classical cryptography i cs432 - security in computing copyright © 2005, 2010 by scott orr and the...
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Classical Cryptography IClassical Cryptography I
CS432 - Security in Computing
Copyright © 2005, 2010 by Scott Orr and the Trustees of Indiana University
Section OverviewSection Overview
Cryptography TypesCryptography Types
Shifts and Keyword Substitution CiphersShifts and Keyword Substitution Ciphers
Polyalphabetic Substitution CiphersPolyalphabetic Substitution Ciphers
Perfect Ciphers and Random Number Perfect Ciphers and Random Number
GenerationGeneration
ReferencesReferences
Security in Computing, 3Security in Computing, 3rdrd Ed. Ed. Chapter 2 (pgs. 35-66)Chapter 2 (pgs. 35-66)
Online ResourcesOnline Resources The Codebook CD, Simon Singh, Simon Singh
Message ComponentsMessage Components
Source: Gilbert HeldUnderstanding Data Communications, 1st Ed.
Possible Intruder GoalsPossible Intruder Goals
Intercept itIntercept it Modify itModify it Fabricate an authentic looking Fabricate an authentic looking
messagemessage Block it (deny access to)Block it (deny access to)
Classical CryptographyClassical Cryptography
Alphabetic SubstitutionsAlphabetic Substitutions ShiftsShifts Mono-Alphabetic ReplacementsMono-Alphabetic Replacements Poly-Alphabetic ReplacementsPoly-Alphabetic Replacements One-Time PadsOne-Time Pads
TranspositionsTranspositions Most were stream ciphers Most were stream ciphers
Basic Encryption ProcessBasic Encryption Process
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EncryptionEncryption DecryptionDecryption
PlaintextPlaintext PlaintextPlaintextCiphertextCiphertext
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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Symmetric Key EncryptionSymmetric Key Encryption
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EncryptionEncryption
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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DecryptionDecryption
Shared Shared KeyKey
PlaintextPlaintext PlaintextPlaintextCiphertextCiphertext
Asymmetric Key EncryptionAsymmetric Key Encryption
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EncryptionEncryption
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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DecryptionDecryption
Recipient’sRecipient’sPublic KeyPublic Key
Recipient’sRecipient’sPrivate KeyPrivate Key
PlaintextPlaintext PlaintextPlaintextCiphertextCiphertext
CRYPTANALYSIS TOOLSCRYPTANALYSIS TOOLS
Math and statistical Math and statistical techniquestechniques
Properties of Properties of languageslanguages
ComputersComputers
Ingenuity and luckIngenuity and luck
Encrypted messagesEncrypted messages
Known encryption Known encryption algorithmsalgorithms
Intercepted plaintextIntercepted plaintext
Cribs - Data known Cribs - Data known or suspected to beor suspected to bein enciphered in enciphered messagesmessages
Source: Lance HoffmanGeorge Washington University
The Alphabet & Modular The Alphabet & Modular ArithmeticArithmetic
A B C D E F G H I J K L MA B C D E F G H I J K L M0 1 2 3 4 5 6 7 8 9 10 11 120 1 2 3 4 5 6 7 8 9 10 11 12
N O P Q R S T U V W X Y ZN O P Q R S T U V W X Y Z13 14 15 16 17 18 19 20 21 22 23 24 2513 14 15 16 17 18 19 20 21 22 23 24 25
Arithmetic operation Arithmetic operation mod 26 = [0,25]mod 26 = [0,25]
SubstitutionSubstitution
Substitution CiphersSubstitution Ciphers
MonoalphabeticMonoalphabetic
OtherOther
KeywordKeywordShiftShift
PolyalphabeticPolyalphabetic
Source: Richard SpillmanPacific Lutheran University
Caesar Cipher - Simple Shift Caesar Cipher - Simple Shift
This is a cipher algorithm that transforms This is a cipher algorithm that transforms each Plaintext character into a Ciphertext each Plaintext character into a Ciphertext character shifted a fixed distance down character shifted a fixed distance down the alphabet the alphabet The key is the distance of the shiftThe key is the distance of the shift For example, a key of 3 would replace each For example, a key of 3 would replace each
Plaintext “a” with “d”, each “b” with “e”, etc.Plaintext “a” with “d”, each “b” with “e”, etc. Easy for children to use as a secret code, Easy for children to use as a secret code,
but obvious pattern is its major weaknessbut obvious pattern is its major weakness
Caesar Cipher ExampleCaesar Cipher Example
If the key is 5 then the Plaintext If the key is 5 then the Plaintext alphabet becomes the Ciphertext alphabet becomes the Ciphertext alphabet shown below:alphabet shown below:
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 za b c d e f g h i j k l m n o p q r s t u v w x y zf g h i j k l m n o p q r s t u v w x y z a b c d ef g h i j k l m n o p q r s t u v w x y z a b c d e
t h i st h i s
yy mm nn xx
Source: Richard SpillmanPacific Lutheran University
DECRYPTING CAESAR CIPHERSDECRYPTING CAESAR CIPHERS
Break between words. Blank translatedBreak between words. Blank translatedto self reveals small wordsto self reveals small words
Double letter. No QQ pairs in English!Double letter. No QQ pairs in English!
