We investigate the security of Diffie-Hellman key exchange as used in popular Internet protocols and find it to be less secure than widely believed. First, we present Logjam, a novel flaw in TLS that lets a man-in-the-middle downgrade connections to “export-grade” Diffie-Hellman. To carry out this attack, we implement the number field sieve discrete logarithm algorithm. After a week-long precomputation for a specified 512-bit group, we can compute arbitrary discrete logarithms in that group in about a minute. We find that 82% of vulnerable servers use a single 512-bit group, and that 8.4% of Alexa Top Million HTTPS sites are vulnerable to the attack. In response, major browsers have changed to reject short groups. We go on to consider Diffie-Hellman with 768- and 1024-bit groups. We estimate that even in the 1024-bit case, the computations are plausible given nation-state resources. A small number of fixed or standardized groups are used by millions of servers; performing precomputation for a single 1024-bit group would allow passive eavesdropping on 18% of popular HTTPS sites, and a second group would allow decryption of traffic to 66% of IPsec VPNs and 26% of SSH servers. A close reading of published NSA leaks shows that the agency’s attacks on VPNs are consistent with having achieved such a break. We conclude that moving to stronger key exchange methods should be a priority for the Internet community.<\/p>\n","protected":false},"excerpt":{"rendered":"
We investigate the security of Diffie-Hellman key exchange as used in popular Internet protocols and find it to be less secure than widely believed. First, we present Logjam, a novel flaw in TLS that lets a man-in-the-middle downgrade connections to “export-grade” Diffie-Hellman. To carry out this attack, we implement the number field sieve discrete logarithm […]<\/p>\n","protected":false},"featured_media":0,"template":"","meta":{"msr-url-field":"","msr-podcast-episode":"","msrModifiedDate":"","msrModifiedDateEnabled":false,"ep_exclude_from_search":false,"_classifai_error":"","footnotes":""},"msr-content-type":[3],"msr-research-highlight":[],"research-area":[13558],"msr-publication-type":[193715],"msr-product-type":[],"msr-focus-area":[],"msr-platform":[],"msr-download-source":[],"msr-locale":[268875],"msr-post-option":[],"msr-field-of-study":[246691,248383,253840,253843,253858,253849,253855,253852,253846,248542],"msr-conference":[],"msr-journal":[],"msr-impact-theme":[],"msr-pillar":[],"class_list":["post-215136","msr-research-item","type-msr-research-item","status-publish","hentry","msr-research-area-security-privacy-cryptography","msr-locale-en_us","msr-field-of-study-computer-science","msr-field-of-study-computer-security","msr-field-of-study-diffie-hellman-key-exchange","msr-field-of-study-discrete-logarithm","msr-field-of-study-eavesdropping","msr-field-of-study-forward-secrecy","msr-field-of-study-ipsec","msr-field-of-study-key-exchange","msr-field-of-study-precomputation","msr-field-of-study-server"],"msr_publishername":"","msr_edition":"","msr_affiliation":"","msr_published_date":"2018-12-18","msr_host":"","msr_duration":"","msr_version":"","msr_speaker":"","msr_other_contributors":"","msr_booktitle":"","msr_pages_string":"","msr_chapter":"","msr_isbn":"","msr_journal":"Communications of The ACM","msr_volume":"62","msr_number":"","msr_editors":"","msr_series":"","msr_issue":"1","msr_organization":"","msr_how_published":"","msr_notes":"","msr_highlight_text":"","msr_release_tracker_id":"","msr_original_fields_of_study":"","msr_download_urls":"","msr_external_url":"","msr_secondary_video_url":"","msr_longbiography":"","msr_microsoftintellectualproperty":1,"msr_main_download":"215491","msr_publicationurl":"","msr_doi":"","msr_publication_uploader":[{"type":"file","viewUrl":"https:\/\/www.microsoft.com\/en-us\/research\/uploads\/prod\/2020\/09\/CACMZanella2019.pdf","id":"692643","title":"cacmzanella2019","label_id":"243132","label":0},{"type":"doi","viewUrl":"false","id":"false","title":"10.1145\/3292035","label_id":"243106","label":0}],"msr_related_uploader":[{"type":"file","viewUrl":"https:\/\/www.microsoft.com\/en-us\/research\/wp-content\/uploads\/2016\/04\/imperfect-forward-secrecy-ccs15.pdf","id":"215491","title":"imperfect-forward-secrecy-ccs15-pdf","label_id":"243118","label":0}],"msr_attachments":[{"id":215491,"url":"https:\/\/www.microsoft.com\/en-us\/research\/wp-content\/uploads\/2016\/04\/imperfect-forward-secrecy-ccs15.pdf"}],"msr-author-ordering":[{"type":"text","value":"David Adrian","user_id":0,"rest_url":false},{"type":"user_nicename","value":"Karthik Bhargavan","user_id":32491,"rest_url":"https:\/\/www.microsoft.com\/en-us\/research\/wp-json\/microsoft-research\/v1\/researchers?person=Karthik Bhargavan"},{"type":"text","value":"Zakir Durumeric","user_id":0,"rest_url":false},{"type":"text","value":"Pierrick Gaudry","user_id":0,"rest_url":false},{"type":"text","value":"Matthew Green","user_id":0,"rest_url":false},{"type":"text","value":"J. 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