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TLS 1.3 certificate compression (RFC 8879) never reached the certificates installed from Lua. OpenSSL only sends a CompressedCertificate for a chain that was pre-compressed beforehand: get_compressed_certificate_alg() picks an algorithm only among the ones whose comp_cert entry is populated. nginx populates those for ssl_certificate when ssl_certificate_compression is on, and ngx.ssl.clear_certs() drops them together with the certificates they belong to, so a dynamic chain always went out in the clear. Add ngx_http_lua_ffi_ssl_compress_certs(), which pre-compresses the chain the connection currently holds. SSL_compress_certs() cannot do this from a certificate callback: it reaches ssl_get_cert_to_compress(), which gives up unless SSL_in_before(), and by then the handshake has started. So compress on a throw-away connection holding the same chain and install the result with SSL_set1_compressed_cert(), which has no such restriction. Requires OpenSSL 3.2.0 or later, where the RFC 8879 API arrived; every other TLS library gets the version error instead.
u5surf
marked this pull request as ready for review
September 25, 2026 21:25
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Fixes #2516.
Problem
With
ssl_certificate_compression on;(nginx 1.29.1+) and OpenSSL 3.2+, a statically configured certificate is sent as a TLS 1.3CompressedCertificate(RFC 8879). One installed fromssl_certificate_by_lua*is not.OpenSSL sends only a chain that was pre-compressed beforehand —
get_compressed_certificate_alg()chooses among the algorithms whosecomp_cert[]entry is populated. nginx populates those at configuration time forssl_certificate;ngx.ssl.clear_certs()drops them together with the certificates they belong to, andset_cert()does not create new ones.Approach
SSL_compress_certs()cannot do this from a certificate callback: it reachesssl_get_cert_to_compress(), which gives up unlessSSL_in_before(), and by then the handshake has started. Implemented that way first, it failed on every handshake.ngx_http_lua_ffi_ssl_compress_certs(r, alg, err)therefore compresses on a throw-awaySSLmade from the connection'sSSL_CTX— a fresh object is in thebeforestate — and installs the result on the live connection withSSL_set1_compressed_cert(), which checks onlysc->serverand so is allowed mid-handshake.SSL_CTX_compress_certs()works the same way internally.algis the RFC 8879 number, or 0 for every algorithm the library has, matchingSSL_CTX_compress_certs(). LibreSSL, BoringSSL/AWS-LC and OpenSSL < 3.2.0 getat least OpenSSL 3.2.0 required but found ....The Lua API
ngx.ssl.compress_certs()is in openresty/lua-resty-core#540.Verification
nginx 1.31.6 with this module against an OpenSSL 3.5.4 configured with
zlib-dynamic, certificate installed from Lua, observed withopenssl s_client -trace:ssl_certificate_compression on;aloneCertificate, Length=1566ssl.compress_certs("zlib")CompressedCertificate, Length=1396Tests
t/194-ssl-cert-compression.tcovers the dynamic certificate, an unknown algorithm, and compressing with no certificate on the connection. Whether OpenSSL was built with zlib, brotli or zstd is not visible fromnginx -V, so the first block accepts either outcome unlessTEST_NGINX_CERT_COMP_ALGS=1says the library has one; the other two are deterministic everywhere.I hereby granted the copyright of the changes in this pull request
to the authors of this lua-nginx-module project.