Logjam (computer security): Difference between revisions

Revision as of 23:14, 6 December 2023

Revision as of 05:46, 16 January 2024

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"The TLS protocol 1.2 and earlier, when a DHE_EXPORT ciphersuite is enabled on a server but not on a client, does not properly convey a DHE_EXPORT choice, which allows man-in-the-middle attackers to conduct cipher-downgrade attacks by rewriting a ClientHello with DHE replaced by DHE_EXPORT and then rewriting a ServerHello with DHE_EXPORT replaced by DHE, aka the 'Logjam' issue."</ref>

The authors also estimated the feasibility of the attack against 1024-bit Diffie–Hellman primes. By design, many Diffie–Hellman implementations use the same pre-generated [[prime number|prime]] for their field. This was considered secure, since the [[discrete loglogarithm problem]] is still considered hard for big enough primes even if the group is known and reused. The researchers calculated the cost of creating logjam precomputation for one 1024-bit prime at hundreds of millions of USD, and noted that this was well within range of the FY2012 $10.5 billion [[U.S. Consolidated Cryptologic Program]] (which includes [[NSA]]). Because of the reuse of primes, generating precomputation for just one prime would break two-thirds of [[VPN]]s and a quarter of all [[Secure Shell|SSH]] servers globally. The researchers noted that this attack fits claims in leaked NSA papers that NSA is able to break much current cryptography. They recommend using primes of 2048 bits or more as a defense or switching to [[elliptic-curve Diffie–Hellman]] (ECDH).<ref name="paper" />

Claims on the practical implications of the attack were however disputed by security researchers Eyal Ronen and [[Adi Shamir]] in their paper "Critical Review of Imperfect Forward Secrecy".<ref>{{Cite web | url=http://www.wisdom.weizmann.ac.il/~eyalro/RonenShamirDhReview.pdf | first1=Eyal | last1=Ronen | first2=Adi | last2=Shamir | title=Critical Review of Imperfect Forward Secrecy | date=October 2015 | journal= | access-date=2022-04-30 | archive-date=2021-12-11 | archive-url=https://web.archive.org/web/20211211100114/https://www.wisdom.weizmann.ac.il/~eyalro/RonenShamirDhReview.pdf | url-status=live }}</ref>