[PATCH 2 of 4] QUIC: trial packet decryption in response to invalid key update

[Maxim Dounin]

# HG changeset patch
# User Sergey Kandaurov <pluknet at nginx.com>
# Date 1707911734 -14400
#      Wed Feb 14 15:55:34 2024 +0400
# Node ID 1bf1b423f26853d0453ee30fa4ed4467ea05af7c
# Parent  2ed3f57dca0a664340bca2236c7d614902db4180
QUIC: trial packet decryption in response to invalid key update.

Inspired by RFC 9001, Section 6.3, trial packet decryption with the current
keys is now used to avoid a timing side-channel signal.  Further, this fixes
segfault while accessing missing next keys (ticket #2585).

diff --git a/src/event/quic/ngx_event_quic_protection.c b/src/event/quic/ngx_event_quic_protection.c
--- a/src/event/quic/ngx_event_quic_protection.c
+++ b/src/event/quic/ngx_event_quic_protection.c
@@ -1144,8 +1144,19 @@ ngx_quic_decrypt(ngx_quic_header_t *pkt,
         key_phase = (pkt->flags & NGX_QUIC_PKT_KPHASE) != 0;
 
         if (key_phase != pkt->key_phase) {
-            secret = &pkt->keys->next_key.client;
-            pkt->key_update = 1;
+            if (pkt->keys->next_key.client.ctx != NULL) {
+                secret = &pkt->keys->next_key.client;
+                pkt->key_update = 1;
+
+            } else {
+                /*
+                 * RFC 9001,  6.3. Timing of Receive Key Generation.
+                 *
+                 * Trial decryption to avoid timing side-channel.
+                 */
+                ngx_log_debug0(NGX_LOG_DEBUG_EVENT, pkt->log, 0,
+                               "quic next key missing");
+            }
         }
     }