2.
What is Cryptography?
2
The term cryptography comes from Greek kryptos which
means "hidden, secret"; and gráphō "writing".
Cryptography is the practice and study of hiding information
like means of converting information from its
normal, comprehensible form into an incomprehensible
format, rendering it unreadable without secret knowledge.
Modern cryptography intersects the disciplines of
mathematics, computer science, and engineering.
3.
Terminology
3
Plaintext: plaintext is information a sender wishes to
transmit to a receiver. It is the input fed into a encryption
algorithm.
4.
Terminology (contd.)
4
Cipher: a cipher is an algorithm for performing encryption
or decryption — a series of well-defined steps that can be
followed as a procedure.
5.
Terminology (contd.)
5
Ciphertext: Ciphertext is the unintelligible gibberish
obtained as a result after applying a cipher on a plaintext.
6.
Terminology (contd.)
6
Encryption: encryption is the process of transforming
information (plaintext) using an algorithm (cipher) to make it
unreadable to anyone except those possessing special
knowledge, usually referred to as a key. The result of the
process is encrypted information (ciphertext).
7.
Terminology (contd.)
7
Decryption: the reverse of encryption, to make the encrypted
information readable again (i.e. to make it unencrypted).
Key: a key is a piece of information which is generated at
the time of encryption and is required to decrypt the
encrypted data.
8.
Application of Cryptography
8
Defence forces: To ensure secrecy of
communication.
Intelligence Agencies: To decrypt intercepted
communication among terrorist outfits and other
countries.
E-Commerce (online shopping, net banking):
To ensure secrecy of confidential information like
credit card numbers during transactions.
10.
Classical Ciphers
10
Substitution: a substitution cipher is a method of
encryption by which units of plaintext are replaced with
ciphertext according to a sequential order.
Caesar’s shift cipher
Atbash
ROT13
Affine
Transposition: the units of the plaintext are rearranged
in a different and usually quite complex order.
Caesar’s Box Cipher
Columnar transposition
11.
Substitution: Caesar’s Shift Cipher
11
Caesar’s shift cipher is one of the simplest substitution cipher.
It is a type of substitution cipher in which each letter in the plaintext is replaced by a
letter some fixed number of positions down the alphabet.
For example, with a shift of 3, A would be replaced by D, B would become E, and so on.
Plain:
Cipher:
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
(the shift parameter is +3)
12.
Substitution: Atbash Cipher
12
Atbash is another simple substitution cipher .
It consists in substituting the first letter for the last, the second for
the one before last, and so on, therefore reversing the alphabet.
Plain: abcdefghijklmnopqrstuvwxyz
Cipher: ZYXWVUTSRQPONMLKJIHGFEDCBA
First 13 letters: A|B|C|D|E|F|G|H|I|J|K|L|M
Last 13 Letters: Z|Y|X|W|V|U|T|S|R|Q|P|O|N
13.
Substitution: ROT13 Cipher
13
ROT13: Applying ROT13 to a piece of text merely requires
examining its alphabetic characters and replacing each one by the
letter 13 places further along in the alphabet, wrapping back to the
beginning if necessary.
14.
Substitution: ROT13 Cipher
14
ROT13 is its own inverse; that is, to undo ROT13, the
same algorithm is applied, so the same action can be used
for encoding and decoding.
Only those letters which occur in the English alphabet are
affected; numbers, symbols, whitespace, and all other
characters are left unchanged.
Because there are 26 letters in the English alphabet and
26 = 2 × 13, the ROT13 function is its own inverse:
ROT13(ROT13(x)) = ROT26(x) = x for any text x.
In other words, two successive applications of ROT13
restore the original text (in mathematics, this is sometimes
called an involution; in cryptography, a reciprocal cipher).
15.
Substitution: Affine Cipher
15
In Affine cipher each letter in an alphabet is mapped to
its numeric equivalent and then encrypted using a
simple mathematical function.
The encryption function for this example will be
y = E(x) = (5x + 8) mod 26
16.
Transposition: Caesar’s Box Cipher
16
This cipher requires the encoder to omit any spaces and then
rewrite the plaintext in a square column-wise and read row-wise to
get the cipher text.
So for example to encode the phrase 'What an unusual box', first
omit the spaces to get 'WHATANUNUSUALBOX' and then write
them in a box as follows:
To write this in code, you would then print
'WAULHNSBAUUOTNAX'
17.
Columnar Transposition Cipher
17
In a columnar transposition, the message is written
out in rows of a fixed length, and then read out
column by column, and the columns are chosen in
some scrambled order.
Both the width of the rows and the permutation of
the columns are usually defined by a keyword.
