Cryptography

CONTENTS
• Definition
• Cryptography Issues
• Cryptography Components
• Cryptography Technique
• Cryptography Categories
• Symmetric key cryptography
• Asymmetric key cryptography
• Comparison of Symmetric and Asymmetric key cryptography
• Famous Algorithms/Ciphers

CRYPTOGRAPHY
• A word with Greek origins, means “secret writing”.
• The term to refer to the science and art of transforming
messages to make them secure and immune to attacks.
• Applications of cryptography includes ATM cards, computer
passwords, and electronic commerce.
Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan

CRYPTOGRAPHY ISSUES
• Confidentiality: Only sender, intended receiver should “understand”
message contents.
• End-Point Authentication: Sender and receiver want to confirm
identity of each other.
• Message Integrity: Sender and receiver want to ensure message not
altered (in transit, or afterwards) without detection.
• Message Nonrepudiation: Message nonrepudiation means that a
sender must not be able to deny sending a message that he or she,
in fact, did send.
• Entity Authentication: In entity authentication (or user identification)
the entity or user is verified prior to access to the system resources

PLAINTEXT AND CIPHERTEXT
• The original message, before being transformed, is called
plaintext.
• After the message is transformed, it is called ciphertext.
• An encryption algorithm transforms the plaintext into
ciphertext; a decryption algorithm transforms the ciphertext
back into plaintext.
• Example:
• Plaintext: HELLO
• Ciphertext: KHOOR

CIPHER
• Encryption and Decryption algorithms are referred as ciphers.
• Also used to refer to different categories of algorithms in
cryptography.
• Example (Traditional Substitution Ciphers):
• Monoalphabetic cipher
• Polyalphabetic cipher
• Ciphertext: ABNZF

KEY
• A key is a number (or a set of numbers) that the cipher
operates on, as an algorithm.
• To encrypt a message, we need an encryption algorithm, an
encryption key, and the plaintext.
• To decrypt a message, we need a decryption algorithm, a
decryption key, and the ciphertext. These reveal the original
plaintext.
• Types:
• Shared key, Public key and Private key.

USE OF KEY
• Example:
• Use Monoalphabetic Cipher to encrypt “HELLO” with key = 3.

ALICE, BOB, AND EVE
• Alice is the person who needs to send secure data.
• Bob is the recipient of the data.
• Eve is the person who somehow disturbs the communication
between Alice and Bob.

CRYPTOGRAPHY TECHNIQUE
Plaintext Ciphertext Plaintext
Alice Bob
Eve
Alice’s
Encryption
Key K1
Bob’s
Decryption
Key K2
If m = Plaintext, then
• Ciphertext = K1(m) and
• m = K2(K1(m))

CRYPTOGRAPHY CATEGORIES
• We can divide all the cryptography algorithms (ciphers) into two
groups:
• Symmetric key (also called secret-key) cryptography
algorithms and
• Asymmetric key (also called public-key) cryptography
algorithms.

SYMMETRIC KEY CRYPTOGRAPHY
• 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.

SYMMETRIC KEY CRYPTOGRAPHY
TECHNIQUE
Alice’s
Encryption
Key K
Bob’s
Decryption
Key K
Shared
Secret
Key K
Alice Bob
If m = Plaintext and K is the shared secret key, then
• Ciphertext = K(m) and
• m = K(K(m))

SYMMETRIC KEY CIPHERS
Traditional
Ciphers
Modern
Ciphers
Substitution
Ciphers
Transpositio
n
Ciphers
Mono-alphabetic
Ciphers
Poly-alphabetic
Ciphers
DES, AES,
IDEA, CAST,
RC4, RC5,
etc.
Simple
Ciphers
Round
Ciphers
XOR,
Rotation, S-box,
P-box
etc.

ASYMMETRIC KEY CRYPTOGRAPHY
• 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.
• If Alice wants to send a message to Bob, Alice uses the public key to
encrypt the message. When the message is received by Bob, the
private key is used to decrypt the message.
• Some of the asymmetric algorithms are:
• Diffie-Hellman, RSA, El Gamal, Elliptic Curve Cryptography (ECC), Man-in-the-
Middle Attack etc.

TECHNIQUE
Bob’s
Public
Key K+
Bob’s
Private
Key K-If
Alice Bob
m = Plaintext, then
• Ciphertext = K+(m) and
• m = K-(K+(m))
To Public
Used for encryption
Used for decryption

COMPARISON OF SYMMETRIC AND

SYMMETRIC KEY
CRYPTOGRAPHY
• Only 1 shared key is involved.
• The same key encrypts and
decrypts the plaintext.
• The shared key is kept secret
between Alice and Bob.
• Examples of Symmetric
algorithms:
• DES, 3DES, AES, IDEA,
BLOWFISH, TWOFISH, RC4, RC5,
SAFER etc.
ASYMMETRIC KEY
CRYPTOGRAPHY
• Here 2 keys : a private and a
public key are involved.
• The Public key encrypts the
plaintext while the private
key decrypts it.
• The private key is just kept
secret by the Bob while the
public key is made public.
• Examples of Asymmetric
algorithms:
• Diffie-Hellman, RSA, El Gamal,
Elliptic Curve Cryptography

