Chp 3 Cryptography Part 1

Q. What is the meaning of cryptography?

It means secret writing, from the Greek words.

“kryptos” (hidden)

“graphia” (writing).

Q. What does cryptography do?

It converts readable text (plaintext) into unreadable text (ciphertext) using an encryption process.

Q. What is the main purpose of cryptography?

To provide confidentiality, integrity, authentication, and non-repudiation in communication.

Q. What is confidentiality in cryptography?

Ensures that only authorized people can read the message.

Q. What is integrity in cryptography?

Ensures that a message is not modified during transmission.

Q. What is authentication in cryptography?

Verifies that the message is from a legitimate sender and is original.

Q. What is non-repudiation?

Prevents the sender from denying having sent the message later.

Q. What is plaintext?

The original readable message before encryption.

Q. What is ciphertext?

The encrypted or unreadable version of a plaintext message.

Q. What is encryption?

The process of converting plaintext into ciphertext.

Q. What is decryption?

The process of converting ciphertext back to plaintext.

Q. What is a key in cryptography?

A value used in encryption and decryption to control how data is transformed.

Q. What is cryptanalysis?

The process of breaking a cipher or finding the plaintext without knowing the key.

Q. What is cryptology?

The study that combines cryptography (creating ciphers) and cryptanalysis (breaking ciphers).

Q. What are the two types of cryptosystems?

Symmetric cryptosystem and asymmetric cryptosystem.

Q. What is a symmetric cryptosystem?

A system that uses the same key for both encryption and decryption.

Q. What is an asymmetric cryptosystem?

A system that uses two different keys:

• encryption

• decryption.

Q. What are the two main cipher types?

• Bit stream cipher

• Block cipher

Q. What is a bit stream cipher?

Encrypts one bit of plaintext at a time into a ciphertext bit.

Q. What is a block cipher?

Divides the message into blocks and encrypts each block using an algorithm and key.

Q. What are the two main classical cryptography techniques?

• Substitution cipher

• Transposition cipher

Q. What is a substitution cipher?

Replaces characters in plaintext with other characters or symbols.

Q. What is a transposition cipher?

Rearranges the order of characters in plaintext without changing them.

Q. Give examples of substitution ciphers.

• Easy substitution

• Homophonic substitution

• Polyalphabetic

• Polygram substitution.

Q. Give examples of transposition ciphers.

• Columnar transposition

• Rail fence

• Vernam cipher

Q. What is a mono-alphabetic substitution cipher?

Each plaintext letter is replaced with one fixed ciphertext letter (e.g., Caesar Cipher).

Q. What is a homophonic substitution cipher?

One plaintext letter can map to multiple ciphertext symbols.

Q. What is a polyalphabetic cipher?

Uses multiple substitution alphabets (e.g., Vigenère Cipher).

Q. What is a polygram cipher?

Encrypts blocks of letters instead of individual letters (e.g., Playfair Cipher).

Q. How does the Caesar Cipher work?

Each letter in the plaintext is shifted by a fixed number of positions (e.g., +3).

Q. What is the encryption and decryption formula for Caesar Cipher?

Encryption: C = (P + K) mod 26

Decryption: P = (C - K) mod 26

Q. How many possible keys does the Caesar Cipher have?

26 possible keys (for 26 letters in the alphabet).

Q. Why is the Caesar Cipher considered weak?

It can easily be broken using brute force or frequency analysis.

Q. What is frequency analysis?

A method that analyzes letter frequency to guess the cipher pattern.

Q. What is a homophonic substitution cipher?

A cipher where one plaintext letter can correspond to several ciphertext letters.

Q. What is a polygram substitution cipher?

A cipher that replaces a group of letters with another group of letters.

Q. Give examples of polygram ciphers.

• Playfair Cipher

• Hill Cipher.

Q. What is the rule for pairs in the same row in Playfair Cipher?

Replace each letter with the one to its right (wrap to the start if needed).

Q. What is the rule for pairs in the same column in Playfair Cipher?

Replace each letter with the one below it (wrap to the top if needed).

Q. What is the rule for pairs forming a rectangle in Playfair Cipher?

Replace each letter with the one in the same row and the other’s column.

Q. What letter is used as a filler in Playfair Cipher when letters repeat?

The letter X.

Q. Give examples of polyalphabetic ciphers.

• Vigenère Cipher

• Beaufort Cipher.

Q. How does the Vigenère Cipher work?

It applies multiple Caesar Ciphers using a repeating key.

Q. Why is the Vigenère Cipher stronger than Caesar Cipher?

It reduces letter frequency patterns, making cryptanalysis harder.

Q. What does a transposition cipher do?

It rearranges letter positions to create ciphertext.

Q. Does transposition change letter frequencies?

No, the frequency remains the same as the plaintext.

Q. What is the Rail Fence Cipher?

A type of transposition cipher that writes text diagonally across rows.

Q. How do you read a Rail Fence Cipher?

After writing diagonally, read each row horizontally to form the ciphertext.

Q. How is Rail Fence Cipher decrypted?

Reconstruct the zigzag pattern and read horizontally.