These are learning exercises, frequently held as a competition, but also useful as a learning vehicle after the contest is completed.
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The 2012 DC3 Digital Forensics Challenge is based on 23 individual scenario-based, progressive level exercises as a call to the digital forensics community to pioneer new investigative tools, techniques and methodologies.
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This includes challenges going back several years.
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This is an annual challenge, with previous year's challenges located on their website.
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The Honeynet Project regularly runs challenges. This is a list of them.
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This has the scenario, evidence files, and the winning writeups for reference.
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For Blackhat 2012, Defcon 2012 conferences.