New branch prediction vulnerabilities in OpenSSL and necessary software countermeasures

Onur Aciiçmez, Shay Gueron, Jean Pierre Seifert

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review


Software based side-channel attacks allow an unprivileged spy process to extract secret information from a victim (cryptosystem) process by exploiting some indirect leakage of "side-channel" information. It has been realized that some components of modern computer microarchitectures leak certain sidechannel information and can create unforeseen security risks. An example of such MicroArchitectural Side-Channel Analysis is the Cache Attack -a group of attacks that exploit information leaks from cache latencies [4,7,13,15,18]. Public awareness of Cache Attack vulnerabilities lead software writers of OpenSSL (version 0.9.8a and subsequent versions) to incorporate countermeasures for preventing these attacks. In this paper, we present a new and yet unforeseen side channel attack that is enabled by the recently published Simple Branch Prediction Analysis (SBPA) which is another type of MicroArchitectural Analysis, cf. [2,3]. We show that modular inversion -a critical primitive in public key cryptography -is a natural target of SBPA attacks because it typically uses the Binary Extended Euclidean algorithm whose nature is an input-centric sequence of conditional branches. Our results show that SBPA can be used to extract secret parameters during the execution of the Binary Extended Euclidean algorithm. This poses a new potential risk to crypto-applications such as OpenSSL, which already employs Cache Attack countermeasures. Thus, it is necessary to develop new software mitigation techniques for BPA and incorporate them with cache analysis countermeasures in security applications. To mitigate this new risk in full generality, we apply a security-aware algorithm design methodology and propose some changes to the CRT-RSA algorithm flow. These changes either avoid some of the steps that require modular inversion, or remove the critical information leak from this procedure. In addition, we also show by example that, independently of the required changes in the algorithms, careful software analysis is also required in order to assure that the software implementation does not inadvertently introduce branches that may expose the application to SBPA attacks. These offer several simple ways for modifying OpenSSL in order to mitigate Branch Prediction Attacks.

Original languageEnglish
Title of host publicationCryptography and Coding - 11th IMA International Conference, Proceedings
PublisherSpringer Verlag
Number of pages19
ISBN (Print)9783540772712
StatePublished - 2007
Event11th IMA Conference on Cryptography and Coding - Cirencester, United Kingdom
Duration: 18 Dec 200720 Dec 2007

Publication series

NameLecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics)
Volume4887 LNCS
ISSN (Print)0302-9743
ISSN (Electronic)1611-3349


Conference11th IMA Conference on Cryptography and Coding
Country/TerritoryUnited Kingdom


  • Binary extended euclidean algorithm
  • Branch prediction attacks
  • CRT
  • Cache eviction attacks
  • Modular inversion
  • OpenSSL
  • RSA
  • Side channel attacks
  • Software mitigation methods

ASJC Scopus subject areas

  • Theoretical Computer Science
  • General Computer Science


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