Repeated letters translating to same thingRepeated letters translating to same thing
wuhdwb lpsrvvleohwuhdwb lpsrvvleoh
Source: Lance J. Source: Lance J. HoffmanHoffman
0
2
4
6
8
10
12
14
16
18
a c e g i k m o q s u w y
Pe
rce
nt
CipherEnglish
Frequency DistributionFrequency Distribution
Source: Hoffman & PfleegerSource: Hoffman & Pfleeger
Keyword SubstitutionsKeyword Substitutions
Choose a “key word” such as Choose a “key word” such as countcount Write out the alphabet; then write Write out the alphabet; then write
the keyword directly below the first the keyword directly below the first few letters of the alphabetfew letters of the alphabet
Complete the second row by writing Complete the second row by writing (in order) the unused letters(in order) the unused letters
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 za 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
c o u n t a b d e f g h i j k l m p q r s v w x y zc o u n t a b d e f g h i j k l m p q r s v w x y z
Letter:
Code:
Starting PositionStarting Position
The keyword does not have to start The keyword does not have to start at the beginning of the plaintext at the beginning of the plaintext alphabetalphabet it could start at any letterit could start at any letter for example, “count” could start at “k”for example, “count” could start at “k”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 za b c d e f g h i j k l m n o p q r s t u v w x y zm p q r s v w x y z c o u n t a b d e f g h i j k lm p q r s v w x y z c o u n t a b d e f g h i j k l
Note: the alphabet wraps aroundNote: the alphabet wraps around
Source: SpillmanSource: Spillman
Key Word ExampleKey Word Example
If the keyword is “visit” (note, the If the keyword is “visit” (note, the second “i” is visit is dropped below) second “i” is visit is dropped below) starting at “a” and the plaintext is starting at “a” and the plaintext is “next”, the application is:“next”, the application is:
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 za b c d e f g h i j k l m n o p q r s t u v w x y zv i s t a b c d e f g h j k l m n o p q r u w x y zv i s t a b c d e f g h j k l m n o p q r u w x y z
n e x tn e x t
kk aa xx qqSource: Spillman
Frequency TableFrequency Table
a 3312 7.49 n 2982 6.74b 573 1.29 o 3261 7.37c 1568 3.54 p 1074 2.43d 1602 3.62 q 116 0.26e 6192 14 r 2716 6.14f 966 2.18 s 3072 6.95g 769 1.74 t 4358 9.85h 1869 4.22 u 1329 3i 2943 6.65 v 512 1.16j 119 0.27 w 748 1.69k 206 0.47 x 123 0.28l 1579 3.57 y 727 1.64
m 1500 3.39 z 16 0.04
Letter Frequency Pct. Letter Frequency Pct.