MODERN ROUND CIPHERS
• The ciphers of today are called round ciphers because they involve
multiple rounds, where each round is a complex cipher made up of
the simple ciphers.
• The key used in each round is a subset or variation of the general key
called the round key.
• If the cipher has N rounds, a key generator produces N keys, K1,
K2,...., KN, where K1 is used in round 1, K2 in round 2, and so on.
• Modem symmetric-key ciphers: DES and AES are referred to as block
ciphers because they divide the plaintext into blocks and use the
same key to encrypt and decrypt the blocks.

DES - DATA ENCRYPTION STANDARD
• The algorithm encrypts a 64-bit plaintext block using a 64-bit
key.
• DES has two transposition blocks (P-boxes) and 16 complex
round ciphers (they are repeated).
• Although the 16 iteration round ciphers are conceptually the
same, each uses a different key derived from the original key.
• The initial and final permutations are keyless straight
permutations that are the inverse of each other. The
permutation takes a 64-bit input and permutes them according
to predefined values.

DES TECHNIQUE
Initial Permutation
Round 1
Round 2
Round 16
Final Permutation
Round
Key
Generator
K1
K2
K16
64-Bit Plaintext
64-Bit Ciphertext
64-Bit Key
DES

AES - ADVANCED ENCRYPTION STANDARD
• The Advanced Encryption Standard (AES) was designed because DES's
key was too small.
• AES is designed with three key sizes: 128, 192, or 256 bits.
• Table shows the relationship between the data block, number of
rounds, and key size.
Size of Data Block Number of Rounds Key Size
10 128
128-Bits 12 192
14 256
• The structure and operation of the other configurations are similar.
The difference lies in the key generation.

AES TECHNIQUE
Pre Round
Transformation
Round 1
Round 2
Round N
(slightly different)
Round
Key
Generator
K1
K2
KN
128-Bit Plaintext
128-Bit Ciphertext
Cipher Key
(128-bits for N =10,
192-bits for N =12 or
256-bits for N =14)
AES
K0
• Each round
of AES,
except for
the last, is a
cipher with
four
operations
that are
invertible.
• The last
round has
only three
operations.

RSA ALGORITHM
• It uses two numbers, e and d, having a special relationship to
each other, as the public and private keys.
• Selecting Keys
Bob use the following steps to select the private and public keys:
1. Bob chooses two very large prime numbers p and q.
2. Bob multiplies the above two primes to find n, the modulus for
encryption and decryption. In other words, n = p X q.
3. Bob calculates another number φ = (p -1) X (q - 1).
4. Bob chooses a random integer e. He then calculates d so that d x e = 1
mod φ.
5. Bob announces e and n to the public; he keeps φ and d secret.

RSA ALGORITHM
• Encryption
• Anyone who needs to send a message to Bob can use n and e.
• For example, if Alice needs to send a message to Bob, she can change
the message, usually a short one, to an integer. This is the plaintext.
• She then calculates the ciphertext, using e and n, as c = pe (mod n).
• Alice sends C, the ciphertext, to Bob.
• Decryption
• When Bob receives the ciphertext, he uses his private key d to decrypt
the message:
p = cd (mod n) Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan

CONCLUSION
• By using of encryption techniques a fair unit of confidentiality,
authentication, integrity, access control and availability of data
is maintained.
• Using cryptography Electronic Mail Security, Mail Security, IP
Security, Web security can be achieved.

REFERENCE
CHAPTER 30, CRYPTOGRAPHY
DATA COMMUNICATIONS AND NETWORKING
BY BEHROUZ A. FOROUZAN

MONOALPHABETIC CIPHER
• Use Monoalphabetic Cipher to encrypt “HELLO” with key = 3.
Key=3 means shift each character of plaintext by character at
3 places ahead of it. Hence replacing H by K, E by H, L by O and
O by R in the plaintext.

POLYALPHABETIC CIPHER
• Use Polyalphabetic Cipher to encrypt “HELLO” with key = 3,2.
Divide HELLO in group of 3 characters viz. HEL-LO and use
key=3 to encrypt HEL and key=2 to encrypt LO.
• Ciphertext: KHONB

TRANSPOSITION CIPHER
• Encrypt the message "HELLO MY DEAR" using the key:
• Plaintext: 1234
• Ciphertext: 2413
In encryption, we move the character at position 2 to position 1, the
character at position 4 to position 2, and so on. We first remove the
spaces in the message. We then divide the text into blocks of four
characters. We add a bogus character Z at the end of the third block.
The result is HELL OMYD EARZ. We create a three-block ciphertext
ELHLMDOYAZER.
• Plaintext: HELLO MY DEAR
• Ciphertext: ELHLMDOYAZER

Cryptography

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