Bioinformatics in Computer Security - PowerPoint PPT Presentation

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Bioinformatics in Computer Security

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Masquerade attack - unauthorized user gains access to information ... Sequence alignment allows technology to compare monitored sessions with past user behavior ... – PowerPoint PPT presentation

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Title: Bioinformatics in Computer Security


1
Bioinformatics in Computer Security
  • Sequence Alignment for Masquerade Detection by
    Scott Coull and Boleslaw Szymanski
  • Paul Casey
  • CS 329E
  • April 22, 2008

2
What is Masquerade Detection?
  • Masquerade attack - unauthorized user gains
    access to information resources by assuming the
    identity of legitimate users
  • A masquerade attack can neutralize expensive
    security technologies
  • Sequence alignment allows technology to compare
    monitored sessions with past user behavior

3
Smith-Waterman Algorithm
  • Local alignment places the focus on finding the
    best aligned substrings of two sequences over all
    possible substrings
  • Good for strings where one string is longer than
    the other
  • Useful when searching for areas of functional
    similarity
  • Global alignment attempts to maximize the length
    of the subsequence over the entire length of both
    strings
  • Good for very similar strings with same length

4
Local and Global Alignment
  • Global alignment produces a matrix of distance or
    cost scores smaller is better
  • S1 GCCCTAGCG
  • S2 GCGC_AATG
  • Local alignment produces a matrix of similarity
    scores bigger is better
  • S1 GCG
  • S2 GCG
  • Note Any value reaches negative is reset to zero
    allows algorithm to ignore prefixes
  • Initialization begins with zeroes in first row
    and column
  • Can begin traceback at the maximum score rather
    than bottom right corner

5
Detecting Masquerade Attacks
  • Two strings in masquerade detection
  • Normal usage Signature a1a2am
  • Current usage Test b1b2bn
  • M should be greater than N to create a useful
    profile of user behavior
  • Align two strings to find areas of similarity,
    assume gaps and mismatches may indicate a
    masquerade attack
  • Scoring to reward alignment of commands in the
    user segment but not penalize the misalignment of
    large portions of the signature

6
Alignment Algorithm
  • Diagonal step indicates an alignment between i-1
    symbol in Signature and j-1 symbol in Test.
    Alignment score in (i-1,j-1).
  • Vertical step indicates a gap in Signature
    aligned with j-1 symbol in Test. Gap penalty in
    (i, j-1).
  • Horizontal step indicates a gap in Test aligned
    with i-1 symbol in Signatures. Gap penalty in
    (i-1,j).
  • 2 for exact matches, 1 for mismatches where
    symbol occurred previously, -1 for other
    mismatches
  • O(mn) expensive
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