Image/Data Encryption

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• Image/Data Encryption

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Security Requirements

• Confidentiality Data can be read by only
  authorized parties
• Integrity Data can be modified only by
  authorized parties
• Availability Data are available to authorized
  parties

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Passive Attacks

• Eavesdropping on transmissions
• To obtain information
• Two types
• Release of message contents
• Outsider learns content of transmission
• Traffic analysis
• By monitoring frequency and length of messages,
  the nature of even encrypted communication may be
  guessed
• Difficult to detect but can be prevented

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Active Attacks

• Masquerade Pretending to be a different entity
• Replay passive capture of data and subsequent
  retransmission
• Modification of messages
• Denial of service
• Easy to detect
• Detection may lead to deterrent
• Hard to prevent

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Symmetric Encryption

• Also known as Conventional encryption

Fig. 21.1
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Ingredients

• Plain text
• Input data that is to be encrypted
• Encryption algorithm
• Secret key
• Cipher text
• Encrypted data
• Decryption algorithm

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Requirements for Security

• Strong encryption algorithm
• Opponent should not able to decrypt cipher text
  or work out the key, even if
• algorithm is known, and/or
• number of cipher texts are available together
  with their plain texts
• Sender and receiver must obtain secret key
  securely
• Once key is known, all communication using this
  key is readable

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Encryption Algorithms

• Block ciphers are most commonly used algorithms
• Process plain text in fixed block sizes producing
  block of cipher text of equal size
• Data encryption algorithm (DEA) used in DES
• Triple DEA (TDEA)
• Advanced encryption algorithm (AEA)

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Data Encryption Standard (DES)

• US standard (developed by NIST) adopted in 1997
• 64 bit plain text blocks
• 56 bit key
• Broken in 1998 by Electronic Frontier Foundation
• Special purpose machine
• Less than three days
• DES now worthless

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Triple DES

• Based on TDEA, ANSI X9.17 (1985)
• Incorporated in DES standard in 1999
• Uses 3 keys and 3 executions of DEA algorithm
• C Ek3Dk2Ek1P
• C is cipher text, P is plain text, EKX is
  encryption of X using key K DKY is decryption
  of Y using key K
• Effective key length 112 or 168 bit
• Slow
• Block size (64 bit) too small

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Advanced Encryption Standard

• AES issued as federal information processing
  standard (FIPS 197) in 2001
• Block length 128 bits
• Key lengths 128, 192, and 256 bits
• Security strength 3DES
• Improved efficiency
• Symmetric block cipher

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Message Authentication

• Protection against active attacks
• Falsification of data
• Authentication allows receiver to verify that
  message is authentic
• Message has not been altered
• Message is from authentic source
• Message timeline

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Message Authentication Using Message
Authentication Code
Secret key
Fig. 21.6
Secret key
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One Way Hash Function

• Accepts variable size message and produces fixed
  size tag (message digest)
• Same as MAC
• Unlike MAC, hash function does not require a
  secret key
• Message is authenticated if message digest is
  authentic

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Using One Way Hash
Fig. 21.7
Used for IP security
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Public Key Encryption

• Based on mathematical algorithms
• Asymmetric
• Use two separate keys
• Ingredients
• Plain text
• Encryption algorithm
• Public and private key
• Cipher text
• Decryption algorithm

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Public Key Encryption -Encryption
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Public Key Encryption Authentication
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Public Key Encryption - Operation

• One key made public
• Used for encryption
• Other kept private
• Used for decryption
• Infeasible to determine decryption key given
  encryption key and algorithm
• Either key can be used for encryption, the other
  for decryption

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Steps

• User generates pair of keys
• User places one key in public domain
• To send a message to user, encrypt using public
  key
• User decrypts using private key

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Digital Signature

• Sender encrypts message with their private key
• Receiver can decrypt using senders public key
• This authenticates sender, who is only person who
  has the matching key
• Does not give privacy of data
• Decrypt key is public