cryptology basics

8/7/2019 cryptology basics

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CRYPTOGRAPHY

Cryptography is the practice and study of hiding information .modern day

cryptography includes disciplines of mathematics, computer science and

engineering.

Encryption is the science of changing data so that it is unrecognisable and

useless to an unauthorized person. Decryption is changing it back to its

original form. The most secure techniques use a mathematical algorithm

and a variable value known as a key.

This is the basis of the protection.... if the key (sometimes called a

password) is only known by authorized individual(s), the data cannot be

exposed to other parties.

The three main fields of cryptography are CIPHERS, STEGNOGRAPHY, and

VISUAL CRYPTOGRAPHY

CLASSICAL CRYPTOGRAPHY

MONOALPHABETIC CIPHERS:

The key is such that each alphabetic character of plaintext is mapped to a

unique alphabetic character in the cipher text.


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The examples include:

a)SHIFT CIPHER : this is the most simplest form of encryption .the

characters of plaintext are replaced by characters obtained after shifting

by fixed number in the cipher text .

Also referred to as Caesars cipher as Julius Caesar is supposed to have

used this cipher for military dispatches he wished to keep secret!!!!!

In mathematical terms it can be represented by

Ek(x) =(x+k) mod 26

Dk(x) =(y-k) mod 26 where (x, y belongs toZ26)

It can be seen that x=D k (Ek(x))

For example shifting by 3: plaintext A B C D.......

Cipher text D E F G........

So there can be only 25 possible shift ciphers as shifting by 26 will

amount to same text and shifting by 27 will be same as shifting by 1 and

so on.

b) SUBSTITUTION CIPHERS:

The key is any permutation of letters; need not be a shift, for example

plaintext 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

cipher text C V B N M X Z A S D F G H J K L Q W E R T Y U I O P

So that a b c>> c v b

There can be 26! Different substitution ciphers!!

Examples are Caesars cipher (mentioned above), at bash cipher in which

alphabets are simply reversed a>>z, b >> y, c>> x and so on....

c) AFFINE CRYPTOSYSTEM

Mathematically it can be represented as


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0


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The basic principal in all polyalphabetic cipher is the use of multiple but

different mono alphabetic substitution techniques. In vigenere cipher,

each letter key value will denote the shift to be done for that particular

letter i.e. the n in the shift n cipher.

It will be clearer by an example:

let key = a b cPlaintext= x y z

Cipher text = x z b (shift of 0, 1, 2 respectively)

Another example: key =ad

Plaintext= a b c d

Cipher text = a e c g (shift of 0, 3, 0, 3)

The second example shows us an important technique, if the key is of a

lesser length than the plaintext, then use the key repeatedly till whole

length of the plaintext is traversed.

b) HILL CIPHER

Hill cipher is a polygraphic substitution cipher based on linear algebra

invented by Lester .S.Hill in 1929.

Each letter is first encoded as a number. Often the simplest scheme isused: A = 0, B =1...Z=25, but this is not an essential feature of the cipher.A block ofn letters is then considered as a vector of n dimensions, andmultiplied by an n n matrix, modulo 26.For example

Consider the message 'ACT', and the key below (or GYBNQKURP in

letters):

Since 'A' is 0, 'C' is 2 and 'T' is 19, the message is the vector:

Thus the enciphered vector is given by:

Which corresponds to a cipher text POH.


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In order to decrypt, we turn the cipher text back into a vector, then

simply multiply by the inverse matrix of the key matrix (IFKVIVVMI in

letters)

c) PERMUTATION CIPHER

The permutation cipher is a transposition cipher in which the key is a

permutation. A random permutation of size e is generated. (The larger

the value of e more secure is the cipher).the plaintext is then broken

into segments of size e and the letters within the segment are

permuted according to this key. Because the cipher doesnt changes any

of the characters, the cipher text will have exactly the same letter

frequency as the underlying plaintext .there is a limitation that the

plaintext has to have a length which is in multiple of e.

Example:

Let e = 3.Key = 2 1 3

Plaintext: CRYPTOGRAPHY

Breaking the text: CRY PTO GRA PHY

Cipher text: RCY TPO RGA HPY i.e. RCYTPORGAHP

BLOCK CIPHER

The most important symmetric (meaning the same key is used for both

encryption and decryption ) algorithms are block ciphers .the general

operation of all the block ciphers is the same a given number of bits of

plaintext (a block) is encrypted into a block of cipher text of the same

size. This is in contrast to stream ciphers which encrypt one bit at a time.

There are several modes for block encryption:


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A) ELECTRONIC CODEBOOK (ECB) MODE

It is the simplest mode of operation for a block cipher. The input data is

padded out to a multiple of the block size, broken into a integer number

of blocks, each of which is encrypted independently using the key. In

addition to simplicity, ECB has the advantage of allowing any block to bedecrypted independently of the others. Thus, lost data blocks do not

affect the decryption of other blocks. The disadvantage of ECB is that it

aids known-plaintext attacks. If the same block of plaintext is encrypted

twice with ECB, the two resulting blocks of cipher text will be the same.

