Editing Network Time Protocol (section)

=== Secure extensions ===

NTP itself includes support for authenticating servers to clients. NTPv3 supports a [[symmetric key]] mode, which is not useful against MITM. The [[public key]] system known as "autokey" in NTPv4 adapted from [[IPSec]] offers useful authentication,<ref name="cGifv"/> but is not practical for a busy server.<ref name="7a6Kk"/> Autokey was also later found to suffer from several design flaws,<ref>{{Cite conference|url=https://www.ietf.org/proceedings/83/slides/slides-83-tictoc-1.pdf|title=Analysis of NTP's Autokey Protocol|author1=Dieter Sibold|author2=Stephen Röttger|conference=IETF 83|date=2012}}</ref> with no correction published, save for a change in the [[message authentication code]].{{Ref RFC|8573}}  Autokey should no longer be used.{{ref RFC|8633|section=4.2}}

'''Network Time Security''' (NTS) is a secure version of NTPv4 with [[Transport Layer Security|TLS]] and [[Authenticated encryption|AEAD]].<ref>{{Cite web|title=nts.time.nl homepage|url=https://nts.time.nl/|access-date=2021-08-19|website=nts.time.nl}}</ref> The main improvement over previous attempts is that a separate "key establishment" server handles the heavy asymmetric cryptography, which needs to be done only once. If the server goes down, previous users would still be able to fetch time without fear of MITM.{{Ref RFC|8915}} NTS is supported by several NTP servers including [[Cloudflare]] and [[Netnod]].<ref>{{Cite web|last=Langer|first=Martin|date=2019-12-05|title=Setting up NTS-Secured NTP with NTPsec|url=https://weberblog.net/setting-up-nts-secured-ntp-with-ntpsec/|access-date=2021-08-19|website=Weberblog.net|language=en-US}}</ref><ref>{{Cite web|title=How to use NTS {{!}} Netnod|url=https://www.netnod.se/time-and-frequency/how-to-use-nts|access-date=2021-08-19|website=Netnod}}</ref> It can be enabled on {{Proper name|chrony}}, NTPsec, and ntpd-rs.<ref>{{cite web |date=13 August 2024 |title=Network Time Security · Cloudflare Time Services docs |url=https://developers.cloudflare.com/time-services/nts/ |access-date=12 January 2025 |website=developers.cloudflare.com |language=en}}</ref>

Microsoft also has an approach to authenticate NTPv3/SNTPv4 packets using a [[Windows domain]] identity, known as MS-SNTP.<ref>{{cite web | url=https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-sntp/8106cb73-ab3a-4542-8bc8-784dd32031cc | title=&#91;MS-SNTP&#93;: Network Time Protocol (NTP) Authentication Extensions | date=24 June 2021 }}</ref> This system is implemented in the reference ntpd and chrony, using [[Samba (software)|samba]] for the domain connection.<ref name="comparison">{{Cite web|title=Comparison of NTP implementations|url=https://chrony.tuxfamily.org/comparison.html|publisher=chrony.tuxfamily.org|accessdate=2019-10-08}}</ref>