Data Compression and Security

1
Data Compression and Security

• Chapter 20,
• Exploring the Digital Domain

2
Digital Compression Concepts

• Compression techniques are used to replace a file
  with another that is smaller
• Decompression techniques expands the compressed
  file to recover original data -- exactly or in
  facsimile
• A pair of compression/decompression techniques
  that work together is called a codec for short

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Types of Codecs

• Codecs that upon decompression always reproduce
  the original file exactly are called lossless
  codecs
• Codecs that reproduce only an approximation of
  the original file upon decompression are called
  lossy codecs
• Codecs that take approximately the same amount of
  time to compress and decompress a file are
  referred to as symmetric codecs
• By contrast, codecs that feature simple fast
  decompression but significantly slower
  compression are called asymmetric codecs

4
Compression Encoding

• Compression is an encoding process that filters
  the original file in several successive stages

5
Codec Methods

• Syntactic encoding (also called entropy encoding)
  methods attempt to reduce the redundancy of
  symbolic patterns in a file without any regard to
  the type of information represented
• Semantic methods consider special properties of
  the type of information represented to reduce
  nonessential information in a file
• Hybrid methods combine both syntactic and
  semantic methods

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Compressing Text and Numerical DataLossless
Syntactic Methods

• Run-Length Encoding (RLE)
• looks for repeated sequences of symbols
• widely used for fax (facsimile) transmissions
• Huffman Codes
• exploits frequency distribution of symbols in a
  source
• adaptive Huffman coding builds its own frequency
  tables rather than use predefined statistics
• Lempel-Ziv-Welch (LZW) compression
• based on recognizing patterns of strings in
  original file
• fast and yields good results (50 typical
  reduction)

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Compressing Images

• GIF (Graphic Interchange Format) Codec
• employs LZW method for lossless compression
• TIFF (Tagged Image File Format) Codec
• lossless syntactic method
• JPEG (Joint Photographics Experts Group) Codec
• umbrella term covering several lossy and lossless
  methods
• baseline method is most commonly used one --
  lossy method based on a hybrid method

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Compressing Video

• Video compression employs both spatial and
  temporal compression
• spatial techniques compress individual frames
• temporal methods compress data in frames over
  time
• QuickTime and AVI (Audio Video Interleaved) are
  two popular (and incompatible with each other)
  formats

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Compressing VideoSome Additional Methods

• DVI (Digital Video Interactive)
• Motion-JPEG
• MPEG (Motion Pictures Experts Group)
• The px64 Standard

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Temporal Compression in Video

• Lossy strategies for eliminating redundancy of
  information between frames employ temporal
  compression -- referred to as interframe
  compression
• Sequence of frames are considered together
• key frames
• difference frames
• Used in QuickTime and DVI

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Temporal Video Compression (contd)

• MPEG and related codecs employ a more complex
  frame-referencing method
• intrapictures (I pictures)
• predicted pictures (P pictures)
• bidirectional pictures (B pictures)

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Compressing Audio

• A widely used method is ADPCM (Adaptive
  Differential Pulse Code Modulation)
• ADPCM
• lossy method
• employs a differencing technique related to those
  used in video compression
• used in DVI
• MP3 employs psychoacoustic methods
• filter out parts of the signal most people do not
  hear
• uses methods to measure the amount of
  quantization that will just mask noise

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Encryption and Data Security

• Cryptography is the art and science of keeping
  message secret
• Encryption techniques convert data into a secret
  code for transmission
• The process of retrieving the original message at
  the receiver is called decryption

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Encryption Keys

• Keys are essential information -- usually a
  numerical parameter(s) -- needed for encryption
  and/or decryption algorithms
• Encryption keys are used to encode plaintext as
  encoded ciphertext
• Decryption keys are used to decode ciphertext and
  recover the original plaintext
• Decryption keys are sometimes discovered by brute
  force methods employing computers to search large
  potential key spaces

