SSL: support for multiple curves (ticket #885). OpenSSL 1.0.2+ allows configuring a curve list instead of a single curve previously supported. This allows use of different curves depending on what client supports (as available via the elliptic_curves extension), and also allows use of different curves in an ECDHE key exchange and in the ECDSA certificate. The special value "auto" was introduced (now the default for ssl_ecdh_curve), which means "use an internal list of curves as available in the OpenSSL library used". For versions prior to OpenSSL 1.0.2 it maps to "prime256v1" as previously used. The default in 1.0.2b+ prefers prime256v1 as well (and X25519 in OpenSSL 1.1.0+). As client vs. server preference of curves is controlled by the same option as used for ciphers (SSL_OP_CIPHER_SERVER_PREFERENCE), the ssl_prefer_server_ciphers directive now controls both.

--- a/src/mail/ngx_mail_ssl_module.c
+++ b/src/mail/ngx_mail_ssl_module.c
@@ -11,7 +11,7 @@
 
 
 #define NGX_DEFAULT_CIPHERS     "HIGH:!aNULL:!MD5"
-#define NGX_DEFAULT_ECDH_CURVE  "prime256v1"
+#define NGX_DEFAULT_ECDH_CURVE  "auto"
 
 
 static void *ngx_mail_ssl_create_conf(ngx_conf_t *cf);