFIQCAYTQOWC XYMWM
SEMEFCFWYEYQETRLI QYCGMTWCWFBSMYFPLRXTQY EEXMRULUK
SGWFPTLRQAERL UVPMVYQYCXTWFQLMTELSFJ PQEHMOZCIWCIW
FPZZSLMAEZ IQVLQMZVPPXAWCSMZMORVG VVQSZETRLQZPBJAZ
VQIYXE WWOICCGDWHQMMVOWSGNTJP FPPAYBIYBJUTWRLQKLLL
MD PYVACDCFQNZPIFPPKSDVPT IDGXMQQVEBMQALKEZMGCVK U
ZKIZBZLIUAMMVZ
The unknown message, no knowledge of the key or
the plaintext.
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• Background
• Vigenère uses a priming key with 3 possibilities
• Autokeying One character priming key, then
• use the plaintext for the rest of the key.
• Priming word Select a word to be used as
• the key, use it in repetition (e.g., BATBATBAT)
• for the length of the message.
• Priming text Selected text (e.g., book) or a
• random string as long as the message.
• Use second case a fixed length priming word.

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Step-by-step breakage - 1 1 Look for long
repeating sequences shifts EFIQ, line 1 5,
95 letter shift. PSDLP, occurs twice in line 4,
5 letter shift. WCXYM, line 5 5-6, 20 letter
shift. ETRL, line 6 12, 120 letter shift. 2
Find factors (neglect 1 as a factor) Factors of
95 5, 19 Factors of 5 5 Factors of 20
2, 4, 5, 10, 20 Factors of 120 2, 3, 4, 5, 6,
8, 10, 15, 20
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Step-by-step breakage - 2 3 The only common
factor is 5, so we assume the priming key is 5
letters long k1k2k3k4k5 4 Use frequency
analysis to find k1 knowing that the 1st, 6th,
11th, 16th, characters, etc. are encrypted by
k1. Create a frequency table for letters
encrypted with k1 and compare with the standard
frequency table for English text.
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Step-by-step breakage - 3
10 8 6 4 2 0
A B C D E F G H I J K L M N O P Q
R S T U V W X Y Z
Frequency distribution of ciphertext for k1
encryption.
77
Step-by-step breakage - 4
10 8 6 4 2 0
A B C D E F G H I J K L M N O P Q
R S T U V W X Y Z
Frequency distribution of a plaintext with the
same Number of letters (normalized) as the
ciphertext.
78
Step-by-step breakage - 5
10 8 6 4 2 0
E F G H I J K L M N O P Q R S T U
V W X Y Z A B C
Frequency distribution of ciphertext for k1
encryption, shifted left 4 letters to get best
distribution match. Indicates k1 is most likely
E.
79
Step-by-step breakage to end Process is
repeated to find k2, k3, k4, k5. Have reduced
the problem to solving a set of mono- alphabetic
substitution ciphers, 1 for each k. Depended on
the statistics of the ciphertext being similar
to the statistics of an equal length plaintext
message. The full priming key found was EMILY.
This was done by Charles Babbage. The message
was Tennysons poem The Vision of Sin.
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The Vision of Sin - begins Sit thee down and
have no shame Cheek by jowl and knee by knee What
care I for any name What for order or degree Let
me screw thee up a peg Let me loose thy tongue
with wine Callest thou that thing a leg Which is
thinnest thine or mine .
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Summary - Breaking Ciphers Brute force searches
(Caesar shift offset shift search breaks the
key). Determining the length and structure of
the priming key using frequency analysis
(Vigenère break). Operational procedures
failures (Enigma break). Is there any secure
cipher?
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• Secure Ciphers
• 1918 U. S. Army research three new ideas.
• Randomize the key (dont use real words).
• Make the key as long as the message.
• Dont ever re-use the key.
• Result secure message.
• This fully secures the Vigenère cipher.
• Called a one-time pad.

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• Secure Cipher Problems
• One-time pads are secure, but
• Random numbers are hard to generate.
• New key for each message means huge pads.
• Key distribution is a problem since key cant
• be re-used, must be replaced frequently.
• These difficulties are huge and hard to overcome
• in a practical world (think e-commerce)
• used only for highly secure environments.
• Used to secure the U.S.-Russian hotline.