For example, the word ZEBRAS is of length 6 (so
the rows are of length 6), and the permutation is
defined by the alphabetical order of the letters in the
keyword.
In this case, the order would be "6 3 2 4 1 5".
18.
Columnar Transposition Cipher
18
For example, suppose we use the keyword ZEBRAS and the
message WE ARE DISCOVERED. FLEE AT ONCE.
In a columnar transposition, we write this into the grid as:
Z
6
W
I
R
E
E
E
3
E
S
E
A
B
2
A
C
D
T
R
4
R
O
F
O
A
1
E
V
L
N
S
5
D
E
E
C
This results in the following ciphertext:
EVLN ACDT ESEA ROFO DEEC WIREE
19.
Rotor Machine
19
A rotor machine is an electro-mechanical device used for
encrypting and decrypting secret messages.
The primary component is a set of rotors which are rotating disks
with an array of electrical contacts on either side. The wiring
between the contacts implements a fixed substitution of
letters, scrambling them in some complex fashion.
The most widely known rotor cipher device is the German Enigma
machine used during World War II.
20.
Modern Ciphers
20
Private Key ciphers are a class of algorithms for
cryptography that use identical cryptographic keys
for both decryption and encryption.
A single secret key shared by sender and receiver
(which must also be kept private)
The sender and receiver must securely share a key
in advance.
21.
Private Key Ciphers
21
Stream Ciphers: A stream cipher is a private key
cipher where plaintext bits are combined with a
pseudorandom cipher bit stream
(keystream), typically by an exclusive-or (XOR)
operation.
E.g. A5/1 is a stream cipher used to provide over-the-air
communication (mainly voice) privacy in the GSM cellular
telephone standard
Turing is a stream cipher developed at Qualcomm for CDMA.
22.
Private Key Ciphers
22
Block Ciphers: A block cipher is a private key cipher operating on
fixed-length groups of bits, called blocks. A block cipher
encryption algorithm might take (for example) a 128-bit block of
plaintext as input, and output a corresponding 128-bit block of
ciphertext. The exact transformation is controlled using a second
input — the secret key.
E.g. DES, AES
23.
Public Key Encryption
23
The distinguishing technique used in public key cryptography is
the use of asymmetric key algorithms, where the key used to
encrypt a message is not the same as the key used to decrypt it.
The asymmetric key algorithms are used to create a
mathematically related key pair: a secret private key and a
published public key.
24.
Public Key Encryption
24
Unlike symmetric key algorithms, it does not require
a secure initial exchange of one or more secret keys
to both sender and receiver.
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.
The keys are related mathematically, but the private
key cannot be feasibly (i.e. in actual or projected
practice) derived from the public key.
25.
Public Key Encryption
25
Messages are encrypted with the recipient's public
key and can only be decrypted only with the
corresponding private key.
26.
Public Key Encryption
26
It was the discovery of such algorithms which
revolutionized the practice of cryptography beginning
in the middle 1970s.
E.g. RSA (which stands for Rivest, Shamir and
Adleman who first publicly described it), PGP (short
for Pretty Good Privacy)
27.
Decryption Techniques
27
Brute force attack is a strategy used to break the
encryption of data which involves traversing all
possible keys until the correct key is found.
The selection of an appropriate key length depends
on the practical feasibility of performing a brute force
attack.
The resources required for a brute force attack scale
exponentially with increasing key size, not linearly.
As a result, doubling the key size for an algorithm
does not simply double the required number of
operations, but rather squares them.
28.
Decryption Techniques
28
Dictionary attack is a technique for defeating a cipher
or authentication mechanism by trying to determine its
decryption key or passphrase by searching likely
possibilities.
Dictionary attack uses a brute-force technique of
successively trying all the words in an exhaustive list
called a dictionary (from a pre-arranged list of values).
In contrast with a normal brute force attack, where a
large proportion key space is searched systematically, a
dictionary attack tries only those possibilities which are
most likely to succeed, typically derived from a list of
words like a dictionary (hence the phase dictionary
attack), or easily-predicted variations on words, such as
appending a digit.
29.
Conclusion
29
Cryptography is a very important field of study in
today’s world whether in our daily lives or in national
security.
It is evolving at a rapid pace.
Theoretically, no cipher can provide absolute
secrecy. Given enough time and computational
power, any encrypted data can be deciphered.
Only practical limitations (time and computing
resources) makes an encryption technique
sufficiently secure.
30.
THANK YOU
30
A Seminar by:
Anik Datta
Avijit Majhi
Diptesh Saha
Kousik Roy
Somaditya Basak
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