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Introduction to Symmetric Block Cipher

Introduction to Symmetric Block Cipher. Jing Deng Based on Prof. Rick Han’s Lecture Slides Dr. Andreas Steffen’s Security Tutorial. Encryption. Decryption. Encryption. Decryption. plaintext. ciphertext. plaintext. Key K A. Key K B. Cryptography. plaintext. ciphertext. plaintext.

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Introduction to Symmetric Block Cipher

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  1. Introduction to Symmetric Block Cipher Jing Deng Based on Prof. Rick Han’s Lecture Slides Dr. Andreas Steffen’s Security Tutorial

  2. Encryption Decryption Encryption Decryption plaintext ciphertext plaintext Key KA Key KB Cryptography plaintext ciphertext plaintext • 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

  3. Encryption Decryption plaintext ciphertext plaintext Key KA Key KB=KA Symmetric-Key Cryptography Secure Key Distribution • 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)

  4. Cryptanalysis • 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.

  5. 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

  6. Examples • Simple and non-secure ciphers • Shift Cipher – Caesar Cipher • Affine Cipher • Vigenere Cipher • Hill Cipher • Information-secure cipher • One-Time Pad

  7. Encryption Decryption plaintext ciphertext plaintext Key KA Key KB Confusion and Diffusion • Terms courtesy of Claude Shannon, father of Information Theory • “Confusion” = Substitution • a -> b • Caesar cipher • “Diffusion” = Transposition or Permutation • abcd -> dacb • DES

  8. Confusion and Diffusion (2) • “Confusion” : a classical Substitution Cipher Courtesy: Andreas Steffen • Modern substitution ciphers take in N bits and substitute N bits using lookup table: called S-Boxes

  9. Confusion and Diffusion (3) • “Diffusion” : a classical Transposition cipher Courtesy: Andreas Steffen • modern Transposition ciphers take in N bits and permute using lookup table : called P-Boxes

  10. Block Cipher • Divide input bit stream into n-bit sections, encrypt only that section, no dependency/history between sections Courtesy: Andreas Steffen • In a good block cipher, each output bit is a function of all n input bits and all k key bits

  11. 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

  12. Example: DES (2) • DES • 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

  13. Example: DES (3)

  14. 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)

  15. 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

  16. 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

  17. 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

  18. Encryption Mode (OFB)

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