--- title: RFC 9849: TLS Encrypted Client Hello | daily.dev description: RFC 9849 specifies the TLS Encrypted Client Hello (ECH) mechanism, which encrypts the ClientHello message under a server public key to prevent metadata... canonical: https://daily.dev/posts/rfc-9849-tls-encrypted-client-hello-exjfuobix twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: RFC 9849: TLS Encrypted Client Hello | daily.dev og:description: RFC 9849 specifies the TLS Encrypted Client Hello (ECH) mechanism, which encrypts the ClientHello message under a server public key to prevent metadata... og:url: https://daily.dev/posts/rfc-9849-tls-encrypted-client-hello-exjfuobix og:image: https://api.daily.dev/og/posts/exjfUObIx.png og:image:alt: RFC 9849: TLS Encrypted Client Hello og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # RFC 9849: TLS Encrypted Client Hello **[Hacker News](https://daily.dev/sources/hn)** · 6 min read · 0 upvotes · 0 comments ## Summary RFC 9849 specifies the TLS Encrypted Client Hello (ECH) mechanism, which encrypts the ClientHello message under a server public key to prevent metadata leakage. The spec details how a client-facing server processes incoming ECH extensions: it collects candidate ECHConfig values, attempts decryption using HPKE (SetupBaseR/context.Open), verifies cipher suite and version compatibility, and forwards the decrypted ClientHelloInner to the backend server. It also covers HelloRetryRequest handling, fallback behavior when decryption fails (including GREASE ECH), and requirements for retry_configs in EncryptedExtensions. Servers must abort with specific alerts for malformed or inconsistent ECH parameters. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Similar posts on daily.dev - [ECH Is Done, But Can We Make It Work?](https://daily.dev/posts/ech-is-done-but-can-we-make-it-work--hltj5jdpk) · Feisty Duck · 0 upvotes · 0 comments - [Encrypted Client Hello \(ECH\) is now supported on Vercel CDN](https://daily.dev/posts/encrypted-client-hello-ech-is-now-supported-on-vercel-cdn-ugddpiunl) · Vercel · 0 upvotes · 0 comments - [OpenSSL 4.0 Alpha 1 Released With Encrypted Client Hello "ECH" & Other Features](https://daily.dev/posts/openssl-4-0-alpha-1-released-with-encrypted-client-hello-ech-other-features-mequ8p2e0) · Phoronix · 0 upvotes · 0 comments --- Tags: [#security](https://daily.dev/tags/security), [#cryptography](https://daily.dev/tags/cryptography) [View this post on daily.dev](https://daily.dev/posts/rfc-9849-tls-encrypted-client-hello-exjfuobix) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"RFC 9849: TLS Encrypted Client Hello","url":"https://daily.dev/posts/rfc-9849-tls-encrypted-client-hello-exjfuobix","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/rfc-9849-tls-encrypted-client-hello-exjfuobix"},"datePublished":"2026-03-04T11:01:44.014Z","dateModified":"2026-08-24T06:51:43.028Z","description":"RFC 9849 specifies the TLS Encrypted Client Hello (ECH) mechanism, which encrypts the ClientHello message under a server public key to prevent metadata...","image":"https://media.daily.dev/image/upload/s--qPvKM23u--/f_auto/v1722860399/public/Placeholder%2009","thumbnailUrl":"https://media.daily.dev/image/upload/s--qPvKM23u--/f_auto/v1722860399/public/Placeholder%2009","isAccessibleForFree":true,"articleSection":"Hacker News","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"Hacker News","logo":"https://media.daily.dev/image/upload/t_logo,f_auto/v1/logos/hn","url":"https://daily.dev/sources/hn"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/rfc-9849-tls-encrypted-client-hello-exjfuobix","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"security,cryptography","timeRequired":"PT6M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"Hacker News","item":"https://daily.dev/sources/hn"},{"@type":"ListItem","position":3,"name":"RFC 9849: TLS Encrypted Client Hello"}]} ```