Editing Advanced Encryption Standard (section)

=== Side-channel attacks ===
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[[Side-channel attack]]s do not attack the cipher as a [[black box]], and thus are not related to cipher security as defined in the classical context, but are important in practice. They attack implementations of the cipher on hardware or software systems that inadvertently leak data. There are several such known attacks on various implementations of AES.

In April 2005, [[Daniel J. Bernstein|D.&nbsp;J. Bernstein]] announced a cache-timing attack that he used to break a custom server that used [[OpenSSL]]'s AES encryption.<ref name="bernstein_timing">{{cite web |url=http://cr.yp.to/papers.html#cachetiming |title=Index of formal scientific papers |publisher=Cr.yp.to |access-date=2008-11-02 |url-status=live |archive-url=https://web.archive.org/web/20080917042758/http://cr.yp.to/papers.html#cachetiming |archive-date=2008-09-17}}</ref> The attack required over 200 million chosen plaintexts.<ref>{{cite web |url=http://www.schneier.com/blog/archives/2005/05/aes_timing_atta_1.html |title=AES Timing Attack |author=Bruce Schneier |date=17 May 2005 |access-date=2007-03-17 |archive-url=https://web.archive.org/web/20070212015727/http://www.schneier.com/blog/archives/2005/05/aes_timing_atta_1.html |archive-date=12 February 2007 |url-status=live}}</ref> The custom server was designed to give out as much timing information as possible (the server reports back the number of machine cycles taken by the encryption operation). However, as Bernstein pointed out, "reducing the precision of the server's timestamps, or eliminating them from the server's responses, does not stop the attack: the client simply uses round-trip timings based on its local clock, and compensates for the increased noise by averaging over a larger number of samples."<ref name="bernstein_timing" />

In October 2005, Dag Arne Osvik, [[Adi Shamir]] and [[Eran Tromer]] presented a paper demonstrating several cache-timing attacks against the implementations in AES found in OpenSSL and Linux's <code>dm-crypt</code> partition encryption function.<ref>{{cite book |chapter-url=http://www.wisdom.weizmann.ac.il/~tromer/papers/cache.pdf |title=The Cryptographer's Track at RSA Conference 2006 |chapter=Cache Attacks and Countermeasures: the Case of AES |date=2005-11-20 |author=Dag Arne Osvik |author2=Adi Shamir |author3=Eran Tromer |series=Lecture Notes in Computer Science |volume=3860 |pages=1–20 |access-date=2008-11-02 |doi=10.1007/11605805_1 |isbn=978-3-540-31033-4 |url-status=live |archive-url=https://web.archive.org/web/20060619221046/http://www.wisdom.weizmann.ac.il/%7Etromer/papers/cache.pdf |archive-date=2006-06-19}}</ref> One attack was able to obtain an entire AES key after only 800&nbsp;operations triggering encryptions, in a total of 65&nbsp;milliseconds. This attack requires the attacker to be able to run programs on the same system or platform that is performing AES.

In December 2009 an attack on some hardware implementations was published that used [[differential fault analysis]] and allows recovery of a key with a complexity of 2<sup>32</sup>.<ref>{{cite journal |url=http://eprint.iacr.org/2009/581.pdf |title=A Diagonal Fault Attack on the Advanced Encryption Standard |author=Dhiman Saha |author2=Debdeep Mukhopadhyay |author3=Dipanwita RoyChowdhury|author3-link=Dipanwita Roy Chowdhury |access-date=2009-12-08 |journal=IACR Cryptology ePrint Archive |archive-url=https://web.archive.org/web/20091222070135/http://eprint.iacr.org/2009/581.pdf |archive-date=22 December 2009 |url-status=live}}</ref>

In November 2010 Endre Bangerter, David Gullasch and Stephan Krenn published a paper which described a practical approach to a "near real time" recovery of secret keys from AES-128 without the need for either cipher text or plaintext. The approach also works on AES-128 implementations that use compression tables, such as OpenSSL.<ref>{{cite journal |url=http://eprint.iacr.org/2010/594.pdf |title=Cache Games – Bringing Access-Based Cache Attacks on AES to Practice |author=Endre Bangerter |author2=David Gullasch |author3=Stephan Krenn |name-list-style=amp |date=2010 |journal=IACR Cryptology ePrint Archive |url-status=live |archive-url=https://web.archive.org/web/20101214092512/http://eprint.iacr.org/2010/594.pdf |archive-date=2010-12-14}}</ref> Like some earlier attacks, this one requires the ability to run unprivileged code on the system performing the AES encryption, which may be achieved by malware infection far more easily than commandeering the root account.<ref>{{cite web |url=http://news.ycombinator.com/item?id=1937902 |title=Breaking AES-128 in realtime, no ciphertext required |publisher=Hacker News |access-date=2012-12-23 |url-status=live |archive-url=https://web.archive.org/web/20111003193004/http://news.ycombinator.com/item?id=1937902 |archive-date=2011-10-03}}</ref>

In March 2016, C. Ashokkumar, Ravi Prakash Giri and Bernard Menezes presented a side-channel attack on AES implementations that can recover the complete 128-bit AES key in just 6–7 blocks of plaintext/ciphertext, which is a substantial improvement over previous works that require between 100 and a million encryptions.<ref>{{Cite conference |date=12 May 2016 |title=Highly Efficient Algorithms for AES Key Retrieval in Cache Access Attacks |conference=2016 IEEE European Symposium on Security and Privacy (EuroS&P) |last1=Ashokkumar |first1=C. |pages=261–275 |last2=Giri |first2=Ravi Prakash |last3=Menezes |first3=Bernard |location=Saarbruecken, Germany |doi=10.1109/EuroSP.2016.29}}</ref> The proposed attack requires standard user privilege and key-retrieval algorithms run under a minute.

Many modern CPUs have built-in [[AES instruction set|hardware instructions for AES]], which protect against timing-related side-channel attacks.<ref>{{cite conference |last1=Mowery |first1=Keaton |last2=Keelveedhi |first2=Sriram |last3=Shacham |first3=Hovav |conference=CCS'12: the ACM Conference on Computer and Communications Security |date=19 October 2012 |location=Raleigh, North Carolina, USA |pages=19–24 |title=Are AES x86 cache timing attacks still feasible? |url=https://cseweb.ucsd.edu/~kmowery/papers/aes-cache-timing.pdf |archive-url=https://web.archive.org/web/20170809152309/http://cseweb.ucsd.edu/~kmowery/papers/aes-cache-timing.pdf |archive-date=2017-08-09 |doi=10.1145/2381913.2381917}}</ref><ref>{{cite web |url=https://www.intel.in/content/dam/doc/white-paper/enterprise-security-aes-ni-white-paper.pdf |title=Securing the Enterprise with Intel AES-NI |access-date=2017-07-26 |url-status=live |archive-url=https://web.archive.org/web/20130331041411/http://www.intel.in/content/dam/doc/white-paper/enterprise-security-aes-ni-white-paper.pdf |archive-date=2013-03-31 |website=[[Intel Corporation]]}}</ref>