SSL: support for compressed server certificates with OpenSSL.

The ssl_certificate_compression directive allows to send compressed
server certificates.  In OpenSSL, they are pre-compressed on 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.

2 files changed, 19 insertions, 0 deletions

diff --git a/src/http/modules/ngx_http_ssl_module.c b/src/http/modules/ngx_http_ssl_module.c
index dbfe5c08b..fbf4ab871 100644
--- a/src/http/modules/ngx_http_ssl_module.c
+++ b/src/http/modules/ngx_http_ssl_module.c
@@ -124,6 +124,13 @@ static ngx_command_t  ngx_http_ssl_commands[] = {
       0,
       NULL },
 
+    { ngx_string("ssl_certificate_compression"),
+      NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_CONF_FLAG,
+      ngx_conf_set_flag_slot,
+      NGX_HTTP_SRV_CONF_OFFSET,
+      offsetof(ngx_http_ssl_srv_conf_t, certificate_compression),
+      NULL },
+
     { ngx_string("ssl_dhparam"),
       NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_CONF_TAKE1,
       ngx_conf_set_str_slot,
@@ -621,6 +628,7 @@ ngx_http_ssl_create_srv_conf(ngx_conf_t *cf)
      */
 
     sscf->prefer_server_ciphers = NGX_CONF_UNSET;
+    sscf->certificate_compression = NGX_CONF_UNSET;
     sscf->early_data = NGX_CONF_UNSET;
     sscf->reject_handshake = NGX_CONF_UNSET;
     sscf->buffer_size = NGX_CONF_UNSET_SIZE;
@@ -658,6 +666,9 @@ ngx_http_ssl_merge_srv_conf(ngx_conf_t *cf, void *parent, void *child)
     ngx_conf_merge_value(conf->prefer_server_ciphers,
                          prev->prefer_server_ciphers, 0);
 
+    ngx_conf_merge_value(conf->certificate_compression,
+                         prev->certificate_compression, 0);
+
     ngx_conf_merge_value(conf->early_data, prev->early_data, 0);
     ngx_conf_merge_value(conf->reject_handshake, prev->reject_handshake, 0);
 
@@ -792,6 +803,13 @@ ngx_http_ssl_merge_srv_conf(ngx_conf_t *cf, void *parent, void *child)
         {
             return NGX_CONF_ERROR;
         }
+
+        if (ngx_ssl_certificate_compression(cf, &conf->ssl,
+                                            conf->certificate_compression)
+            != NGX_OK)
+        {
+            return NGX_CONF_ERROR;
+        }
     }
 
     conf->ssl.buffer_size = conf->buffer_size;
diff --git a/src/http/modules/ngx_http_ssl_module.h b/src/http/modules/ngx_http_ssl_module.h
index 8650fab93..9b26529fa 100644
--- a/src/http/modules/ngx_http_ssl_module.h
+++ b/src/http/modules/ngx_http_ssl_module.h
@@ -18,6 +18,7 @@ typedef struct {
     ngx_ssl_t                       ssl;
 
     ngx_flag_t                      prefer_server_ciphers;
+    ngx_flag_t                      certificate_compression;
     ngx_flag_t                      early_data;
     ngx_flag_t                      reject_handshake;