The document provides a comprehensive overview of cryptography, detailing its definition, historical context, and importance in securing communication and protecting information. It describes various cryptographic techniques, including symmetric and asymmetric key cryptography, hash functions, and encryption methods such as stream and block ciphers. Additionally, it covers cryptanalysis, the study of breaking cryptographic systems, and the intricacies of different cipher techniques, including the Caesar cipher and Playfair cipher.

1. UNIT 2 : CRYPTOGRAPHY
Information system Security
Dr. Pallawi Bulakh

2. Contents
• 2.1 Techniques
• 2.2 Mathematical Foundation
• 2.3 Steam Ciphers
• 2.4 Block Ciphers
• 2.5 Cryptanalysis

3. Introduction to Cryptography
• Cryptography is the process of hiding or coding information so that only the
person a message was intended for can read it.
• The art of cryptography has been used to code messages for thousands of years
and continues to be used in bank cards, computer passwords, and ecommerce.
• Definition: Cryptography is the practice and study of techniques for securing
communication and protecting information from unauthorized access. It
involves encoding messages or data in such a way that only authorized parties
can access it.

4. Cryptography
• The word "cryptography" is derived from the Greek kryptos, meaning
hidden.
• The prefix "crypt-" means "hidden" or "vault," and the suffix "-graphy"
stands for "writing."

5. Cryptography
• Importance: Cryptography ensures data privacy, confidentiality, and
integrity. It also plays a vital role in securing communication channels,
such as emails, messages, and financial transactions, in the digital world.

6. Cryptography
 Historical Context: Cryptography has been used since ancient
civilizations (e.g., Caesar Cipher used by Julius Caesar) to protect
sensitive information.
 Modern cryptography, however, relies heavily on mathematical
algorithms to protect digital data.

7. Cryptograpgy

8. Features Of Cryptography
• Confidentiality: Information can only be accessed by the person for whom it is intended
and no other person except him can access it.
• Integrity: Information cannot be modified in storage or transition between sender and
intended receiver without any addition to information being detected.
• Non-repudiation: The creator/sender of information cannot deny his intention to send
information at a later stage.
• Authentication: The identities of the sender and receiver are confirmed. As well
destination/origin of the information is confirmed.
• Interoperability: Cryptography allows for secure communication between different
systems and platforms.
• Adaptability: Cryptography continuously evolves to stay ahead of security threats and
technological advancements.

9. Types Of Cryptography
• 1. Symmetric Key Cryptography
• 2. Hash Functions
• 3. Asymmetric Key Cryptography

10. 1. Symmetric Key Cryptography
• It is an encryption system where the sender and receiver of a message
use a single common key to encrypt and decrypt messages.
• Symmetric Key cryptography is faster and simpler but the problem is that
the sender and receiver have to somehow exchange keys securely.
• The most popular symmetric key cryptography systems are
Data Encryption Systems (DES) and Advanced Encryption Systems (AES) .

12. 2. Hash Functions
• There is no usage of any key in this algorithm. A hash value with a
fixed length is calculated as per the plain text which makes it
impossible for the contents of plain text to be recovered.
• Many operating systems use hash functions to encrypt passwords.

13. 3. Asymmetric Key Cryptography
• In Asymmetric Key Cryptography, a pair of keys is used to encrypt and decrypt
information.
• A sender’s public key is used for encryption and a receiver’s private key is used
for decryption.
• Public keys and Private keys are different. Even if the public key is known by
everyone the intended receiver can only decode it because he alone knows his
private key.
• The most popular asymmetric key cryptography algorithm is the RSA algorithm.

15. Applications of Cryptography
• Computer passwords:
• Secure web browsing:
• Electronic Signatures:
• Authentication:
• Cryptocurrencies:
• End-to-end Internet Encryption:

16. Caesar cipher
• The Caesar cipher is a simple encryption technique that was used by
Julius Caesar to send secret messages to his allies.
• It works by shifting the letters in the plaintext message by a certain
number of positions, known as the “shift” or “key”.
• The Caesar Cipher technique is one of the earliest and simplest
methods of encryption techniques.

17. Caesar cipher
• It’s simply a type of substitution cipher,
i.e., each letter of a given text is
replaced by a letter with a fixed number
of positions down the alphabet.
• For example with a shift of 1, A would
be replaced by B, B would become C,
and so on.