n = 44232
Frequency GroupingsFrequency Groupings
HighHighEE
TT
A O N I R SA O N I R S
HH
MediumMedium D L U C MD L U C M
LowLow P F Y W G B VP F Y W G B V
RareRare J K Q X ZJ K Q X Z
There is usually a 2% drop between the high and mediumThere is usually a 2% drop between the high and mediumFrequency letter groupsFrequency letter groups
Source: Richard SpillmanSource: Richard Spillman
English CharacteristicsEnglish Characteristics
Three vowels: “a”, “I”, “o” tend to avoid Three vowels: “a”, “I”, “o” tend to avoid each other each other
Vowels make up 40% of messagesVowels make up 40% of messages High-frequency letters make up 70% of High-frequency letters make up 70% of
messagemessage 80% of letters before “n” are vowels80% of letters before “n” are vowels ““h” frequently proceeds “e” but rarely h” frequently proceeds “e” but rarely
followsfollows
Source: Richard SpillmanSource: Richard Spillman
Digrams and TrigramsDigrams and Trigrams
Top 10 DigramsTop 10 Digrams
THTH ININHEHE ERERRERE ONONESES ANANATAT TITI
Top 10 TrigramsTop 10 Trigrams
THETHE INGINGTHATHA ANDANDHATHAT IONIONENTENT YOUYOUTHITHI FORFOR
These are just as prevalent as individual charactersThese are just as prevalent as individual charactersand can be subjected to the same kind of relativeand can be subjected to the same kind of relativefrequency analysis ~ some like qp just don’t exist.frequency analysis ~ some like qp just don’t exist.
Source: Source: Making, Breaking CodesMaking, Breaking CodesBy Paul GarrettBy Paul Garrett
Polyalphabetic SubstitutionsPolyalphabetic Substitutions Monoalphabetic ciphers produce the same Monoalphabetic ciphers produce the same
distributions as plaintext. To flatten the ciphertext distributions as plaintext. To flatten the ciphertext distribution, try combining two ciphers so that letters distribution, try combining two ciphers so that letters of high and low frequency will map to the same cipher of high and low frequency will map to the same cipher letter.letter. ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWXYZADGJMPSVYBEHKNQTWZCFILORUXADGJMPSVYBEHKNQTWZCFILORUX
3a mod 26 above for odd positions3a mod 26 above for odd positions
ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWXYZNSXCHMRWBQLQVAFKPUZEJOTYDINSXCHMRWBQLQVAFKPUZEJOTYDI
(5a + 13) mod 26 above for even positions(5a + 13) mod 26 above for even positions
TREAT YIMPO SSIBL ETREAT YIMPO SSIBL E encrypts to encrypts toFUMNF DYVTF CZYSH HFUMNF DYVTF CZYSH H
Vigenère CipherVigenère Cipher
This is an example of a This is an example of a polyalphabetic cipher where the polyalphabetic cipher where the substitution pattern variessubstitution pattern varies that is, a plaintext “e” may be replaced that is, a plaintext “e” may be replaced
by a ciphertext “p” one time and a by a ciphertext “p” one time and a ciphertext “w” anotherciphertext “w” another
the Vigenère cipher does this using a the Vigenère cipher does this using a Vigenère tableVigenère table
Vigenère TableVigenère Table
The table The table lists the keylists the keycharacters characters onontop and thetop and theplaintextplaintextcharacters characters ononthe sidethe side
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 za 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 zb 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 an c d e f g h i j k l m n o p q r s t u v w x y z a bd d e f g h i j k l m n o p q r s t u v w x y z a b ce 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 df f g h i j k l m n o p q r s t u v w x y z a b c d e g g h i j k l m n o p q r s t u v w x y z a b c d e f h h i j k l m n o p q r s t u v w x y z a b c d e f g i i j k l m n o p q r s t u v w x y z a b c d e f g h j j k l m n o p q r s t u v w x y z a b c d e f g h i k k l m n o p q r s t u v w x y z a b c d e f g h i j l l m n o p q r s t u v w x y z a b c d e f g h i j k m m n o p q r s t u v w x y z a b c d e f g h i j k l n n o p q r s t u v w x y z a b c d e f g h i j k l m o o p q r s t u v w x y z a b c d e f g h i j k l m n p p q r s t u v w x y z a b c d e f g h i j k l m n o q q r s t u v w x y z a b c d e f g h i j k l m n o p r r s t u v w x y z a b c d e f g h i j k l m n o p q s s t u v w x y z a b c d e f g h i j k l m n o p q r t t u v w x y z a b c d e f g h i j k l m n o p q r s u u v w x y z a b c d e f g h i j k l m n o p q r s t v v w x y z a b c d e f g h i j k l m n o p q r s t u w w x y z a b c d e f g h i j k l m n o p q r s t u v a x y z a b c d e f g h i j k l m n o p q r s t u v w y y z a b c d e f g h i j k l m n o p q r s t u v w x z z 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
