M.Sridevi II-M.Sc (computer science)

 Plaintext:
This is often the initial intelligible message
or information that's fed into the algorithmic
program as input.
 Encryption algorithm:
 The cryptography algorithmic
program performs numerous substitutions
and transformations on the plaintext.

 Secret key:
The key key's conjointly input to
the coding algorithmic program.
The key's a worth freelance of the plaintext
and of the algorithmic program.
The algorithmic program can turn out a
special output betting on the
precise key getting used at the time. the
precise substitutions and transformations
performed by the algorithmic program rely
on the key.

 Cipher text:
It depends on the plaintext and also
the secret key. For a given
message, 2 totally different completely
different} keys can turn out 2 different cipher
texts. The cipher text is Associate in
Nursing apparently random stream of
knowledge and, because it stands, is
unintelligible.

 Decryption algorithm: This
is primarily the cryptography rule run in
reverse. It takes the ciphertext and also
the secret key and produces the
first plaintext.

 Thisis primarily the cryptography rule run
in reverse.
 It takes the ciphertext and also the secret
key and produces the first plaintext.
 Cryptographic systems area
unit characterized 3 freelance dimensions.
The type of operation is
employed for remodeling plaintext to cipher
text.

 All encoding algorithmic rule area
unit supported 2 general principles:
substitution. within which every
component within the plaintext(bit,
letter, cluster of bits or letter) is mapped into
another component and transposition. within
which components within the plaintext area
unit rearranged. the basic demand is that
no info be lost(i.e., that each
one operations area unit reversible). Most
system, reffered to as product systems, involve
multiple stages of substitution and
transpositions.

 The number of keys used. If each sender
and receiver use identical key, the system
is stated as symmetrical , single-key,
secret-key, or standard encoding. If the
sender and receiver use totally
different keys, the system
is stated as uneven, two-key, public-
key encoding.

 The means within which the plaintext is
processed. A block cipher processes the input
one block of parts at a
time, manufacturing Associate in
Nursing output block for every input block. A
stream cipher processes the
input parts ceaselessly. manufacturing outp
ut one part at a time, because it goes on.

 Cryptanalysis:
cryptology attacks have faith in the
character of the algorithmic
program and maybe some information of the
overall characteristics of the plaintext or
maybe some sample plaintext-cipher text
pairs.
 This sort of attacks exploits the characteristics
of the algorithmic programto aim to deduce a
particular plaintext or to deduce the
key getting used.

 Brute-Force attack:
The attacker tries every possible key on a
piece of cipher-text until an intelligible
translation into plaintext is obtained. On
average, half of all possible keys must be
tried to achieve success.

 Is one during which the letters of
plaintext are replaced by alternative letters
or by numbers or symbols.
 If the plaintext is viewed as a sequence of
bits, then substitution
involves exchange plaintext bit patterns with
ciphertext bit patterns

 Simplest and earliest best-known use of a
substitution cipher used by general.
 Involves replacement every letter of the
alphabet with the letter
standing 3 places additional down the alphabet
 Alphabet is wrapped around in order that the
letter following Z may be a
 plain: meet me after the toga
party
 cipher: PHHW PH DIWHU WKH WRJD
SDUWB

 Can define transformation as:
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
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 C
 Mathematically give each letter a number
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
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 25
 Algorithm 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 ) mod 26
 Where k takes on a value in the range 1 to 25;
 the decryption algorithm is simply:
 p = D(k , C ) = (C - k ) mod 26

 With only 25 possible keys, the caeser cipher is
so far from secure. A dramatic increase in the
key space can achieved by allowing arbitrary
substitution. Before proceeding, we define the
term permutation. A Permutation of finite set of
elements S is an ordered sequence of all the
elements of S, with each element appearing
exactly once. For example, if S={a, b, c}, there
are six permutations of S.
abc, acb, bac, bca, cab, cba,

 If the “cipher” line may be any permutation of
the twenty six alphabetic characters, then
there square measure 26!
or larger than 4x1026 potential keys. this is
often ten orders of magnitude larger than
the key house for DES
 Approach is named as a monoalphabetic
substitution cipher as a result of one cipher
alphabet is employed per message

 The known multiple
letter cryptography cipher is that
the playfair, that treats diagrams within
the plaintext as single units
and interprets these units into ciphertext
diagrams

 The playfair algorithm is based on the use of
5x5 matrix of letters constructed using a
keyword.

 Hill cipher is developed by the man of
science Lester Hill in 1929. Strength is that
it fully hides single-letter frequencies.
 The use of a bigger matrix hides a lot
of frequency info
 A 3x3 Hill cipher hides not solely single-
letter however additionally two-letter
frequency info try other relevant Tools

 This example will rely on some linear
algebra and some number theory.
The key for a hill cipher is a matrix e.g.

 In the higher than case, we've got taken the
dimensions to be 3×3, but it will be any size
(as long because it is square).
 Assume we wish to inscribe the message
ATTACK AT DAWN. To inscribe this, we'd
like to interrupt the message into chunks of
three. we tend to currently take the
primary three characters from our plaintext, ATT
and produce a vector that corresponds to the
letters (replace A with 0, B with 1 ... Z with 25
etc.) to get: [0 nineteen 19] (this is ['A' 'T' 'T']).

 To get our ciphertext we perform a matrix
multiplication (you may need to
revise matrix multiplication if this doesn't
make sense):

 This method is performed for
all three letter blocks within the plaintext.
The plaintext might have to be
compelled tobe soft with
some further letters to
create positive that there's a
full range of blocks.
 Now for the tricky part, the decryption.
We need to find an inverse matrix modulo
26 to use as our 'decryption key'. i.e.

we want something that will take 'PFO'
back to 'ATT'. If our 3 by 3 key matrix is
called K, our decryption key will be the 3
by 3 matrix K-1, which is the inverse of K.
To find K-1 we have to use a bit of maths.
It turns out that K-1 above can be calculated
from our key

M.Sridevi II-M.Sc (computer science)

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M.Sridevi II-M.Sc (computer science)