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SUMMARY:Defragmentation of compressed bitstreams for H.264/AVC video evide
 nce recovery - Andrew  Lewis (University of Cambridge)
DTSTART:20110318T160000Z
DTEND:20110318T163000Z
UID:TALK30306@talks.cam.ac.uk
CONTACT:Markus Kuhn
DESCRIPTION:The latest generation of video cameras use the H.264/AVC compr
 ession format to store video data efficiently. Because the files are usual
 ly larger than a single block of storage\, they may be split up into many 
 fragments when they are written to disk or memory. Fragmentation can occur
  when no sufficiently long runs of contiguous blocks are available\, and a
 lso in flash memory devices which try to avoid wearing out individual bloc
 ks by distributing writes.\n\nFragmentation is a problem for forensic inve
 stigators whenever block allocation information is inconvenient to access 
 or completely absent. Files might be deleted but intact in unallocated spa
 ce\, or fragmented in flash memories with inaccessible wear-levelling meta
 data. Compressed bitstreams are particularly difficult to defragment based
  on content alone because they are produced by algorithms which try to rem
 ove redundancy.\n\nIn this talk\, I present a general-purpose algorithm fo
 r location and defragmentation of compressed bitstreams with arbitrarily p
 ermuted blocks\, without relying on detailed information about multimedia 
 containers or filesystem layout. The algorithm uses an efficient syntax-ch
 ecking parser as part of a specialized search algorithm\, taking advantage
  of remaining redundancy in compressed bitstreams due to restrictions on s
 yntax.\n\nI demonstrate the algorithm on Baseline profile H.264/AVC bitstr
 eams\, and show that it can locate and defragment video files from a varie
 ty of video sources\, memories and block sizes.\n
LOCATION:Computer Laboratory\, William Gates Building\, Room FW11
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