Unit-1B

Presented by :
Prof Ghanshyam I Prajapati
Dept of CE & IT,
SVMIT - Bharuch

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 Important Key Points…..
• An original message is known as the plaintext, while the
coded message is called the ciphertext.
• The process of converting from plaintext to ciphertext is
known as enciphering or encryption; restoring the
plaintext from the ciphertext is deciphering or decryption.
• The many schemes used for encryption constitute the area
of study known as cryptography. Such a scheme is known
as a cryptographic system or a cipher.
• Techniques used for deciphering a message without any
knowledge of the enciphering details fall into the area of
cryptanalysis.
• Cryptanalysis is what the layperson calls “breaking the
code.”
• The areas of cryptography and cryptanalysis together are
called cryptology.
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 Symmetric Cipher Model

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• There are two requirements for secure use of conventional
encryption:

1. We need a strong encryption algorithm. At a minimum, we

would like the algorithm to be such that an opponent who
knows the algorithm and has access to one or more ciphertexts
would be unable to decipher the ciphertext or figure out the
key. This requirement is usually stated in a stronger form: The
opponent should be unable to decrypt ciphertext or discover
the key even if he or she is in possession of a number of
ciphertexts together with the plaintext that produced each
ciphertext.
2. Sender and receiver must have obtained copies of the secret
key in a secure fashion and must keep the key secure. If
someone can discover the key and knows the algorithm, all
communication using this key is readable.
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Cryptographic systems are characterized along three independent dimensions:
1. The type of operations used for transforming plaintext to ciphertext.
All encryption algorithms are based on two general principles:
substitution, in which each element in the plaintext (bit, letter, group of
bits or letters) is mapped into another element, and transposition, in which
elements in the plaintext are rearranged. The fundamental requirement is
that no information be lost (i.e., that all operations are reversible). Most
systems, referred to as product systems, involve multiple stages of
substitutions and transpositions.
2. The number of keys used.
If both sender and receiver use the same key, the system is referred to
as symmetric, single-key, secret-key, or conventional encryption. If the
sender and receiver use different keys, the system is referred to as
asymmetric, two-key, or public-key encryption.
3. The way in which the plaintext is processed.
A block cipher processes the input one block of elements at a time,
producing an output block for each input block. A stream cipher processes
the input elements continuously, producing output one element at a time, as
it goes along.
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Cryptanalysis:

Cryptanalytic attacks rely on the nature of the algorithm

plus perhaps some knowledge of the general characteristics of
the plaintext or even some sample plaintext–ciphertext pairs.
This type of attack exploits the characteristics of the algorithm
to attempt to deduce a specific plaintext or to deduce the key
being used.

Brute-force attack:

The attacker tries every possible key on a piece of

ciphertext until an intelligible translation into plaintext is
obtained. On average, half of all possible keys must be tried to
achieve success.
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 Types of Attacks on Encrypted Messages

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• An encryption scheme is unconditionally secure if the
ciphertext generated by the scheme does not contain
enough information to determine uniquely the corres-
ponding plaintext, no matter how much ciphertext is
available.
• Therefore, all that the users of an encryption algorithm
can strive for is an algorithm that meets one or both of the
following criteria:
(1) The cost of breaking the cipher exceeds the value of
the encrypted information.
(2) The time required to break the cipher exceeds the
useful lifetime of the information.
• An encryption scheme is said to be computationally
secure if either of the foregoing two criteria are met.
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• The two basic building blocks of all encryption
techniques are :

** substitution

** transposition.

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 Substitution Techniques
• A substitution technique is one in which the letters of
plaintext are replaced by other letters or by numbers or
symbols. If the plaintext is viewed as a sequence of bits,
then substitution involves replacing plaintext bit patterns
with ciphertext bit patterns.
• Different Flavors:
– Caesar Cipher
– Monoalphabetic Ciphers
– Playfair Cipher
– Hill Cipher
– Polyalphabetic Ciphers
– One-Time Pad

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 Caesar Cipher
• The earliest known, and the simplest, use of a
substitution cipher was by Julius Caesar.
• The Caesar cipher involves replacing each letter of
the alphabet with the letter standing three places
further down the alphabet.
• For example,
plain: meet me after the toga party
cipher: PHHW PH DIWHU WKH WRJD SDUWB

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 Monoalphabetic Ciphers
• Better than Caeser Cipher
• For each character of alphabet, assign different-abrupt
concerned character
• Example:
a – P, b-Y, c-E, …. z-Q etc…

• Monoalphabetic ciphers are easy to break because

they reflect the frequency data of the original
alphabet.
• A countermeasure is to provide multiple substitutes,
known as homophones, for a single letter.
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 Playfair Cipher
• The best-known multiple-letter encryption cipher is
the Playfair, which treats digrams in the plaintext as
single units and translates these units into ciphertext
digrams.
• The Playfair algorithm is based on the use of a 5 * 5
matrix of letters constructed using a keyword.
• In this case, the keyword is monarchy. The matrix is
constructed by filling in the letters of the keyword
(minus duplicates) from left to right and from top to
bottom, and then filling in the remainder of the matrix
with the remaining letters in alphabetic order. The
letters I and J count as one letter.
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 Plaintext is encrypted two
letters at a time, according to
the following rules:

1. Repeating plaintext letters that are in the same pair are separated with
a filler letter, such as x, so that balloon would be treated as ba lx lo
on.
2. Two plaintext letters that fall in the same row of the matrix are each
replaced by the letter to the right, with the first element of the row
circularly following the last. For example, ar is encrypted as RM.
3. Two plaintext letters that fall in the same column are each replaced by
the letter beneath, with the top element of the column circularly
following the last. For example, mu is encrypted as CM.
4. Otherwise, each plaintext letter in a pair is replaced by the letter that
lies in its own row and the column occupied by the other plaintext
letter. Thus, hs becomes BP and ea becomes IM (or JM, as the
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encipherer wishes).
• The Playfair cipher is a great advance over simple
monoalphabetic ciphers.

