Introduction to Symmetric Block Cipher
Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
seminar class
Active In SP

Posts: 5,361
Joined: Feb 2011
16-02-2011, 09:33 AM

.ppt   7000_1.ppt (Size: 344.5 KB / Downloads: 262)
Introduction to Symmetric Block Cipher
• Encryption algorithm also called a cipher
• Cryptography has evolved so that modern encryption and decryption use secret keys
• Cryptographic algorithms can be openly published
• Only have to protect the keys
Symmetric-Key Cryptography
• Both sender and receiver keys are the same: KA=KB
• The keys must be kept secret and securely distributed
• Thus, also called “Secret Key Cryptography”
• Data Encryption Standard (DES)
• Brute force: try every key
• Ciphertext-only attack:
• Attacker knows ciphertext of several messages encrypted with same key (but doesn’t know plaintext).
• Possible to recover plaintext (also possible to deduce key) by looking at frequency of ciphertext letters
• Known-plaintext attack:
• Attackers observes pairs of plaintext/ciphertext encrypted with same key.
• Possible to deduce key and/or devise algorithm to decrypt ciphertext.
Cryptanalysis (2)
• Chosen-plaintext attack:
• Attacker can choose the plaintext and look at the paired ciphertext
• Attacker has more control than known-plaintext attack and may be able to gain more info about key
• Adaptive Chosen-Plaintext attack:
• Attacker chooses a series of plaintexts, basing the next plaintext on the result of previous encryption
• Examples
• Differential cryptanalysis – DES is resistant it
• Linear cryptanalysis
• Cryptanalysis attacks often exploit the redundancy of natural language
• Lossless compression before encryption removes redundancy
• Simple and non-secure ciphers
– Shift Cipher – Caesar Cipher
– Affine Cipher
– Vigenere Cipher
– Hill Cipher
• Information-secure cipher
– One-Time Pad
Confusion and Diffusion
• Terms courtesy of Claude Shannon, father of Information Theory
• “Confusion” = Substitution
• a -> b
• Caesar cipher
• “Diffusion” = Transposition or Permutation
• abcd -> dacb
Confusion and Diffusion (2)
Confusion” : a classical Substitution Cipher
• Modern substitution ciphers take in N bits and substitute N bits using lookup table: called S-Boxes
Confusion and Diffusion (3)
Diffusion” : a classical Transposition cipher
• modern Transposition ciphers take in N bits and permute using lookup table : called P-Boxes
Block Cipher
• Divide input bit stream into n-bit sections, encrypt only that section, no dependency/history between sections
In a good block cipher, each output bit is a function of all n input bits and all k key bits
Example: DES
• Data Encryption Standard (DES)
• Encodes plaintext in 64-bit chunks using a 64-bit key (56 bits + 8 bits parity)
• Uses a combination of diffusion and confusion to achieve security
• Was cracked in 1997
• Parallel attack – exhaustively search key space
• Decryption in DES – it’s symmetric! Use KA again as input and then the same keys except in reverse order
Example: DES (2)
• 64-bit input is permuted
• 16 stages of identical operation
• differ in the 48-bit key extracted from 56-bit key - complex
• R2= R1 is encrypted with K1 and XOR’d with L1
• L2=R1, …
• Final inverse permutation stage
Beyond DES
• Triple-DES: put the output of DES back as input into DES again with a different key, loop again: 3*56 = 168 bit key
• Advanced Encryption Standard (AES)
– Requirements:
• shall be designed so that the key length may be increased as needed.
• block size n = 128 bits, key size k = 128, 192, 256 bits
– Candidates: MARS, twofish, RC6, Serpent, Rijndael
– successor (Rijndael)
Encryption Mode (ECB)
• Electronic Code Book (ECB) mode for block ciphers of a long digital sequence
• Vulnerable to replay attacks: if an attacker thinks block C2 corresponds to $ amount, then substitute another Ck
• Attacker can also build a codebook of <Ck, guessed Pk> pairs
Encryption Mode (CBC)
• Cipher Block Chaining (CBC) mode for block ciphers
• Inhibits replay attacks and codebook building: identical input plaintext Pi =Pk won’t result in same output code due to memory-based chaining
• IV = Initialization Vector – use only once
Stream Cipher
• Stream ciphers
• Rather than divide bit stream into discrete blocks, as block ciphers do, XOR each bit of your plaintext continuous stream with a bit from a pseudo-random sequence
• At receiver, use same symmetric key, XOR again to extract plaintext
Encryption Mode (OFB)


Important Note..!

If you are not satisfied with above reply ,..Please


So that we will collect data for you and will made reply to the request....OR try below "QUICK REPLY" box to add a reply to this page

Quick Reply
Type your reply to this message here.

Image Verification
Please enter the text contained within the image into the text box below it. This process is used to prevent automated spam bots.
Image Verification
(case insensitive)

Possibly Related Threads...
Thread Author Replies Views Last Post
  Introduction to Cloud Computing project uploader 4 4,678 22-03-2014, 05:27 PM
Last Post: navasfiroz
  Introduction to Multimedia Systems seminar projects maker 0 501 28-09-2013, 01:15 PM
Last Post: seminar projects maker
  AN INTRODUCTION TO TCP/IP REPORT seminar projects maker 0 451 26-09-2013, 04:31 PM
Last Post: seminar projects maker
  Introduction to Functions PPT seminar projects maker 0 380 25-09-2013, 12:45 PM
Last Post: seminar projects maker
  Open Core Protocol ( OCP ) An Introduction to Interface Specification seminar projects maker 0 536 24-09-2013, 12:29 PM
Last Post: seminar projects maker
  Tutorial On Introduction to 8085 Architecture and Programming seminar projects maker 0 390 20-09-2013, 03:52 PM
Last Post: seminar projects maker
  Introduction to Image Processing PPT seminar projects maker 0 512 13-09-2013, 04:56 PM
Last Post: seminar projects maker
  Introduction to Expert Systems PPT project girl 1 677 12-09-2013, 04:01 PM
Last Post: seminar projects maker
  INTRODUCTION TO DATABASE MANAGEMENT SYSTEMS seminar paper 3 1,558 12-09-2013, 09:39 AM
Last Post: seminar projects maker
  Introduction to Web Services PPT seminar projects maker 0 549 11-09-2013, 03:46 PM
Last Post: seminar projects maker