Certificate compression #788
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Certificate compression is supported since OpenSSL 3.2, it is enabled
automatically as negotiated in a TLSv1.3 handshake.
Using certificate compression and decompression in runtime may be
suboptimal in terms of CPU and memory consumption in certain typical
scenarios, hence it is disabled by default on both server and client
sides. It can be enabled with ssl_conf_command and similar directives
in upstream as appropriate, for example:
ssl_conf_command Options RxCertificateCompression;
ssl_conf_command Options TxCertificateCompression;
Compressing server certificates requires additional support, this is
addressed separately.
The ssl_certificate_compression directive allows to send compressed server certificates, which are pre-compressed on on nginx startup. To simplify configuration, the SSL_OP_NO_TX_CERTIFICATE_COMPRESSION option is automatically cleared if certificates were pre-compressed. SSL_CTX_compress_certs() may return an error in legitimate cases, e.g., when none of compression algorithms is available or if the resulting compressed size is larger than the original one, thus it is silently ignored. Certificate compression is supported in Chrome with brotli only, in Safari with zlib only, and in Firefox with all listed algorithms. It is supported since Ubuntu 24.10, which has OpenSSL with enabled zlib and zstd support. The actual list of algorithms supported in OpenSSL depends on how the library was configured; it can be brotli, zlib, zstd as listed in RFC 8879.
BoringSSL provides two callbacks to register on SSL context for each compression algorithm, which is used to send and receive compressed server certificates, as appropriate. This change adds support for compression with zlib, enabled with the ssl_certificate_compression directive. Compressed certificates are cached in SSL context on the first callback invocation.
| zstream.next_out = tmp.data; | ||
| zstream.avail_out = tmp.len; | ||
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| rc = deflateInit(&zstream, Z_DEFAULT_COMPRESSION); |
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As the result of compression is cached and certificates are usually small (few kb), I think it would be better to use Z_BEST_COMPRESSION.
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Z_DEFAULT_COMPRESSION - is a compromise between two evils: non-efficient and slow compression.
If proven useful, a separate directive can be introduced, similar to gzip_comp_level.
arut
reviewed
Jul 29, 2025
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| ngx_log_error(NGX_LOG_WARN, ssl->log, 0, | ||
| "\"ssl_certificate_compression\" ignored, not supported"); |
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A similar function ngx_ssl_early_data() has different text:
ngx_log_error(NGX_LOG_WARN, ssl->log, 0,
"\"ssl_early_data\" is not supported on this platform, "
"ignored"); The same text exists in ngx_ssl_session_ticket_keys(), but the example above looks more relevant.
arut
reviewed
Jul 29, 2025
| #ifdef SSL_OP_NO_RX_CERTIFICATE_COMPRESSION | ||
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| if (SSL_CTX_compress_certs(ssl->ctx, 0) != 0) { | ||
| SSL_CTX_clear_options(ssl->ctx, SSL_OP_NO_TX_CERTIFICATE_COMPRESSION); |
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Why not disable tx compression in the if (!enable) {...} block above instead of disabling it for all new ssl context and enabling it back here. Also, seems like enabling tx compression is completely safe anyway.
double "on" |
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See also trac ticket #2546 for the previous patch series discussion and some numbers linked from there.