A keyword is selected and it is A keyword is selected and it is repeatedly written above the plaintextrepeatedly written above the plaintext EXAMPLE: using the keyword “hold”EXAMPLE: using the keyword “hold”
Each column forms a keyword/plaintext Each column forms a keyword/plaintext letter pair which is used in the Vigenère letter pair which is used in the Vigenère table to determine the ciphertext lettertable to determine the ciphertext letter
Vigenère Cipher StepsVigenère Cipher Steps
h o l d h o l d h o l d h o l dh o l d h o l d h o l d h o l dt h i s t h e p l a i n t e x tt h i s t h e p l a i n t e x t
Vigenère ExampleVigenère Example
Using the keyword “hold”Using the keyword “hold”
h o l d h o l d h o l d h o l dh o l d h o l d h o l d h o l dt h i s t h e p l a i n t e x tt h i s t h e p l a i n t e x t
a b c d e f g h i . . .a a b c d e f g h ib b c d e f g h i j . . .n c d e f g h i j k . . .d d e f g h i j k l . . .e e f g h i j k l m . . .f f g h i j k l m n . . .g g h i j k l m n o . . .h h i j k l m n o p . . .i i j k l m n o p q . . .j j k l m n o p q r . . . k k l m n o p q r s . . .l l m n o p q r s t . . .m m n o p q r s t u . . .n n o p q r s t u v . . .o o p q r s t u v w . . .p p q r s t u v w x . . .q q r s t u v w x y . . .r r s t u v w x y z . . .s s t u v w x y z a . . .t t u v w x y z a b . . .u u v w x y z a b c . . .
a
So, “t” becomes “a” butSo, “t” becomes “a” butat the end “t” becomes “w”at the end “t” becomes “w”
w
Cryptanalysis of Cryptanalysis of PolyalphabeticsPolyalphabetics
While difficult, these are not immuneWhile difficult, these are not immune
Basic strategy is to determine the Basic strategy is to determine the number of alphabets used to number of alphabets used to encrypt, and then…encrypt, and then… break message into its monoalphabetic break message into its monoalphabetic
components andcomponents and solve each of these as beforesolve each of these as before
KASISKI METHODKASISKI METHODfor repeated patternsfor repeated patterns
Relies on frequency of letter patterns such Relies on frequency of letter patterns such asas-th, -ing, in-, un-, re-, of, and, to-th, -ing, in-, un-, re-, of, and, to
If message enciphered with n alphabets in If message enciphered with n alphabets in cycliccyclicrotation and a word appears k times in rotation and a word appears k times in plaintext,plaintext,it should be enciphered approximately k/n it should be enciphered approximately k/n timestimesfrom same alphabetfrom same alphabet
KASISKI METHODKASISKI METHODExample using Dickens' workExample using Dickens' work
dicke nsdic kensd ickedicke nsdic kensd icken sdick enn sdick ensdi ckens dickesdi ckens dickeitwas thebe stoft imesitwas thebe stoft imesi twast hei twast hewor stoft imesiwor stoft imesi
nsdic kensd icken sdick ensdi ckens dicke nsdicnsdic kensd icken sdick ensdi ckens dicke nsdictwast heage ofwis domit wastn eageo ffool ishnetwast heage ofwis domit wastn eageo ffool ishne
kekensd ickennsd icken sdick ensdi cke sdick ensdi ckens dicke nns dicke nsdic kensdsdic kensdssssitw astheitw asthe epoch ofbel ief epoch ofbel iefit wasth eit wasth eepoc hofinepoc hofin
IT WAS THEIT WAS THE is encrypted using keyword is encrypted using keyword nsdickennsdicken three times three timesabove, once in the first line, twice in the third lineabove, once in the first line, twice in the third line
These all appear as identical 8-character ciphertextThese all appear as identical 8-character ciphertextpatterns. Distance between repeated patterns is a multiplepatterns. Distance between repeated patterns is a multipleof keyword length. Any repeated pattern over 3 charactersof keyword length. Any repeated pattern over 3 charactersis probably not accidental.is probably not accidental.
