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Data Compression and Security
Chapter 20,
Exploring the Digital Domain
Digital Compression Concepts
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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
Types of Codecs
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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
Compression Encoding
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Compression is an encoding process that
filters the original file in several successive
stages
Codec Methods
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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
Compressing Text and Numerical Data
Lossless Syntactic Methods
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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)
Compressing Images
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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
Compressing Video
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Video compression employs both spatial
and temporal compression
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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
Compressing Video
Some Additional Methods
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DVI (Digital Video Interactive)
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Motion-JPEG
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MPEG (Motion Pictures Experts Group)
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The px64 Standard
Temporal Compression in Video
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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
Temporal Video Compression (cont’d)
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MPEG and related codecs employ a more complex
frame-referencing method
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intrapictures (I pictures)
predicted pictures (P pictures)
bidirectional pictures (B pictures)
Compressing Audio
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A widely used method is ADPCM (Adaptive
Differential Pulse Code Modulation)
ADPCM
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lossy method
employs a differencing technique related to those used in
video compression
used in DVI
MP3 employs psychoacoustic methods
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filter out parts of the signal most people do not hear
uses methods to measure the amount of quantization that
will just mask noise
Encryption and Data Security
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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
Encryption Keys
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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
Symmetric or Secret Key Ciphers
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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
Asymmetric or Public Key Ciphers
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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)
Asymmetric or Public Key
Ciphers Illustrated
More on Public Key Methods
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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
Authentication
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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:
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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)
Encryption and National Security
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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
Encryption and National Security
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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
Summary
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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
Summary (cont’d)
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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
Summary (cont’d)
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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:
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secret key (symmetric key) ciphers
public key (asymmetric key) ciphers
Summary (cont’d)
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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