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Symmetric or Secret Key Ciphers

• Secret key ciphers use a single secret key (or
  set of keys) for both encryption and decryption
• The secret key must be transferred securely in
  order for secret key methods to be secure
• Data Encryption Standard (DES) is a US government
  sponsored secret key cipher. DES uses a 56-bit
  key.
• International Data Encryption Algorithm (IDEA)
  has been proposed to replace DES. It uses a
  128-bit key.
• Longer keys make it more difficult for brute
  force discovery of the secret key

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Asymmetric or Public Key Ciphers

• The first practical public key algortihm was
  published by Rivest, Shamir, and Adleman in 1976
  and is know as RSA (for their last names)
• Public key ciphers employ an algortihm with two
  keys -- a public key and a private key
• A sender looks up the recipient's public key and
  uses it to encode a message
• The recipient then decodes the message with his
  or her private key (this private key is necessary
  to decode the message)

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Asymmetric or Public Key Ciphers Illustrated
18
More on Public Key Methods

• No attempt is made to keep secret the actual
  encryption and decryption algorithms for public
  key methods -- security depends on only the
  recipient knowing his or her private key
• Public key ciphers are more secure than secret
  key ciphers, but are not as efficient since they
  require longer keys and more computing in the
  encryption and decryption processes
• For sake of efficiency, sometimes secret key
  encryption is used and the secret key is
  communicated employing public key methods -- the
  combination of a secret key encoded message and
  public key encoded value of the secret key is
  called a digital envelope

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Authentication

• The process used to verify the identity of a
  respondent is called authentication
• Authentication is very important for electronic
  commerce and other network transactions
• Authentication exploits the symmetry of public
  and private keys
• To authenticate that a person is who they say
  they are
• send that person a nonsense message and ask them
  to encode it with their private key and return it
  to you
• when the message is returned, if the person is
  who they claim to be, you should be able to
  recover your nonsense message using their public
  key (which presumably you know)

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Encryption and National Security

• An escrowed secret key cipher is a secret key
  cipher in which a trusted third party controls
  the secret key.
• DES is an example of such a cipher, and the US
  government holds the escrowed 56-bit secret key
• The International Trafficking in Arms Regulation
  (ITAR) prohibits the export of secret key cipher
  systems with secret keys longer than 40 bits

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Encryption and National Security

• A major governments can break ciphers with
  40-bit or shorter keys by brute force
• Limiting longer secret key ciphers is an attempt
  to retain the ability to break codes when this is
  deemed necessary for national security
• The ITAR law has been debated for a number of
  years
• Public key ciphers have complicated the debate
  further -- and it continues
• The basic issue is privacy versus the national
  security

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Summary

• Compressing data means reducing the effective
  size of a data file for storage or transmission
• Particular paired compression/decompression
  methods are called codecs
• Codecs that cannot reproduce the original file
  exactly are called lossy methods those that
  reproduce the original exactly are called
  lossless methods
• Text and numbers usually require lossless methods
• Images, video, and sound codecs are usually lossy

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Summary (contd)

• Syntactic methods attempt to reduce the
  redundancy of symbolic patterns in a file without
  any regard to the type of information represented
• Semantic methods exploit characteristics inherent
  in the type of information being represented
• The use of codecs is not an exact science -- the
  effectiveness and suitability of any method will
  depend on the exact nature of the original file
  and the intended use for the compressed file

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Summary (contd)

• With the increasing access to and ease of
  transmitting sensitive and confidential
  information come significant security risks
• Encryption techniques are used to encode messages
  for secure transmission
• The two primary encryption/decryption methods
  are
• secret key (symmetric key) ciphers
• public key (asymmetric key) ciphers

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Summary (contd)

• Public key ciphers are more secure, but secret
  key ciphers are more efficient
• Public key encryption is used for authentication
  over computer networks
• An active national (and international) debate
  continues over government control and regulation
  of encryption/decryption methods