Cryptography and Network SecurityPrinciples and PracticeEighth Ed

Cryptography and Network Security:Principles and Practice
Eighth Edition
Chapter 3
Classical Encryption Techniques
Copyright © 2020 Pearson Education, Inc. All Rights Reserved.
Definitions(1 of 2)Plaintext–An original messageCiphertext–
The coded messageEnciphering/encryption–The process of
converting from plaintext tociphertextDeciphering/decryption–
Restoring the plaintext from the ciphertext
Definitions(2 of 2)Cryptography–The area of study of the many
schemes used forencryptionCryptographic system/cipher–A
schemeCryptanalysis–Techniques used for deciphering a
message withoutany knowledge of the enciphering
detailsCryptology–The areas of cryptography and cryptanalysis
Copyright © 2020 Pearson Education, Inc. All Rights
Reserved.Figure 3.1 Simplified Model ofSymmetric Encryption
Symmetric Cipher ModelThere are two requirements for secure
use of conventionalencryption:–A strong encryption algorithm–
Sender and receiver must have obtained copies of thesecret key
in a secure fashion and must keep the keysecure
Reserved.Figure 3.2 Model of SymmetricCryptosystem
Cryptographic SystemsCharacterized along three independent
dimensions:The type of operations used for transforming
plaintext tociphertext–Substitution–TranspositionThe number of
keys used–Symmetric, single-key, secret-key,
conventionalencryption–Asymmetric, two-key, or public-key
encryptionThe way in which the plaintext is processed–Block
cipher–Stream cipher
Cryptanalysis and Brute-Force AttackCryptanalysis–Attack

relies on the nature of the algorithm plus someknowledge of the
general characteristics of theplaintext–Attack exploits the
characteristics of the algorithm toattempt to deduce a specific
plaintext or to deduce thekey being usedBrute-force attack–
Attacker tries every possible key on a piece ofciphertextuntil an
intelligible translation into plaintext isobtained–On average,
half of all possible keys must be tried toachieve success
Table 3.1 Types of Attacks onEncrypted Messages
Type of Attack
Known to Cryptanalyst
Ciphertext OnlyEncryption algorithmCiphertext
Known PlaintextEncryption algorithmCiphertextOne or more
plaintext–ciphertext pairs formed with the secret key
Chosen PlaintextEncryption algorithmCiphertextPlaintext
message chosen by cryptanalyst, together with its
correspondingciphertextgenerated with the secret key
Chosen CiphertextEncryption algorithmCiphertextCiphertext
chosen by cryptanalyst, together with its corresponding
decryptedplaintext generated with the secret key
Chosen TextEncryption algorithmCiphertextPlaintext message
chosen by cryptanalyst, together with its
correspondingciphertextgenerated with the secret keyCi phertext
chosen by cryptanalyst, together with its corresponding
decryptedplaintext generated with the secret key
Encryption Scheme SecurityUnconditionally secure–No matter
how much time an opponent has, it isimpossible for him or her
to decrypt theciphertextsimply because the required information
is not thereComputationally secure–The cost of breaking the
cipher exceeds the value ofthe encrypted information–The time
required to break the cipher exceeds theuseful lifetime of the
information
Brute-Force AttackInvolves trying every possible key until an
intelligibletranslation of theciphertextinto plaintext is

obtainedOn average, half of all possible keys must be tried
toachieve successTo supplement the brute-force approach, some
degree ofknowledge about the expected plaintext is needed,
andsome means of automatically distinguishing plaintext
fromgarble is also needed
Strong EncryptionThe termstrongencryptionrefers to encryption
schemesthat make it impractically difficult for unauthorized
personsor systems to gain access to plaintext that has
beenencryptedProperties that make an encryption algorithm
strong are:–Appropriate choice of cryptographic algorithm–Use
of sufficiently long key lengths–Appropriate choice of
protocols–A well-engineered implementation–Absence of
deliberately introduced hidden flaws
Substitution TechniqueIs one in which the letters of plaintext
are replaced by otherletters or by numbers or symbolsIf the
plaintext is viewed as a sequence of bits, thensubstitution
involves replacing plaintext bit patterns withciphertextbit
patterns
Caesar CipherSimplest and earliest known use of a substitution
cipherUsed by Julius CaesarInvolves replacing each letter of the
alphabet with theletter standing three places further down the
alphabetAlphabet is wrapped around so that the letter following
Zis A
plain: meet me after the toga party
cipher: PHHW PH DIWHU WKH WRJD SDUWB
Caesar Cipher AlgorithmCan define transformation as:
a b c d e f g hij k l m n o p q r s t u v w x y z
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
CMathematically give each letter a numberb c d e f g hij k l m n
o p q r s t u v w x y z
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
24 25Algorithm can be expressed as:

c = E(3, p) = (p + 3) mod (26)A shift may be of any amount, so
that the general Caesar algorithm is:
C = E(k , p ) = (p + k ) mod26Where k takes on a value in the
range 1 to 25; the decryption algorithm issimply:
p = D(k , C ) = (C − k ) mod26
Figure 3.3 Brute-Force Cryptanalysisof Caesar Cipher
Sample of Compressed Text
Figure 3.4Sample of Compressed Text
MonoalphabeticCipherPermutation–Of a finite set of
elementsSis an ordered sequence ofall the elements ofS, with
each element appearingexactly onceIf the “cipher” line can be
any permutation of the 26alphabetic characters, then there are
26! orgreater than4 x 1026possible keys–This is 10 orders of
magnitude greater than the keyspace for DES–Approach is
referred to as amonoalphabeticsubstitutioncipher because a
single cipher alphabet isused per message
Figure 3.5 Relative Frequency ofLetters in English Text
MonoalphabeticCiphersEasy to break because theyreflect the
frequency data of theoriginal alphabetCountermeasure is to
providemultiple substitutes(homophones) for a single
letterDigram–Two-letter combination–Most common
isthTrigram–Three-letter combination–Most frequent isthe
PlayfairCipherBest-known multiple-letter encryption
cipherTreatsdigramsin the plaintext as single units andtranslates
these units intociphertextdigramsBased on the use of a 5×5
matrix of letters constructedusing a keywordInvented by British
scientist Sir Charles Wheatstone in1854Used as the standard
field system by the British Army inWorld War I and the U.S.
Army and other Allied forcesduring World War II

