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Attacks On Cryptosystems
Information now controls almost every aspect of modern life,
not just business
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Let's consider the
many types of threats that information regularly encounters
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An assault can therefore be active or passive
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Passive attack techniques include, for
instance, intercepting and listening in on the communication
channel
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Passive
assaults are typically linked to information theft
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Passive information attack is more dangerous
than stealing of objects since information theft may go
undetected by the owner
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For example,
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Modifying the information in an unauthorized manner
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Alteration of authentication data such as originator
name or timestamp associated with information
Unauthorized deletion of data
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Cryptography provides many tools and techniques for
implementing cryptosystems capable of preventing most of the
attacks described above
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The attacker's
capabilities are determined by his presumptions and
environmental information
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Details of the Encryption Scheme
The design of a cryptosystem is based on the following two
cryptography algorithms −
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Public Algorithms − With this option, all the details of
the algorithm are in the public domain, known to
everyone
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In case of proprietary algorithms, security is ensured through
obscurity
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Secondly, they allow communication among closed group only
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Also, according to Kerckhoff’s principle, the algorithm is
preferred to be public with strength of encryption lying in
the key
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Availability of Ciphertext
We are aware that after the plaintext has been converted to
ciphertext, it is sent across an insecure public channel (like

email), where it is then sent
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Availability of Plaintext and Ciphertext
This assumption is not as obvious as other
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Some such possible circumstances are
−
The attacker influences the sender to convert plaintext
of his choice and obtains the ciphertext
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The attacker has access to
corresponding ciphertext gathered from open channel
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Using this key, he can generate pairs of corresponding
plaintexts and ciphertexts
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The attacker simply
has to learn the secret decryption key to get the plaintext
because the algorithm is already known to the general public
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The attacked system is regarded as broken or
compromised if the attacker discovers the key
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He does
not have access to corresponding plaintext
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Occasionally, the encryption key can be determined
from this attack
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Known Plaintext Attack (KPA) − In this method, the
attacker knows the plaintext for some parts of the
ciphertext
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This may be done by
determining the key or via some other method
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Chosen Plaintext Attack (CPA) − In this method, the
attacker has the text of his choice encrypted
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This
simplifies his task of determining the encryption key
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A popular public key cryptosystem, RSA
is also vulnerable to chosen-plaintext attacks
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In simplest
method of this attack, attacker builds a dictionary of
ciphertexts and corresponding plaintexts that he has

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learnt over a period of time
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Brute Force Attack (BFA) − In this method, the attacker
tries to determine the key by attempting all possible
keys
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The attacker knows the
ciphertext and the algorithm, now he attempts all the
256 keys one by one for decryption
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Birthday Attack − This attack is a variant of brute-force
technique
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When students in a class are asked about
their birthdays, the answer is one of the possible 365
dates
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Then to find the next student whose birthdate
is 3rd Aug, we need to enquire 1
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Similarly, if the hash function produces 64 bit hash
values, the possible hash values are 1
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By
repeatedly evaluating the function for different inputs,
the same output is expected to be obtained after about
5
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If the attacker is able to find two different inputs that
give the same hash value, it is a collision and that hash
function is said to be broken
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o Host A wants to communicate to host B,
hence requests public key of B
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o Thus, whatever host A sends to host B, the
attacker is able to read
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o The attacker sends his public key as A’s public
key so that B takes it as if it is taking it from A
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Instead, it is launched to exploit the
weakness in physical implementation of the
cryptosystem
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By measuring such timings, it is be possible
to know about a particular computation the processor
is carrying out
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Power Analysis Attacks − These attacks are similar to
timing attacks except that the amount of power
consumption is used to obtain information about the
nature of the underlying computations
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Practicality of Attacks
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The majority of the assaults on cryptosystems detailed here are
academic in nature because they originate from the academic
world
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For instance, in a chosen-ciphertext attack, the attacker needs a
prohibitively large number of pairs of plaintext and ciphertext
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Nonetheless, the fact that any attack exists should be a cause of
concern, particularly if the attack technique has the potential for
improvement
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We compared cryptography to a toolkit, with
different cryptographic algorithms serving as the fundamental
tools
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This chapter expands on this technique's use in creating different
types of cryptosystems
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The only security service these systems provide is
confidentiality of information
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These earlier cryptographic systems are also referred to as
Ciphers
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Caesar Cipher
It is a mono-alphabetic cipher wherein each letter of the
plaintext is substituted by another letter to form the ciphertext
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This cryptosystem is generally referred to as the Shift Cipher
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For this type of scheme, both sender and receiver agree on a
‘secret shift number’ for shifting the alphabet
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The name ‘Caesar Cipher’ is occasionally used to describe the
Shift Cipher when the ‘shift of three’ is used
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• The plaintext letter is then encrypted to the ciphertext
letter on the sliding ruler underneath
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In this case, the
plaintext ‘tutorial’ is encrypted to the ciphertext
‘WXWRULDO’
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He then replaces the ciphertext letter by the plaintext
letter on the sliding ruler underneath
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To
decrypt a message encoded with a Shift of 3, generate
the plaintext alphabet using a shift of ‘-3’ as shown
below −