BBB

Cc BLOCKBBBBB

b) CIPHER BLOCK CHAINING (CBC) MODE

CBC is the most commonly used mode of operation for a block cipher.

Prior to encryption, each block of plaintext is XOR-ed with the prior block

of cipher text. After decryption, the output of the cipher must then be

XOR-ed with the previous cipher text to recover the original plaintext.

The first block of plaintext is XOR-ed with an initialization vector (IV),

which is usually a block of random bits transmitted in the clear. CBC is

more secure than ECB because it effectively scrambles the plaintext prior


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to each encryption step. Since the cipher text is constantly changing, two

identical blocks of plaintext will encrypt to two different blocks of cipher

text. The disadvantage of CBC is that the encryption of a data block

becomes dependent on all the blocks prior to it. A lost block of data will

also prevent decoding of the next block of data.

[XOR is bitwise exclusive or operator. It takes two bit pattern of equal

length and performs the logical XOR operation on each pair of

corresponding bits. The result is each position is 1, if the two bits are

different, and 0 if they are the same. ]

MODERN CRYPTOGRAPHY: PUBLIC KEYCRYPTOGRAPHY

The distinguishing technique used in public key cryptography is the use

ofasymmetric key algorithms, where the key used to encrypt a message

is not the same as the key used to decrypt it. Each user has a pair

of cryptographic keys a public key and a private key. The private key is

kept secret, whilst the public key may be widely distributed. Messages


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CRYPTOGRAPHICATTACKS

Acryptographicattack isamethodforcircumventingthesecurityofa

cryptographicsystembyfindingaeakness inacode,

cipher, cryptographicprotocol orkeymanagementscheme.

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TYPES OF CRYPTOGRAPHIC ATTACK

Known Plaintext and Cipher text Only Attacks: A known plaintext

attack is an attack where a cryptanalyst has access to a plaintext and the

corresponding cipher text and seeks to discover a correlation between

the two.

A cipher text-only attack is an attack where a cryptanalyst has access to a

cipher text but does not have access to corresponding plaintext. With

simple ciphers, such as the Caesar Cipher, frequency analysis can be used

to break the cipher.

Chosen Plaintext and Chosen Cipher text Attacks: A chosen

plaintext attack is an attack where a cryptanalyst can encrypt a plaintext

of his choosing and study the resulting cipher text. This is most commonagainst asymmetric cryptography, where a cryptanalyst has access to a

public key.

A chosen cipher text attack is an attack where a cryptanalyst chooses a

cipher text and attempts to find a matching plaintext. This can be done

with a decryption oracle (a machine that decrypts without exposing the

key). This is also often performed on attacks versus public key

encryption; it begins with a cipher text and searches for matching

publicly-posted plaintext data.

Brute Force Attacks: A brute force attack systematically attempts everypossible key. It is most often used in a known plaintext or cipher text-

only attack.


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STEGANOGRAPHY

Steganography is the science of hiding information. Whereas the goal of

cryptography is to make the data unreadable by a third party, the role of

steganography is to hide the data from a third party. in Greek it means

covered writing .There are a large number of steganographic methods ,

ranging from invisible ink and microdots to secreting a hidden message in

the second letter of each word of a large body of text and spread

spectrum radio communication.

LEAST SIGNIFICANT BIT (LSB) INSERTION:The simplest approach to hiding data within an image file is called least

significant bit (LSB) insertion. In this method, we can take the binary

representation of the hidden data and overwrite the LSB of each byte

within the cover image. If we are using 24-bit colour, the amount of

change will be minimal and indiscernible to the human eye. As an

example, suppose that we have three adjacent pixels (nine bytes) with

the following RGB encoding:

10010101 00001101 11001001

10010110 00001111 11001010

10011111 00010000 11001011

Now suppose we want to hide the following 9 bits of data (the hidden

data is usually compressed prior to being hidden): 101101101. If we

overlay these 9 bits over the LSB of the 9 bytes above, we get the

following (where bits in bold have been changed):

10010101 00001100 1100100110010111 00001110 11001011

10011111 00010000 11001011

Note that we have successfully hidden 9 bits but at a cost of only

changing 4, or roughly 50%, of the LSBs.

This description is meant only as a high-level overview. Similar methods

can be applied to 8-bit colour but the changes, as the reader might

imagine, are more dramatic. Gray-scale images, too, are very useful for

steganographic purposes


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Genome steganography - encoding a hidden message in a strand of

human DNA.

Hiding in Text - information hidden in documents by manipulating the

positions of lines and words, hiding the data in html files

Hiding in Disk Space - hiding the data in unused or reserved space.

Hiding the Data in Software and Circuitry - data can be hidden in the

layout of the code distributed in a program or the layout of the electronic

circuit on a board.

Information Hiding in Images - ranges from least significant bit insertion

to masking and filtering to applying more sophisticated image processing

algorithms.

Hiding in Network Packets - hidden in packets transmitted through the

internet.

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