PlayfairKey MatrixFill in letters of keyword (minus duplicates)
from left to rightand from top to bottom, then fill in the
remainder of thematrix with the remaining letters in alphabetic
orderUsing the keyword MONARCHY:
M
O
N
A
R
C
H
Y
B
D
E
F
G
I/J
K
L
P
Q
S
T
U
V
W
X
Z
Figure 3.6 Relative Frequency ofOccurrence of Letters
Hill CipherDeveloped by the mathematician Lester Hill in
1929Strength is that it completely hides single-letter
frequencies–The use of a larger matrix hides more
frequencyinformation–A 3 x 3 Hill cipher hides not only single-

letter but alsotwo-letter frequency informationStrong against
aciphertext-only attack but easily brokenwith a known plaintext
attack
Polyalphabetic CiphersPolyalphabetic substitution cipher –
Improves on the simplemonoalphabetictechnique byusing
differentmonoalphabeticsubstitutions as oneproceeds through
the plaintext messageAll these techniques have the following
features incommon:–A set of relatedmonoalphabeticsubstitution
rules isused–A key determines which particular rule is chosen
for agiven transformation
VigenèreCipherBest known and one of the simplest
polyalphabeticsubstitution ciphersIn this scheme the set of
relatedmonoalphabeticsubstitution rules consists of the 26
Caesar ciphers withshifts of 0 through 25Each cipher is denoted
by a key letter which is theciphertextletter that substitutes for
the plaintext letter a
Example ofVigenèreCipherTo encrypt a message, a key is
needed that is as long asthe messageUs ually, the key is a
repeating keywordFor example, if the keyword isdeceptive, the
message “weare discovered save yourself” is encrypted as:
key:deceptivedeceptivedeceptive
plaintext:wearediscoveredsaveyourself
ciphertext: ZICVTWQNGRZGVTWAVZHCQYGLMGJ
VigenèreAutokeySystemA keyword is concatenated with the
plaintext itself toprovide a running keyExample:
key:deceptivewearediscoveredsav
plaintext:wearediscoveredsaveyourself
ciphertext: ZICVTWQNGKZEIIGASXSTSLVVWLAEven this
scheme is vulnerable to cryptanalysis –Because the key and the
plaintext share the samefrequency distribution of letters, a
statistical techniquecan be applied

VernamCipher
Figure 3.7VernamCipher
One-Time PadImprovement toVernamcipherproposed by an
Army SignalCorp officer, JosephMauborgneUse a random key
that is aslong as the message so thatthe key need not be
repeatedKey is used to encrypt anddecrypt a single message
andthen is discardedEach new message requires anew key of the
same length asthe new messageScheme is unbreakable–Produces
random outputthat bears no statisticalrelationship to
theplaintext–Because theciphertextcontains no
informationwhatsoever about theplaintext, there is simplyno
way to break the code
DifficultiesThe one-time pad offers complete security but, in
practice, has twofundamental difficulties:–There is the practical
used system might require millions of randomcharacters on a
regular basis–
message to be sent, a key of equal length is neededby both
sender and receiverBecause of these difficulties, the one-time
pad is of limited utility–Useful primarily for low-bandwidth
channels requiring very highsecurityThe one-time pad is the
only cryptosystem that exhibitsperfectsecrecy(see Appendix F)
Rail Fence CipherSimplest transposition cipherPlaintext is
written down as a sequence of diagonals andthen read off as a
sequence of rowsTo encipher the message “meet me after the
toga party”with a rail fence of depth 2, we would write:
m e m a t r h t g p r y
e t e f e t e o aat
Encrypted message is:
MEMATRHTGPRYETEFETEOAAT
Row Transposition CipherIs a more complex transpositionWrite
the message in a rectangle, row by row, and read themessage

off, column by column, but permute the order ofthe columns –
The order of the columns then becomes the key to thealgorithm
Key:4 3 1 2 5 6 7
Plaintext: a tta c k p
o s t p o n e
d u n til t
w o a mx y z
Ciphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZ
SummaryPresent an overview of the main concepts of
symmetriccryptographyExplain the difference between
cryptanalysis and brute-force attackUnderstand the operation of
amonoalphabeticsubstitutioncipherUnderstand the operation of a
polyalphabetic cipherPresent an overview of the Hill cipher
Reserved.Copyright
Cybersecurity is critical to protecting an organization’s
infrastructure. Even within the cybersecurity field, several
people may be responsible for ensuring an organization's
infrastructure is protected.
Locate and integrate at least two quality, academic resources on
how to apply change management principles to infrastructure
protection. You may also use government websites, such
as Cybersecurity from the National Institute of Standards and
Technology.
Please respond to the following in a post of at least 200 words:
Explain the purpose of change management and how it applies
to infrastructure protection.
Describe the methods organizations use to determine whether
changes have been made to the infrastructure.
Outline the process to be followed prior to integrating any
changes into a production environment.

Cryptography and Network SecurityPrinciples and PracticeEighth Ed

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