CSCE 715: Network Systems Security

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CSCE 715Network Systems Security

• Chin-Tser Huang
• huangct_at_cse.sc.edu
• University of South Carolina

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AES Cipher Rijndael

• Process data as 4 groups of 4 bytes (State)
• Has 9/11/13 rounds in which state undergoes
• byte substitution (1 S-box used on every byte)
• shift rows (permute bytes between groups/columns)
  
• mix columns (subs using matrix multiply of
  groups)
• add round key (XOR state with key material)
• Initial XOR key material incomplete last round
• All operations can be combined into XOR and table
  lookups, hence very fast and efficient

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AES Encryption and Decryption
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AES Data Structure
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AES Encryption Round
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Byte Substitution

• A simple substitution of each byte
• Uses one table of 16x16 bytes containing a
  permutation of all 256 8-bit values
• Each byte of state is replaced by byte in
  corresponding row (left 4 bits) and column (right
  4 bits)
• eg. byte 95 is replaced by row 9 col 5 byte,
  which is 2A
• S-box is constructed using a defined
  transformation of the values in GF(28)

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Shift Rows

• Circular byte shift in each row
• 1st row is unchanged
• 2nd row does 1 byte circular shift to left
• 3rd row does 2 byte circular shift to left
• 4th row does 3 byte circular shift to left
• Decryption does shifts to right
• Since state is processed by columns, this step
  permutes bytes between the columns

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Mix Columns

• Each column is processed separately
• Each byte is replaced by a value dependent on all
  4 bytes in the column
• Effectively a matrix multiplication in GF(28)
  using prime poly m(x) x8x4x3x1

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Add Round Key

• XOR state with 128 bits of the round key
• Again processed by column (though effectively a
  series of byte operations)
• Inverse for decryption is identical since XOR is
  own inverse, just with correct round key
• Designed to be as simple as possible

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AES Key Expansion

• Take 128-bit (16-byte) key and expand into array
  of 44/52/60 32-bit words
• Start by copying key into first 4 words
• Then loop creating words that depend on values in
  previous and 4 places back
• in 3 of 4 cases just XOR these together
• every 4th has S-box rotate XOR constant of
  previous before XOR together
• Designed to resist known attacks

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AES Decryption

• AES decryption is not identical to encryption
  because steps are done in reverse
• But can define an equivalent inverse cipher with
  steps as for encryption
• but using inverses of each step
• with a different key schedule
• Works since result is unchanged when
• swap byte substitution shift rows
• swap mix columns and add (tweaked) round key

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Implementation Aspects

• Can efficiently implement on 8-bit CPU
• byte substitution works on bytes using a table of
  256 entries
• shift rows is simple byte shifting
• add round key works on byte XORs
• mix columns requires matrix multiply in GF(28)
  which works on byte values, can be simplified to
  use a table lookup

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Implementation Aspects

• Can efficiently implement on 32-bit CPU
• redefine steps to use 32-bit words
• can precompute 4 tables of 256-words
• then each column in each round can be computed
  using 4 table lookups 4 XORs
• at a cost of 16Kb to store tables
• Designers believe this very efficient
  implementation was a key factor in its selection
  as the AES cipher

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Points of Vulnerability

• Adversary can eavesdrop from a machine on the
  same LAN
• Adversary can eavesdrop by dialing into
  communication server
• Adversary can eavesdrop by gaining physical
  control of part of external links
• twisted pair, coaxial cable, or optical fiber
• radio or satellite links

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Placement of Symmetric Encryption

• Two major placement alternatives
• Link encryption
• encryption occurs independently on every link
• implies must decrypt traffic between links
• requires many devices, but paired keys
• End-to-end encryption
• encryption occurs between original source and
  final destination
• need devices at each end with shared keys

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Characteristics ofLink and End-to-End Encryption
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Placement of Encryption

• Can place encryption function at various layers
  in OSI Reference Model
• link encryption occurs at layers 1 or 2
• end-to-end can occur at layers 3, 4, 6, 7
• If move encryption toward higher layer
• less information is encrypted but is more secure
• application layer encryption is more complex,
  with more entities and need more keys

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Scope of Encryption
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Traffic Analysis

• When using end-to-end encryption, must leave
  headers in clear so network can correctly route
  information
• Hence although contents are protected, traffic
  patterns are not protected
• Ideally both are desired
• end-to-end protects data contents over entire
  path and provides authentication
• link protects traffic flows from monitoring

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Key Distribution

• Symmetric schemes require both parties to share a
  common secret key
• Need to securely distribute this key
• If key is compromised during distribution, all
  communications between two parties are
  compromised

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Key Distribution Schemes

• Various key distribution schemes for two parties
• A can select key and physically deliver to B
• third party C can select and deliver key to A and
  B
• if A and B have shared a key previously, can use
  previous key to encrypt a new key
• if A and B have secure communications with third
  party C, C can relay key between A and B

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Key Distribution Scenario
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Key Distribution Issues

• Hierarchies of KDCs are required for large
  networks, but must trust each other
• Session key lifetimes should be limited for
  greater security
• Use of automatic key distribution on behalf of
  users, but must trust system
• Use of decentralized key distribution
• Controlling purposes keys are used for

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Summary of Symmetric Encryption

• Traditional symmetric cryptography uses one key
  shared by both sender and receiver
• If this key is disclosed, communications are
  compromised
• Symmetric because parties are equal
• Provide confidentiality, but does not provide
  non-repudiation

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Next Class

• Asymmetric encryption RSA
• Key management with asymmetric encryption
• Read Chapter 9, 10