18. Caesar cipher
• Here is an example of how to use the Caesar cipher to
encrypt the message “HELLO” with a shift of 3:
1.Write down the plaintext message: HELLO
2.Choose a shift value. In this case, we will use a shift of 3.
3.Replace each letter in the plaintext message with the letter
that is three positions to the right in the alphabet.

19. Caesar cipher
• H becomes K (shift 3 from H)
• E becomes H (shift 3 from E)
• L becomes O (shift 3 from L)
• L becomes O (shift 3 from L)
• O becomes R (shift 3 from O)

20. Monoalphabetic Ciphers
• This is where the ciphers use one rule of substitution
throughout the message.
• This may involve replacing letters with numbers, symbols,
or another set of letters in another order.

21. Playfair Cipher
• The Playfair cipher was the first practical digraph substitution
cipher.
• The scheme was invented in 1854 by Charles Wheatstone but was
named after Lord Playfair who promoted the use of the cipher

22. Playfair Cipher
• For the encryption process let us consider the following
example:

23. Playfair Cipher
• The Playfair Cipher Encryption Algorithm:
• The Algorithm consists of 2 steps:

24. Playfair Cipher
1. Generate the key Square(5×5):
1. The key square is a 5×5 grid of alphabets that acts as the key for
encrypting the plaintext. Each of the 25 alphabets must be unique
and one letter of the alphabet (usually J) is omitted from the table
(as the table can hold only 25 alphabets).
2. If the plaintext contains J, then it is replaced by I.
3. The initial alphabets in the key square are the unique alphabets of
the key in the order in which they appear followed by the remaining
letters of the alphabet in order.
2. Algorithm to encrypt the plain text:
1. The plaintext is split into pairs of two letters (digraphs).
2. If there is an odd number of letters, a Z is added to the last letter.

25. Playfair Cipher
• For example:
PlainText: "instruments"
After Split: 'in' 'st' 'ru' 'me' 'nt' 'sz’

26. Playfair Cipher
• 1. Pair cannot be made with same letter. Break the letter in
single and add a bogus letter to the previous letter.
• Plain Text: “hello”
• After Split: ‘he’ ‘lx’ ‘lo’
• Here ‘x’ is the bogus letter.

27. Playfair Cipher
• 2. If the letter is standing alone in the process of pairing, then add
an extra bogus letter with the alone letter
• Plain Text: “helloe”
• AfterSplit: ‘he’ ‘lx’ ‘lo’ ‘ez’
• Here ‘z’ is the bogus letter.

28. Playfair Cipher
• Rules for Encryption:
• If both the letters are in the same column: Take the
letter below each one (going back to the top if at the
bottom).
For example:
Diagraph: "me"
Encrypted Text: cl
Encryption:
m -> c
e -> l

29. Playfair Cipher
• If both the letters are in the same row: Take the letter to
the right of each one (going back to the leftmost if at the
rightmost position).
For example:
Diagraph: "st"
Encrypted Text: tl
Encryption:
s -> t
t -> l

30. Playfair Cipher
• If neither of the above rules is true: Form a rectangle
with the two letters and take the letters on the horizontal
opposite corner of the rectangle.
Diagraph: "nt"
Encrypted Text: rq
Encryption:
n -> r
t -> q

31. Playfair Cipher- example
Plain Text: "instrumentsz"
Encrypted Text: gatlmzclrqtx
Encryption:
i -> g
n -> a
s -> t
t -> l
r -> m
u -> z
m -> c
e -> l
n -> r
t -> q
s -> t
z -> x

32. Playfair Cipher-example

33. Asymmetric key cryptography,
• It is also known as public-key cryptography.
• Is a method of encryption where two different but mathematically related keys are used:
• a public key for encryption and
• a private key for decryption.
• This approach provides a solution to the key distribution problem that is inherent in
symmetric key cryptography.

34. Asymmetric key cryptography

35. Characteristics of Asymmetric Key Cryptography
• Security Responsibility
• In asymmetric cryptography, the burden of security primarily falls on the receiver, like Bob.
• Bob must generate both a private and a public key, with the public key distributed to the community.
• Distribution occurs through a public-key channel, which doesn’t need secrecy but requires authentication and integrity to
prevent impersonation.
• Unique Key Pairs
• Bob and Alice can’t share the same key pair for two-way communication.
• Each entity in the community, including Bob and Alice, must create its own private and public keys.
• Alice uses Bob’s public key to encrypt messages to him, while she needs her own key pair for responses.
• Key Management
• Bob needs only one private key to receive messages from anyone in the community.
• Alice, on the other hand, needs multiple public keys—one for each entity she communicates with.
• This means Alice requires a collection of public keys for effective communication.