Kasiski Method cont’dKasiski Method cont’dAlthough many 2-letter combinations are coincidental, Although many 2-letter combinations are coincidental, the probability of 4-letter coincidences is only the probability of 4-letter coincidences is only 0.00000210.0000021Once a repeated phrase has been found, compute the Once a repeated phrase has been found, compute the distance to the next occurrence and determine the distance to the next occurrence and determine the factors for that distance.factors for that distance.Repeat as necessary and determine most likely factorsRepeat as necessary and determine most likely factors
Starting Starting Distance fromDistance from FactorsFactors
PositionPosition Previous Previous
2020 ----------- ----------- --------------------------
8383 63 (83-20) 63 (83-20) 3, 7, 9, 21, 633, 7, 9, 21, 63
104104 21 (104-83) 21 (104-83) 3, 7, 213, 7, 21
3 or 7
Steps in the Kasiski MethodSteps in the Kasiski Method
Identify repeated patterns of 3 or more charactersIdentify repeated patterns of 3 or more characters
For each pattern, note the position at which eachFor each pattern, note the position at which eachinstance of the pattern beginsinstance of the pattern begins
Note the difference between starting points ofNote the difference between starting points ofsuccessive instancessuccessive instances
Compute factors of each difference; key length isCompute factors of each difference; key length islikely to be one of the factors that appears oftenlikely to be one of the factors that appears often
Then try to divide message into pieces encipheredThen try to divide message into pieces encipheredwith same alphabetwith same alphabet
Index of CoincidenceIndex of Coincidence
Once a key length is selected (3 or Once a key length is selected (3 or 7), divide the encrypted message 7), divide the encrypted message into that into that number of sub-messages.number of sub-messages.
Compare frequency distributions to Compare frequency distributions to English to determine whether a English to determine whether a particular set was used to encrypt.particular set was used to encrypt.
M1 = {c1,c4,c7,… } M2 = {c2,c5,c8,… } M3 = {c3,c6,c9,… }
Roughness of Distribution Roughness of Distribution of English Text of English Text
0
2
4
6
8
10
12
14
16
a c e g i k m o q s u w y
Per
cent
English
Flat 1/26
Index of CoincidenceIndex of Coincidence
NUMBER OFNUMBER OFALPHABETSALPHABETS
INDEX OFINDEX OFCOINCIDENCECOINCIDENCE
11 0.0680.068
22 0.0520.052
33 0.0470.047
44 0.0440.044
55 0.0440.044
1010 0.0410.041
largelarge 0.0380.038
z
ai
ii
nn
FreqFreqIC
)1(
)1(
If we have lots of ciphertext AND underlying If we have lots of ciphertext AND underlying plaintext has a fairly standard distribution of plaintext has a fairly standard distribution of letters, THEN can use IC:letters, THEN can use IC:
Decrypting Decrypting PolyalphabeticsPolyalphabetics
Use Kasiski method to predict likelyUse Kasiski method to predict likelynumber of enciphering alphabets. Ifnumber of enciphering alphabets. Ifit does not work, then encryption isit does not work, then encryption isprobably not simply a polyalphabeticprobably not simply a polyalphabeticsubstitution.substitution.
Separate ciphertext into appropriateSeparate ciphertext into appropriatesubsets and independently compute ICsubsets and independently compute ICfor each subset (should be near 0.068)for each subset (should be near 0.068)
Use frequency analysis on each subsetUse frequency analysis on each subset
The Perfect Substitution The Perfect Substitution CipherCipher
Use many alphabets to produce a perfectly flat Use many alphabets to produce a perfectly flat distribution with no recognizable pattern for distribution with no recognizable pattern for the choice of any alphabet at any given point.the choice of any alphabet at any given point.
Suppose the Vigenère Tableau were extended Suppose the Vigenère Tableau were extended infinitely with a random keyinfinitely with a random key
Would defy the Kasiski Method. Any repeat Would defy the Kasiski Method. Any repeat encryptions would be purely coincidentalencryptions would be purely coincidental
IC = 0.038 suggesting a totally random IC = 0.038 suggesting a totally random encryption.encryption.
One-time PadsOne-time Pads
Called the perfect cipher because it uses an Called the perfect cipher because it uses an arbitrarily long encryption keyarbitrarily long encryption key
Sender and receiver are provided a book of Sender and receiver are provided a book of keys and encryption tableaus. If each key keys and encryption tableaus. If each key has length = 20, then a 300 letter message has length = 20, then a 300 letter message would require 15 keys pasted adjacently. would require 15 keys pasted adjacently. After encryption and subsequent decryption, After encryption and subsequent decryption, both sender and receiver destroy the keys.both sender and receiver destroy the keys.
No key is ever used twice.No key is ever used twice.
