Cryptography

Basic Concepts:-
• Cryptography - the art or science encompassing
the principles and methods of transforming an
intelligible message into one that is
unintelligible, and then retransforming that
message back to its original form.
• Plaintext - the original intelligible message.
• Cipher Text - the transformed message.

• Cipher - an algorithm for transforming an
intelligible message into one that is
unintelligible by transposition and/or
substitution methods
• Key - some critical information used by the
cipher, known only to the sender and receiver
• Encipher (encode) - the process of converting
plaintext to cipher text using a cipher and a key

• Decipher (decode) - the process of
converting cipher text back into plaintext
using a cipher and a key.
• Cryptanalysis - the study of principles and
methods of transforming an unintelligible
message back into an intelligible message
without knowledge of the key. Also called
codebreaking.

Applications:-
• ATM cards,
• Computer passwords,
• Electronic commerce, etc.

Symmetric-Key
• In symmetric-key cryptography, the
same key is used by both parties. The
sender uses this key and an encryption
algorithm to encrypt data; the receiver
uses the same key and the
corresponding decryption algorithm to
decrypt the data.

Asymmetric-Key
• In asymmetric or public-key cryptography, there
are two keys: a private key and a public key.
The private key is kept by the receiver. The
public key is announced to the public. The
public key that is used for encryption is
different from the private key that is used for
decryption. The public key is available to the
public; the private key is available only to an
individual.

Symmetric-Key Ciphers:-
• Traditional Ciphers: These are character
oriented ciphers.

Substitution Cipher:-
• A substitution cipher substitutes one symbol
with another. If the symbols in the plaintext
are alphabetic characters, we replace one
character with another. For example, we can
replace character A with D, and character T
with Z. If the symbols are digits (0 to9), we can
replace 3 with 7, and 2 with 6. Substitution
ciphers can be categorized as either
monoalphabetic or polyalphabetic ciphers.

Monoalphabetic Cipher:-
• In a monoalphabetic cipher, a character (or a
symbol) in the plaintext is always changed to
the same character (or symbol) in the ciphertext
regardless of its position in the text. For
example, if the algorithm says that character A
in the plaintext is changed to character D, every
character A is changed to character D. In other
words, the relationship between characters in
the plaintext and the ciphertext is a one-to-one
relationship.

Polyalphabetic Cipher:-
• In a polyalphabetic cipher, each occurrence
of a character can have a different
substitute. The relationship between a
character in the plaintext to a character in
the ciphertext is a one-to-many
relationship. For example, character A
could be changed to D in the beginning of
the text, but it could be changed to N at
the middle.

Example:-
• The following shows a plaintext and its
corresponding ciphertext. Is the cipher
monoalphabetic?
Plaintext: HELLO
Ciphertext: KHOOR
• The cipher is probably monoalphabetic
because both occurrences of L's are
encrypted as O's

• The following shows a plaintext and its
corresponding ciphertext. Is the cipher
monoalphabetic?
Plaintext: HELLO
Ciphertext: ABNZF
• The cipher is not monoalphabetic because each
occurrence of L is encrypted by a different
character. The first L is encrypted as N; the
second as Z.

Shift Cipher:-
• The simplest monoalphabetic cipher is probably the
shift cipher. In this cipher, the encryption
algorithm is "shift key characters down," with key
equal to some number. The decryption algorithm is
"shift key characters up." For example, if the key is
5, the encryption algorithm is "shift 5 characters
down" (toward the end of the alphabet). The
decryption algorithm is "shift 5 characters up"
(toward the beginning of the alphabet). Of course,
if we reach the end or beginning of the alphabet,
we wrap around.

Example:-
• Use the shift cipher with key = 15 to
encrypt the message HELLO.
• We encrypt one character at a time. Each
character is shifted 15 characters down.
Letter H is encrypted to W. Letter E is
encrypted to T. The first L is encrypted to
A. The second L is also encrypted to A.
And 0 is encrypted to D. The cipher text is
WTAAD.

• Use the shift cipher with key =15 to
decrypt the message "WTAAD.“
• We decrypt one character at a time. Each
character is shifted 15 characters up.
Letter W is decrypted to H. Letter T is
decrypted to E. The first A is decrypted to
L. The second A is decrypted to L. And,
finally, D is decrypted to O. The plaintext
is HELLO

Transcomposition Cipher:-
• In a transposition cipher, there is no
substitution of characters; instead, their
locations change. A character in the first
position of the plaintext may appear in the
tenth position of the ciphertext. A character in
the eighth position may appear in the first
position. In other words, a transposition cipher
reorders the symbols in a block of symbols.

• Key in a transposition cipher is a mapping between
the position of the symbols in the plaintext and cipher
text. For example, the following shows the key using
a block of four characters:
Plaintext: 2 4 31
Ciphertext: 1 2 3 4
• In encryption, we move the character at position 2 to
position 1, the character at position 4 to position 2,
and so on. In decryption, we do the reverse

Example:-
• Encrypt the message “DEAR" using the
key:
Plaintext: 2 4 31
Ciphertext: 1 2 3 4
• Solution: RDAE

Simple Modern Ciphers:-
• The traditional ciphers we have studied so far
are character-oriented. With the advent of the
computer, ciphers need to be bit-oriented. This is
so because the information to be encrypted is
not just text; it can also consist of numbers,
graphics, audio, and video data. It is
convenient to convert these types of data into a
stream of bits, encrypt the stream, and then
send the encrypted stream.

XOR Cipher:-
• An XOR operation needs two data inputs
plaintext, as the first and a key as the second.
In other words, one of the inputs is the block to
be the encrypted, the other input is a key; the
result is the encrypted block. Note that in an
XOR cipher, the size of the key, the plaintext,
and the ciphertext are all the same. XOR ciphers
have a very interesting property: the encryption
and decryption are the same.

Rotation Cipher:-
• Another common cipher is the rotation cipher, in
which the input bits are rotated to the left or right.
The rotation cipher can be keyed or keyless. In keyed
rotation, the value of the key defines the number of
rotations; in keyless rotation the number of rotations
is fixed.

Other Types:-
• Modern Round Ciphers
• Data Encryption Standard (DES)
• Advanced Encryption Standard (AES)
• Electronic Code Book
• RSA (Rivest, Shamir and Adleman)
• Diffie-Hellman

Cryptography

More Related Content

What's hot (20)

Similar to Cryptography (20)

Cryptography