cososo-ltd · DavidCozens · Jul 30, 2026 · Jul 29, 2026
diff --git a/README.md b/README.md
@@ -10,21 +10,18 @@ It builds on a baseline that simulates the sort of device you might be adding th
 measures itself: see [docs/baseline.md](docs/baseline.md) for what the baseline is, how the
 figures are made, and how to run it.
 
-## This stage — Private SD-ELEMENT
+## This stage — Mutual TLS
 
-RFC 5424 records carry structured data: named groups of key/value pairs a collector parses rather
-than pattern-matches out of the message text. Everything used so far has been standard elements,
-which mean the same on any device. This stage defines one of our own — which is where a product
-says what only it knows, and `app/syslog/SyslogPipelineSd.c` is all it takes to write one.
+The device presents its own certificate, and the collector moves to 6515 — the port that requires
+one, and refuses any client that cannot produce it.
 
-`logPipeline@32473` reports the protection the record's own path was under: `transport="tls"`,
-`atRest="hmac-sha256"`, under our IANA enterprise number so a private name cannot collide with
-anyone else's. A collector can confirm that rather than assume it, and alert on a device whose
-pipeline has weakened.
+Server-authenticated TLS proves the device is talking to the right collector. Mutual TLS also
+proves to the collector which device is talking, so a record is attributable to the holder of a
+provisioned key rather than to anything that could reach the port.
 
 <!-- STAGE-COST:START (generated by scripts/gen-cost-table.py — do not edit by hand) -->
 
-**Cost above baseline: Flash +13,548 B, RAM +35,688 B.**
+**Cost above baseline: Flash +13,624 B, RAM +37,740 B.**
 
 <!-- STAGE-COST:END -->
 
@@ -56,6 +53,7 @@ committed as [`run-report.md`](run-report.md), and rewritten by every stage.
 | TLS | a collector the device authenticates, and records no longer readable on the wire | +13,076 | +35,664 |
 | HMAC at rest | stored records that cannot be edited undetected, not merely checked for corruption | +13,424 | +35,684 |
 | Private SD-ELEMENT | a record that states the protection its own log pipeline was under | +13,548 | +35,688 |
+| Mutual TLS | a collector that knows which device sent the record, not just that one did | +13,624 | +37,740 |
 
 *Deltas are bytes above the baseline, which is itself Flash 350,308 B, RAM 111,192 B.*
 

diff --git a/app/AppConfig.h b/app/AppConfig.h
@@ -30,6 +30,6 @@
  * headroom, not spare capacity: buffer_alloc hands out contiguous space, so a
  * buffer only a little over the peak fails on fragmentation rather than on
  * capacity. Applied again wherever more is asked of mbedTLS. */
-#define SIMULATED_APP_MBEDTLS_HEAP_BYTES (53 * 1024)
+#define SIMULATED_APP_MBEDTLS_HEAP_BYTES (55 * 1024)
 
 #endif /* APP_CONFIG_H */
diff --git a/app/syslog/Syslog.c b/app/syslog/Syslog.c
@@ -51,7 +51,7 @@
  * the resolver numeric-only — no DNS, so no LWIP_DNS and no DNS resolver
  * component to compile. */
 #define SYSLOG_COLLECTOR_HOST "10.0.2.2"
-#define SYSLOG_COLLECTOR_PORT ((uint16_t) 6514U)
+#define SYSLOG_COLLECTOR_PORT ((uint16_t) 6515U)
 
 /* Absorbs records logged while the service task is busy sending. Many devices can
  * reduce this further: the store holds the backlog, so the ring only has to cover
@@ -147,13 +147,20 @@ void Syslog_Start(void)
 
     struct SolidSyslogLwipRawTcpStreamConfig tcpConfig = {.Sleep = SyslogSleep};
 
+    /* Both must be set: either one NULL disables mTLS silently, which is why the
+     * pipeline element reports what was configured rather than what was intended. */
+    struct mbedtls_x509_crt* clientChain = DeviceCertStore_ClientChain();
+    struct mbedtls_pk_context* clientKey = DeviceCertStore_ClientKey();
+
     /* ServerName must match the name in the collector's certificate. */
     struct SolidSyslogMbedTlsStreamConfig tlsConfig = {
         .Transport = SolidSyslogLwipRawTcpStream_Create(&tcpConfig),
         .Sleep = SyslogSleep,
         .Rng = DeviceCertStore_Rng(),
         .CaChain = DeviceCertStore_CaChain(),
         .ServerName = SYSLOG_COLLECTOR_HOST,
+        .ClientCertChain = clientChain,
+        .ClientKey = clientKey,
     };
 