Security Value
Caesar Cipher is not a secure cryptosystem because there are
only 26 possible keys to try out
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Simple Substitution Cipher
It is an improvement to the Caesar Cipher
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For example, A
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Y
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Y……B
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Permutation is nothing
but a jumbled up set of alphabets
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The sender and the
receiver may choose any one of these possible permutation as a
ciphertext alphabet
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Process of Simple Substitution Cipher
• Write the alphabets A, B, C,
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• The sender and the receiver decide on a randomly
selected permutation of the letters of the alphabet
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For
encryption, sender replaces each plaintext letters by
substituting the permutation letter that is directly
beneath it in the table
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In this example, the chosen

permutation is K,D, G,
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The plaintext ‘point’ is
encrypted to ‘MJBXZ’
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On receiving the ciphertext, the receiver, who also
knows the randomly chosen permutation, replaces
each ciphertext letter on the bottom row with the
corresponding plaintext letter in the top row
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Security Value
•

The Simple Substitution Cipher far outperforms the Caesar
Cipher
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However, the Simple Substitution Cipher is susceptible to
design faults, such as picking an obvious permutation, and as a
result, this cryptosystem is readily cracked
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For example, if ‘A’ is
encrypted as ‘D’, for any number of occurrence in that plaintext,
‘A’ will always get encrypted to ‘D’
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Polyalphabetic Cipher is a substitution cipher in which the cipher
alphabet for the plain alphabet may be different at different
places during the encryption process
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Playfair Cipher
In this scheme, pairs of letters are encrypted, instead of single
letters as in the case of simple substitution cipher
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The key table is
a 5×5 grid of alphabets that acts as the key for encrypting the
plaintext
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If the plaintext contains J,
then it is replaced by I
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In a key table, the first characters (going left to right)
in the table is the phrase, excluding the duplicate letters
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The key table works out to be −

Process of Playfair Cipher
• First, a plaintext message is split into pairs of two
letters (digraphs)
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Let us say we want to
encrypt the message “hide money”
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HI →
QC
K M N P Q
V W X Y Z

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If both letters are in the same row, take the letter to
the right of each one (going back to the left if at the
farthest right)
T U O R I
A L

S B C

‘D’ and ‘E’ are in same row, hence take
D E F G H letter to the right of them to replace
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Using these rules, the result of the encryption of ‘hide money’
with the key of ‘tutorials’ would be −
QC EF NU MF ZV
Decrypting the Playfair cipher is as simple as doing the same
process in reverse
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Security Value
It is also a substitution cipher and is difficult to break compared
to the simple substitution cipher
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The Playfair cipher was used mainly to protect important, yet
non-critical secrets, as it is quick to use and requires no special
equipment
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For example, let’s assume the key is ‘point’
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Thus, the key is: 16 15 9 14 20
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Say ‘point’
is the key
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• The sender wants to encrypt the message, say ‘attack
from south east’
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The key must be less than or equal to the size
of the message
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•

Security Value
Vigenere Cipher was designed by tweaking the standard Caesar
cipher to reduce the effectiveness of cryptanalysis on the
ciphertext and make a cryptosystem more robust
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In the history, it was regularly used for protecting sensitive
political and military information
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Variants of Vigenere Cipher
There are two special cases of Vigenere cipher −
The keyword length is same as plaintect message
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It is more secure than
typical Vigenere cipher
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One-Time Pad
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The circumstances are −
The length of the keyword is same as the length of the
plaintext
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• The keyword is used only once
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Shift Cipher − Easy to Break
In case of Shift cipher, the entire message could have had a shift
between 1 and 25
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However, with each character now having its own
individual shift between 1 and 26, the possible keys grow
exponentially for the message
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It
is a 5 letter text
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That’s for a message
with 5 alphabets
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This makes
it computationally impossible to break the ciphertext by brute
force
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The actual
plaintext alphabets are not replaced
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Then the ciphertext is read vertically as shown
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We arrange this text
horizontally in table with number of column equal to key value
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The ciphertext is obtained by reading column vertically
downward from first to last column
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To decrypt, the receiver prepares similar table
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The number of rows is obtained
by dividing number of total ciphertext alphabets by key value
and rounding of the quotient to next integer value
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To obtain the text, he reads
horizontally left to right and from top to bottom row

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