36. Key Components
• Plaintext: This refers to the original, readable message or data that is inputted into the encryption
algorithm.
• Encryption algorithm: This algorithm transforms the plaintext in various ways.
• Public and private keys: A pair of keys chosen so that if one is used for encryption, the other is
used for decryption. The specific transformations performed depend on whether the public or
private key is provided as input.
• Ciphertext: The encrypted, scrambled message produced as output. It can be find using both the
plaintext and the key, but uf there are different keys then it will give different ciphertexts for the
same message or plaintext.
• Decryption algorithm: This algorithm takes the ciphertext and the corresponding key and
retrieves the original plaintext.

37. What is a Cryptographic Key?
• A cryptographic key is a piece of information used by
cryptographic algorithms to encrypt or decrypt data,
authenticate identities, or generate digital signatures.
• It ensures the security and integrity of the message to be
transmitted by acting as a parameter to control the
cryptographic operations.

38. Cryptanalysis
• Definition: Cryptanalysis is the science of analyzing and breaking
cryptographic systems.
• Importance: Vital for assessing the strength of cryptographic systems.
• Relationship to Cryptography: Focuses on finding weaknesses or
vulnerabilities in cryptographic algorithms.

40. History
•Ancient Times:
•Early examples: Caesar cipher and substitution ciphers.
•World War I & II:
•Famous cryptanalysis events like breaking the Enigma code by Alan Turing
and his team.
•Modern Era:
•Advent of computers and sophisticated algorithms.

41. Types of cryptanalysis
•Classical Cryptanalysis:
•Frequency analysis.
•Pattern recognition.
•Modern Cryptanalysis:
•Brute-force attacks.
•Differential and linear cryptanalysis.
•Quantum Cryptanalysis:
•Potential to break RSA and ECC using quantum computers.

42. Tools:
•Computers and specialized software (e.g., Hashcat).
•Techniques:
•Known-plaintext attack.
•Chosen-ciphertext attack.
•Side-channel attacks.
•Mathematical Foundations:
•Use of algebra, number theory, and probability.

43. What is Stream Cipher?
• A stream cipher encrypts data one bit or one byte at a time rather
than in fixed-size blocks.
• It generates a keystream that is combined with the plaintext to the
produce ciphertext.
• Stream ciphers are made for the scenarios where data needs to be
encrypted in the continuous stream making them suitable for the
real-time applications.

45. Key Features of Stream Ciphers
• Continuous Encryption: The data is encrypted in a stream that runs
continuously, a bit or byte at a time
• Keystream Generation: To create encryption keys, the Stream
ciphers use a pseudorandom keystream generator.
• Efficiency: Stream ciphers are generally more efficient for
encrypting data of variable length and in the streaming applications.
• Examples: RC4, Salsa20, and ChaCha20.

46. What is Block Cipher?
• A block cipher encrypts data in fixed-size blocks usually 64 or 128 bits at a time.
• The encryption algorithm processes each block of data separately using
the cryptographic key to transform the plaintext into the ciphertext.
• Block ciphers function on complex mathematical computation and permutation to
ensure that the data encrypted is safe.
• The choice of block size does not directly affect the strength of the encryption scheme.
• The strength of the cipher depends upon the key length. However, any size of the block
is acceptable.
• The following aspects can be kept in mind while selecting the size of a block: Avoid very
small block sizes, Do not have very large block sizes, and Multiples of 8-bit.

48. Key Features of Block Ciphers
• Fixed Block Size: The Data is encrypted in a fixed-size block.
• Complex Operations: In block
ciphers, substitution combined with permutation forms the operatio
n to achieve encryption.
• Modes of Operation: Block ciphers employ several modes such
as ECB (Electronic Codebook) and CBC (Cipher Block
Chaining) for enhanced security.

49. Block Cipher Stream Cipher
Block Cipher Converts the plain text into
cipher text by taking plain text’s block at a
time.
Stream Cipher Converts the plain text into
cipher text by taking 1 bit plain text at a
time.
Block cipher uses either 64 bits or more
than 64 bits.
While stream cipher uses 8 bits.
The complexity of block cipher is simple. While stream cipher is more complex.
Block cipher is slow as compared to a
stream cipher.
While stream cipher is fast in comparison
to block cipher.
More secure than stream ciphers when the
same key is used multiple times.
Less secure than block ciphers when the
same key is used multiple times.
Operates on fixed-length blocks of data. Encrypts data one bit at a time.