     /* No EndpointVersion — this collector never moves, so the sender resolves
@@ -183,7 +190,7 @@ void Syslog_Start(void)
         .GetIpAt = SyslogOriginIpAt,
     };
     s_sd[2] = SolidSyslogOriginSd_Create(&originConfig);
-    s_sd[3] = SyslogPipelineSd_Get();
+    s_sd[3] = SyslogPipelineSd_Init((clientChain != NULL) && (clientKey != NULL));
 
     struct SolidSyslogMbedTlsHmacSha256PolicyConfig hmacConfig = {.GetKey = SyslogStoreKey};
 

diff --git a/app/syslog/SyslogPipelineSd.c b/app/syslog/SyslogPipelineSd.c
@@ -8,14 +8,16 @@
 #include "SolidSyslogSdValue.h"
 #include "SolidSyslogStructuredDataDefinition.h"
 
+static const char* s_transport = "tls";
+
 /* A non-zero enterprise number is what makes the SD-ID private: _Begin emits
  * "name@number" for one, a bare IANA "name" for 0. */
 static void SyslogPipelineSd_Format(struct SolidSyslogStructuredData* base, struct SolidSyslogSdElement* element)
 {
     (void) base;
 
     SolidSyslogSdElement_Begin(element, "logPipeline", SYSLOG_ENTERPRISE_NUMBER);
-    SolidSyslogSdValue_String(SolidSyslogSdElement_Param(element, "transport"), "tls");
+    SolidSyslogSdValue_String(SolidSyslogSdElement_Param(element, "transport"), s_transport);
     SolidSyslogSdValue_String(SolidSyslogSdElement_Param(element, "atRest"), "hmac-sha256");
     SolidSyslogSdElement_End(element);
 }
@@ -24,7 +26,8 @@ static void SyslogPipelineSd_Format(struct SolidSyslogStructuredData* base, stru
  * stateless one is a vtable this application owns. */
 static struct SolidSyslogStructuredData s_pipelineSd = {SyslogPipelineSd_Format};
 
-struct SolidSyslogStructuredData* SyslogPipelineSd_Get(void)
+struct SolidSyslogStructuredData* SyslogPipelineSd_Init(bool mutualTls)
 {
+    s_transport = mutualTls ? "mtls" : "tls";
     return &s_pipelineSd;
 }
diff --git a/app/syslog/SyslogPipelineSd.h b/app/syslog/SyslogPipelineSd.h
@@ -5,9 +5,13 @@
 #ifndef APP_SYSLOG_PIPELINE_SD_H
 #define APP_SYSLOG_PIPELINE_SD_H
 
+#include <stdbool.h>
+
 struct SolidSyslogStructuredData;
 
-/** The shared instance, for SolidSyslogConfig.Sd. Stateless, so never NULL. */
-struct SolidSyslogStructuredData* SyslogPipelineSd_Get(void);
+/** Records what the pipeline was configured with and returns the shared instance,
+ *  for SolidSyslogConfig.Sd. Never NULL. @p mutualTls must reflect the stream
+ *  config, not the intent. */
+struct SolidSyslogStructuredData* SyslogPipelineSd_Init(bool mutualTls);
 
 #endif /* APP_SYSLOG_PIPELINE_SD_H */
diff --git a/measurements/mtls.csv b/measurements/mtls.csv
@@ -0,0 +1,13 @@
+# mtls figures (bytes) — captured by scripts/run.sh (CAPTURE=1).
+# The device reads measurements/Baseline.csv as its frozen baseline and reports current-minus-Baseline.
+flash_text,363280
+flash_data,652
+static_bss,148280
+heap_used,4440
+mbedtls_peak,37244
+mbedtls_free,19076
+lwip_mem_free,7576
+lwip_pbufs_free,13
+stack_log,832
+stack_service,3852
+stack_harness,2848
diff --git a/measurements/stages.tsv b/measurements/stages.tsv
@@ -28,3 +28,4 @@ origin-ip	Origin address	the device's own address in the record, which a relay o
 tls	TLS	a collector the device authenticates, and records no longer readable on the wire
 hmac	HMAC at rest	stored records that cannot be edited undetected, not merely checked for corruption
 pipeline-sd	Private SD-ELEMENT	a record that states the protection its own log pipeline was under
+mtls	Mutual TLS	a collector that knows which device sent the record, not just that one did
diff --git a/run-report.md b/run-report.md
@@ -1,4 +1,4 @@
-# solid-syslog-example — run (pipeline-sd)
+# solid-syslog-example — run (mtls)
 
