This PPT explains about the term "Cryptography - Encryption & Decryption". This PPT is for beginners and for intermediate developers who want to learn about Cryptography. I have also explained some famous ciphers like AES, DES and RSA. Do not forget to like.

1. CRYPTOGRAPHY

2. 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

3. DEFINITION

4. 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

5. CRYPTOGRAPHY ISSUES

6. 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

7. CRYPTOGRAPHY COMPONENTS

8. 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
Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan

9. 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
• Plaintext: HELLO
• Ciphertext: ABNZF
Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan

10. 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.
Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan

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

12. 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.
Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan

13. CRYPTOGRAPHY TECHNIQUE

14. 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))

15. CRYPTOGRAPHY CATEGORIES

16. 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.
Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan

17. 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.
Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan

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

19. 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
Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan
Round
Ciphers
XOR,
Rotation, S-box,
P-box
etc.

20. 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.
Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan

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

22. COMPARISON OF SYMMETRIC AND
ASYMMETRIC KEY CRYPTOGRAPHY

23. 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

24. FAMOUS ALGORITHMS/CIPHERS

25. 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.
Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan

26. 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
Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan
to predefined values.

27. 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

28. 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.
Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan

29. 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.

30. 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.
Ref: Ch.30, Data Communications and Networking, Fourth Edition by Behrouz A. Forouzan

31. 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

32. 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.

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

34. THANK YOU

35. EXAMPLES

36. 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.
• Plaintext: HELLO
• Ciphertext: KHOOR

37. 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.
• Plaintext: HELLO
• Ciphertext: KHONB

38. 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