• For one thing, whereas there are only 26 letters, there

are 26 * 26 = 676 digrams, so that identification of
individual digrams is more difficult.

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 Hill Cipher
• This encryption algorithm takes m successive
plaintext letters and substitutes for them m
ciphertext letters.
• The substitution is determined by m linear
equations in which each character is assigned a
numerical value (a = 0, b = 1, c, z = 25).
• For m = 3, the system can be described as
c1 = (k11p1 + k21p2 + k31p3) mod 26
c2 = (k12p1 + k22p2 + k32p3) mod 26
c3 = (k13p1 + k23p2 + k33p3) mod 26

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• As with Playfair, the strength of the Hill cipher is that it
completely hides single-letter frequencies. Indeed, with Hill, the
use of a larger matrix hides more frequency information.
• Thus, a 3 * 3 Hill cipher hides not only single-letter but also two-
letter frequency information.
• Although the Hill cipher is strong against a ciphertext-only
attack, it is easily broken with
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a known plaintext attack.
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 Polyalphabetic Ciphers

• Another way to improve on the simple monoalphabetic

technique is to use different monoalphabetic substitutions
as one proceeds through the plaintext message.
• The general name for this approach is polyalphabetic
substitution cipher.
• All these techniques have the following features in
common:

1. A set of related monoalphabetic substitution rules is used.

2. A key determines which particular rule is chosen for a
given transformation.

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 One-Time Pad

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• In theory, we need look no further for a cipher. The one-time pad
offers complete security but, in practice, has two fundamental
difficulties:
1. There is the practical problem of making large quantities of
random keys. Any heavily used system might require millions
of random characters on a regular basis. Supplying truly random
characters in this volume is a significant task.
2. Even more daunting is the problem of key distribution and
protection. For every message to be sent, a key of equal length
is needed by both sender and receiver. Thus, a mammoth key
distribution problem exists.
• Because of these difficulties, the one-time pad is of limited utility
and is useful primarily for low-bandwidth channels requiring very
high security.
• The one-time pad is the only cryptosystem that exhibits what is
referred to as perfect secrecy.
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 Transposition Techniques
• All the techniques examined so far involve the substitution
of a ciphertext symbol for a plaintext symbol.

• A very different kind of mapping is achieved by performing

some sort of permutation on the plaintext letters.
• This technique is referred to as a transposition cipher.

• The simplest such cipher is the rail fence technique, in

which the plaintext is written down as a sequence of
diagonals and then read off as a sequence of rows.

• For example, to encipher the message “meet me after the

toga party” with a rail fence of depth 2, we write the
following:
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 Rotor Machines
• The example just given suggests that multiple stages of
encryption can produce an algorithm that is significantly
more difficult to cryptanalyze.
• This is as true of substitution ciphers as it is of transposition
ciphers.
• The basic principle of the rotor machine is illustrated in
figure.
• The machine consists of a set of independently rotating
cylinders through which electrical pulses can flow.
• Each cylinder has 26 input pins and 26 output pins, with
internal wiring that connects each input pin to a unique
output pin. For simplicity, only three of the internal
connections in each cylinder are shown.
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 Steganography
A plaintext message may be hidden in one of two ways. The
methods of steganography conceal the existence of the
message, whereas the methods of cryptography render the
message unintelligible to outsiders by various transformations
of the text.

Various other techniques have been used historically; some

examples are the following :
Character marking: Selected letters of printed or typewritten
text are overwritten in pencil. The marks are ordinarily not
visible unless the paper is held at an angle to bright light.
Invisible ink: A number of substances can be used for writing
but leave no visible trace until heat or some chemical is
applied to the paper.
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Pin punctures: Small pin punctures on selected letters are
ordinarily not visible unless the paper is held up in front of a
light.
Typewriter correction ribbon: Used between lines typed with a
black ribbon, the results of typing with the correction tape are
visible only under a strong light.

• Steganography has a number of drawbacks when compared to

encryption. It requires a lot of overhead to hide a relatively few
bits of information, although using a scheme like that proposed in
the preceding paragraph may make it more effective.
• Also, once the system is discovered, it becomes virtually
worthless. This problem, too, can be overcome if the insertion
method depends on some sort of key.
• Alternatively, a message can be first encrypted and then hidden
using steganography.
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• The advantage of steganography is that it can be
employed by parties who have something to lose should
the fact of their secret communication (not necessarily the
content) be discovered.

• Encryption flags traffic as important or secret or may

identify the sender or receiver as someone with something
to hide.

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