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{{Short description|Type of in security engineering}}
In the field of [[security engineering]], an '''oracle attack''' is an attack that exploits the availability of a weakness in a system that can be used as an "[[test oracle|oracle]]" to give a simple go/no go indication to inform attackers how close they are to their goals. The attacker can then combine the oracle with a systematic search of the problem space to complete their attack.<ref>{{cite web|url=http://www.drdobbs.com/understanding-oracle-attacks-on-informat/184405917|title=Understanding Oracle Attacks|publisher=Dr. Dobbs'|author=Jason Coombs|date=2 December 2004|accessdate=2013-10-04}}</ref>

The [[padding oracle attack]], and compression oracle attacks such as [[BREACH]], are examples of oracle attacks, as was the practice of "crib-dragging" in the [[cryptanalysis]] of the [[Enigma machine]]. An oracle need not be 100% accurate: even a small statistical correlation with the correct go/no go result can frequently be enough for a systematic automated attack.

In a '''compression oracle attack''' the use of adaptive [[data compression]] on a mixture of [[chosen plaintext]] and unknown plaintext can result in content-sensitive changes in the length of the compressed text that can be detected even though the content of the compressed text itself is then encrypted. This can be used in protocol attacks to detect when the injected known plaintext is even partially similar to the unknown content of a secret part of the message, greatly reducing the complexity of a search for a match for the secret text. The [[CRIME]] and [[BREACH (security exploit)|BREACH]] attacks are examples of protocol attacks using this phenomenon.

== See also ==
* [[Side-channel attack]]

== References ==
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[[Category:Security engineering]]