SSL: $ssl_curve (ticket #2135).

The variable contains a negotiated curve used for the handshake key
exchange process.  Known curves are listed by their names, unknown
ones are shown in hex.

Note that for resumed sessions in TLSv1.2 and older protocols,
$ssl_curve contains the curve used during the initial handshake,
while in TLSv1.3 it contains the curve used during the session
resumption (see the SSL_get_negotiated_group manual page for
details).

The variable is only meaningful when using OpenSSL 3.0 and above.
With older versions the variable is empty.

src/event/ngx_event_openssl.c

@@ -4733,6 +4733,42 @@ ngx_ssl_get_ciphers(ngx_connection_t *c, ngx_pool_t *pool, ngx_str_t *s)
 }
 
 
+ngx_int_t
+ngx_ssl_get_curve(ngx_connection_t *c, ngx_pool_t *pool, ngx_str_t *s)
+{
+#ifdef SSL_get_negotiated_group
+
+    int  nid;
+
+    nid = SSL_get_negotiated_group(c->ssl->connection);
+
+    if (nid != NID_undef) {
+
+        if ((nid & TLSEXT_nid_unknown) == 0) {
+            s->len = ngx_strlen(OBJ_nid2sn(nid));
+            s->data = (u_char *) OBJ_nid2sn(nid);
+            return NGX_OK;
+        }
+
+        s->len = sizeof("0x0000") - 1;
+
+        s->data = ngx_pnalloc(pool, s->len);
+        if (s->data == NULL) {
+            return NGX_ERROR;
+        }
+
+        ngx_sprintf(s->data, "0x%04xd", nid & 0xffff);
+
+        return NGX_OK;
+    }
+
+#endif
+
+    s->len = 0;
+    return NGX_OK;
+}
+
+
 ngx_int_t
 ngx_ssl_get_curves(ngx_connection_t *c, ngx_pool_t *pool, ngx_str_t *s)
 {

src/event/ngx_event_openssl.h

@@ -256,6 +256,8 @@ ngx_int_t ngx_ssl_get_cipher_name(ngx_connection_t *c, ngx_pool_t *pool,
     ngx_str_t *s);
 ngx_int_t ngx_ssl_get_ciphers(ngx_connection_t *c, ngx_pool_t *pool,
     ngx_str_t *s);
+ngx_int_t ngx_ssl_get_curve(ngx_connection_t *c, ngx_pool_t *pool,
+    ngx_str_t *s);
 ngx_int_t ngx_ssl_get_curves(ngx_connection_t *c, ngx_pool_t *pool,
     ngx_str_t *s);
 ngx_int_t ngx_ssl_get_session_id(ngx_connection_t *c, ngx_pool_t *pool,

src/http/modules/ngx_http_ssl_module.c

@@ -342,6 +342,9 @@ static ngx_http_variable_t  ngx_http_ssl_vars[] = {
     { ngx_string("ssl_ciphers"), NULL, ngx_http_ssl_variable,
       (uintptr_t) ngx_ssl_get_ciphers, NGX_HTTP_VAR_CHANGEABLE, 0 },
 
+    { ngx_string("ssl_curve"), NULL, ngx_http_ssl_variable,
+      (uintptr_t) ngx_ssl_get_curve, NGX_HTTP_VAR_CHANGEABLE, 0 },
+
     { ngx_string("ssl_curves"), NULL, ngx_http_ssl_variable,
       (uintptr_t) ngx_ssl_get_curves, NGX_HTTP_VAR_CHANGEABLE, 0 },
 

src/stream/ngx_stream_ssl_module.c

@@ -269,6 +269,9 @@ static ngx_stream_variable_t  ngx_stream_ssl_vars[] = {
     { ngx_string("ssl_ciphers"), NULL, ngx_stream_ssl_variable,
       (uintptr_t) ngx_ssl_get_ciphers, NGX_STREAM_VAR_CHANGEABLE, 0 },
 
+    { ngx_string("ssl_curve"), NULL, ngx_stream_ssl_variable,
+      (uintptr_t) ngx_ssl_get_curve, NGX_STREAM_VAR_CHANGEABLE, 0 },
+
     { ngx_string("ssl_curves"), NULL, ngx_stream_ssl_variable,
       (uintptr_t) ngx_ssl_get_curves, NGX_STREAM_VAR_CHANGEABLE, 0 },