Computer and Network Security

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Computer and Network Security

• Secret Key Cryptography
• (Chapter 3 and Chapter 4)

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Generic Block Encryption

• Random mapping from a block of plaintext to a
  block of ciphertext
• when any one bit of plaintext changes, every bit
  in ciphertext has 50 chance to change
• Substitution
• Space complexity O(k 2k)
• Permutation
• Space complexity O(k log k)
• Key

3
Example

• Figure 3-1

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Data Encryption Standard (DES)

• Designed by IBM, published by NIST in 1977
• 56-bit key, mapping a 64-bit input block to a
  64-bit output block
• Not secure any more
• Triple DES, 128-bit key

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DES Overview

• Figure 3-2

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Initial/Final Permutations

• Do not increase security
• Page 66

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Generating Per-Round Keys

• Figure 3-5

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A DES Round

• Figure 3-6

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Mangler Function
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Other Secret Key Ciphers

• IDEA International Data Encryption Algorithm
• 64-bit block 128-bit key
• AES Advanced Encryption Standard
• 128-bit block 128-bit, 192-bit, 256-bit keys

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Overall Structure of AES
Nb 4 words block Nk 4, 6, 8 words key Nr
10, 12, 14 rounds encryption
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Primitive Operation 1 Block Substitution
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Primitive Operation 2 MixColumn
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MixColumn Shifted XOR
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Reverse of Block Substitution
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Reverse of MixColumn
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Key Expansion Set 0
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Key Expansion Set i
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Round Operations

• Block substitution
• Row 1 rotated left by 1 byte
• Row 2 rotated left by 2 bytes
• Row 3 rotated left by 3 bytes
• 3. MixColumn

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Decryption

• Reverse the operations
• Keys
• Block substitution
• MixColumn
• Rotation

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RC4

• One-time pad, generated by a stream cipher, which
  is essentially a random number generator

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RC4
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Exercise

• Use rand() and srand() to write a stream cipher
  that takes a key of arbitrary size and a message
  of arbitrary size as input and produces a
  cipher-text output.

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Encrypting Large Message

• Electronic Code Book (ECB)
• Cipher Block Chaining (CBC)
• k-Bit Cipher Feedback Mode (CFB)
• k-Bit Output Feedback Mode (OFB)
• Counter Mode (CTR)

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Electronic Code Book

• Identical ciphertext blocks for identical message
  blocks
• Predictable outcome by exchanging ciphertextblocks

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Cipher Block Chaining (CBC)

• Purpose of IV
• Predictable outcome by changing a bit in
  ciphertext
• Predictable outcome by rearranging ciphertext

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Output Feedback Mode (OFM)

• One-time pad can be generated in advance.
• A missing or inserted ciphertext block garbles
  everything afterwards.
• No block size.
• Predictable outcome by changing a bit in the
  ciphertext.

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Cipher Feedback Mode

• Resynchronization after missing/extra ciphertext
  blocks
• Predictable outcome after a bit in ciphtertext is
  changed, but more susceptible to detection
• No block size

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Counter Mode (CTR)

• One-time pad can be generated in advance.
• Decryption at any point.

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Integrity Generating MAC

• Integrity protection, error detection, and error
  correction
• CRC and MAC
• The value of MAC is dependent on all message
  blocks and their order.
• How about use CFB to compute the residue.

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Privacy and Integrity

• Use one key to generate CBC residual, and append
  the residual to the message
• Use another key to do CBC encryption over the
  whole thing
• Why cant we use the same key?
• How to protect the integrity for other encryption
  modes?

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Double DES or Triple DES

• M -gt E(k1) -gt E(k2) -gt C
• M -gt E(k1) -gt E(k2) -gt E(k3) -gt C
• M -gt E(k1) -gt D(k2) -gt E(k1) -gt C (encryption)
• C -gt D(k1) -gt E(k2) -gt D(k1) -gt m (decryption)

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Triple DES

• CBC outside v.s. inside
• Unpredictable outcome for one bit change
• No self-synchronization
• Parallel encryption

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More Block-Cipher Modes

• Block Chaining Mode
• A message block XORs with all previous ciphertext
  blocks before encryption
• No self-sychronization
• Predictable outcome after one bit change in
  ciphertext

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More Block-Cipher Modes

• Propagating Cipher Block Chaining Mode
• A message block XORs with the previous message
  block and the previous cipher block before
  encryption.
• Encryption and integrity in one pass
• Integrity check does not detect the swap of two
  ciphertext blocks.

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More Block-Cipher Modes

• Cipher Block Chaining with Checksum
• A message block XORs with all previous message
  blocks and the previous cipher block before
  encryption.
• No predictable outcome from a bit change in
  ciphertext
• Encryption and Integrity check in one pass

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Interleaving

• Divide one stream of message blocks to multiple
  streams with interleaving blocks. This allows for
  parallel encryption and decryption.

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More on Stream Cipher

• Linear Congruential Generators
• Xn (a Xn-1 b) mod m
• maximal period generator, a 84589, b 45989, m
  217728
• Not Secure

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More on Stream Cipher

• Feedback Shift Register
• A shift register and a feedback function

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More on Stream Cipher

• Linear Feedback Shift Register
• The feedback function is simply the XOR of
  certain bits in the register
• Maximal period LFSR, (32, 7, 5, 3, 2, 1, 0)