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Block Cipher
Processes messages in blocks, each of which is encrypted/decrypted
Many current ciphers are block ciphers
They have a broader range of applications
Block Ciphers look like an extremely large substitution
LENGTH: 64-BITS, 128-BITS
Stream Cipher
Processes messages a bit or byte at a time when encrypting/decrypting
LENGTH: BIT OR BYTE
Stream cipher is faster than Block Cipher
When an encryption algorithm takes the input bit by bit or byte by byte, this is called ……… ?
A Stream Cipher
A Comparison between Stream and Block Cipher
Memorize the photo attached
Resource @Neso Academy on YT
Video:
Block Cipher Principles
Most symmetric block ciphers are based on Feistel Cipher Structure
Needed to decrypt cipher text to recover messages efficiently
What does an Ideal Block Cipher look like?
It takes a 4-bit input and results in a 4-bit output
An ideal block cipher is a cryptographic algorithm that encrypts fixed-size blocks of plaintext into ciphertext using a key.
It should be resistant to attacks like differential and linear cryptanalysis.
Claude Shannon
He introduced the idea of substitution-permutation (S-P) networks
Substitution-Permutation(S-P) Networks
S-P nets are based on 2 primitive cryptographic operations
The two operations are: (S-box) for Substitution and (P-box) for Permutation
They provide Confusion and Diffusion of message & key
Diffusion
Dissipates statistical structure of plaintext over the bulk of cipher text
1-bit change in the plain text (PT) leads to a significant effect on the cipher text (CT) and that’s Diffusion
EXAMPLE: Transposition OR Permutation
Confusion
Makes relationship between cipher text and key as complex as possible
EXAMPLE: Substitution
Combining (S&P) elements results in obtaining:
Diffusion and Confusion
Data Encryption Standard (DES)
The most widely used block cipher in the world
Encrypts 64-bit data using 56-bit key
Has controversy over its security
DES exhibits a strong Avalanche Effect
DES Decryption
With Feistel Design, the same encryption steps but using SUB KEYS in REVERSE ORDER
Differential Cryptanalysis
Powerful method to analyze block ciphers
Used to analyze most current block ciphers with varying degrees of success
Linear Cryptanalysis
Must be iterated over rounds, with decreasing probabilities
Based on finding linear approximations
What does a Timing Attack exploit?
A timing attack exploits the fact that an encryption or decryption algorithm often takes slightly different amounts of time on different inputs.
REVIEW QUESTIONS FROM THE BOOK! (THEORY)
Question 1:
What is the difference between a stream cipher and a block cipher?
Answer:
A stream cipher processes messages a bit or byte at a time while encrypting/decrypting.
A block cipher processes messages in blocks, each of which is encrypted/decrypted
REVIEW QUESTIONS FROM THE BOOK! (THEORY)
Question 2:
Briefly define the terms Substitution and Permutation.
Answer:
Substitution is a cryptographic operation that replaces one element with another based on a predefined rule or mapping.
Permutation rearranges the order or position of elements in a sequence or block.
REVIEW QUESTIONS FROM THE BOOK! (THEORY)
Question 3:
What is the strict avalanche criterion for the Feistel F function?
Answer:
The strict avalanche criterion refers to the property of a Feistel F function, where a small change in input results in a significant and unpredictable change in output, exhibiting STRONG DIFFUSION.
REVIEW QUESTIONS FROM THE BOOK! (THEORY)
Question 4:
Which parameters and design choices determine the actual algorithm of a Feistel cipher?
Answer:
The actual algorithm of a Feistel cipher is determined by choices such as:
1- The number of rounds
2- The size of the blocks
3- The key schedule
4- The round function
5- The specific key used
REVIEW QUESTIONS FROM THE BOOK! (THEORY)
Question 5:
What are the critical aspects of Feistel cipher design?
Answer:
Choice of the round function
The number of rounds for sufficient security
The key management
The overall balance between confusion and diffusion properties
MCQ QUESTIONS ONLINE LINKS
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