8/7/2019 cryptology basics
1/11
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.
8/7/2019 cryptology basics
2/11
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
8/7/2019 cryptology basics
3/11
0
8/7/2019 cryptology basics
4/11
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.
8/7/2019 cryptology basics
5/11
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:
8/7/2019 cryptology basics
6/11
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
8/7/2019 cryptology basics
7/11
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
8/7/2019 cryptology basics
8/11
areencrypted
iththerecipient'public
eyandcanonl bedecrypted
iththecorrespondingpri ate
ey.
he
eysarerelated athematically,
butthepriate
eycannotbeeasibly
i.e., inactual orprojected
practice)deri ed romthepublic
ey. t
asthediscoveryofsuch
algorithms
hichrevolutioniedthepracticeofcryptographybeginning
inthemiddle
s.Public
eycryptography isafundamental and
idelyusedtechnologyaroundthe
orld, and istheapproach
hichunderlies
suchnternetstandardsas
ransportayer
ecurity
)successorto
), PGPandGPG
nanasymmetric
eyencryptionscheme, anyonecanencryptmessages
usingthepublic
ey, butonlytheholderofthepairedprivate
eycan
decrypt. ecuritydependsonthesecrecyofthatprivate
ey.
CRYPTOGRAPHICATTACKS
Acryptographicattack isamethodforcircumventingthesecurityofa
cryptographicsystembyfindingaeakness inacode,
cipher, cryptographicprotocol orkeymanagementscheme.
hisprocess
isalsocalled "cryptanalysis .
8/7/2019 cryptology basics
9/11
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.
8/7/2019 cryptology basics
10/11
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
8/7/2019 cryptology basics
11/11
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.