Problems with One-time Problems with One-time PadsPads
Requires absolute synchronization Requires absolute synchronization between sender and receiverbetween sender and receiver
Need exists for an unlimited number Need exists for an unlimited number of keysof keys
Publishing, distributing and securing Publishing, distributing and securing keys is a major problem - an keys is a major problem - an administrative burdenadministrative burden
Use Of Random NumbersUse Of Random Numbers
Approximates one-time padsApproximates one-time pads computer generated random numbers computer generated random numbers
must be scaled to the interval must be scaled to the interval [0, 25][0, 25] Requires complete synchronization Requires complete synchronization
between sender and receiverbetween sender and receiver RN Generators are not truly random, RN Generators are not truly random,
and given enough ciphertext, they and given enough ciphertext, they can be brokencan be broken
Random Number Random Number GeneratorsGenerators
Many encryption algorithms rely on Many encryption algorithms rely on random numbersrandom numbers
RNGs produce long period sequences but RNGs produce long period sequences but the cycle eventually repeatsthe cycle eventually repeats
The The linear congruential RNGlinear congruential RNG is the most is the most common type - requires a common type - requires a seed valueseed value
NEW_RANDNO := (A*OLD_RANDNO + B) NEW_RANDNO := (A*OLD_RANDNO + B) mod mod N N
A, B and N are constants;A, B and N are constants;seed number and B must be prime relative to Nseed number and B must be prime relative to N
The Vernam CipherThe Vernam Cipher
Named after its developer, Gilbert Named after its developer, Gilbert Vernam who worked for AT&TVernam who worked for AT&T
Vernam used a punched paper tape Vernam used a punched paper tape containing a long series of non-containing a long series of non-algorithmic random numbers to algorithmic random numbers to produce the ciphertextproduce the ciphertext
Keys destroyed after a single use to Keys destroyed after a single use to make them immune to analysismake them immune to analysis
Vernam ModelVernam Model
Plaintext
Long Random Number Sequence
CiphertextOriginalPlaintext
Encryption Decryption
denotes an XOR or other combining function
Vernam ExampleVernam Example
V E R N A M C I P H E R21 4 17 13 0 12 2 8 15 7 4 1776 48 16 82 44 3 58 11 60 5 48 8897 52 33 95 44 15 60 19 75 12 52 10519 0 7 17 18 15 8 19 23 12 0 1T A H R S P I T X M A B
plaintextnumeric equivalent+ random number= sum mod 26ciphertext
Probable Word AttacksProbable Word Attacks
Given the structure of the linear Given the structure of the linear congruential RNG, assume the first few congruential RNG, assume the first few ciphertext characters represent some ciphertext characters represent some likely word such as ‘MEMO,’ ‘DATE’ or likely word such as ‘MEMO,’ ‘DATE’ or ‘FROM’‘FROM’
Inserting the numeric equivalents for the Inserting the numeric equivalents for the plaintext probable words, a system of plaintext probable words, a system of simultaneous equations can be simultaneous equations can be developed and solveddeveloped and solved
Long Sequences from BooksLong Sequences from Books
Use the phone book (middle two digits of a Use the phone book (middle two digits of a telephone number make a good RN)telephone number make a good RN) RN RN modmod 26 defines the Vigenère key column 26 defines the Vigenère key column
Use a novel for a nonrepeating keyUse a novel for a nonrepeating key Problem is that both key and plaintext have the Problem is that both key and plaintext have the
same frequency distributionsame frequency distribution also {a,e,i,n,o,t} make up 50% of all letter also {a,e,i,n,o,t} make up 50% of all letter
occurrences in English. Probability that they map occurrences in English. Probability that they map to same subset is 0.25to same subset is 0.25
leads to a reduced Vigenère Tableau and some leads to a reduced Vigenère Tableau and some effective guessingeffective guessing
CRYPTOANALYTIC TOOLSCRYPTOANALYTIC TOOLSFOR SUBSTITUTION CIPHERSFOR SUBSTITUTION CIPHERS
Frequency distributionFrequency distribution
Index of coincidenceIndex of coincidence
Consideration of highly likely lettersConsideration of highly likely lettersand probable wordsand probable words
Pattern analysis and Kasiski approachPattern analysis and Kasiski approach
Persistence, organization, ingenuity, and Persistence, organization, ingenuity, and luckluck