 ## Device (self-measured)
 
@@ -10,14 +10,14 @@
 [device]   first record logged: yes
 [report] --- SolidSyslog cost above baseline (simulated existing application) ---
 [report] key,current,baseline,used_above_baseline
-[report] flash_text,363208,349992,13216
-[report] flash_data,648,316,332
-[report] static_bss,146232,110876,35356
+[report] flash_text,363280,349992,13288
+[report] flash_data,652,316,336
+[report] static_bss,148280,110876,37404
 [report] heap_used,4440,4440,0
-[report] mbedtls_peak,36096,21332,14764
-[report] mbedtls_free,18176,11436,6740
+[report] mbedtls_peak,37244,21332,15912
+[report] mbedtls_free,19076,11436,7640
 [report] lwip_mem_free,7576,7576,0
-[report] lwip_pbufs_free,14,14,0
+[report] lwip_pbufs_free,13,14,-1
 [report] stack_log,832,120,712
 [report] stack_service,3852,52,3800
 [report] stack_harness,2848,2840,8
@@ -29,7 +29,7 @@
 
 ```text
    text	   data	    bss	    dec	    hex	filename
- 363200	    656	 146232	 510088	  7c888	/w/build/baseline-cross/baseline.elf
+ 363272	    660	 148280	 512212	  7d0d4	/w/build/baseline-cross/baseline.elf
 ```
 
 ## Listeners (proved before the device ran)
@@ -47,21 +47,21 @@
 ## Collector (syslog-ng) received
 
 ```text
-wire   <134>1 2026-07-29T10:57:51.850000Z 10.0.2.15 solid-syslog-example - BOOT [meta sequenceId="1" sysUpTime="385"][timeQuality tzKnown="1" isSynced="0"][origin software="solid-syslog-example" swVersion="0.1.0" enterpriseId="32473" ip="10.0.2.15"][logPipeline@32473 transport="tls" atRest="hmac-sha256"] device started
-parsed PRIORITY=134 TIMESTAMP=2026-07-29T10:57:51+00:00 HOSTNAME=10.0.2.15 APP_NAME=solid-syslog-example PROCID= MSGID=BOOT STRUCTURED_DATA=[meta sequenceId="1" sysUpTime="385"][timeQuality tzKnown="1" isSynced="0"][origin software="solid-syslog-example" swVersion="0.1.0" enterpriseId="32473" ip="10.0.2.15"][logPipeline@32473 transport="tls" atRest="hmac-sha256"] MSG=device started
+wire   <134>1 2026-07-29T11:06:52.430000Z 10.0.2.15 solid-syslog-example - BOOT [meta sequenceId="1" sysUpTime="243"][timeQuality tzKnown="1" isSynced="0"][origin software="solid-syslog-example" swVersion="0.1.0" enterpriseId="32473" ip="10.0.2.15"][logPipeline@32473 transport="mtls" atRest="hmac-sha256"] device started
+parsed PRIORITY=134 TIMESTAMP=2026-07-29T11:06:52+00:00 HOSTNAME=10.0.2.15 APP_NAME=solid-syslog-example PROCID= MSGID=BOOT STRUCTURED_DATA=[meta sequenceId="1" sysUpTime="243"][timeQuality tzKnown="1" isSynced="0"][origin software="solid-syslog-example" swVersion="0.1.0" enterpriseId="32473" ip="10.0.2.15"][logPipeline@32473 transport="mtls" atRest="hmac-sha256"] MSG=device started
 ```
 
-## Self-check (vs measurements/pipeline-sd.csv)
+## Self-check (vs measurements/mtls.csv)
 
 ```text
-  OK    flash_text: 363208 (expected 363208, Δ0)
-  OK    flash_data: 648 (expected 648, Δ0)
-  OK    static_bss: 146232 (expected 146232, Δ0)
+  OK    flash_text: 363280 (expected 363280, Δ0)
+  OK    flash_data: 652 (expected 652, Δ0)
+  OK    static_bss: 148280 (expected 148280, Δ0)
   OK    heap_used: 4440 (expected 4440, Δ0)
-  OK    mbedtls_peak: 36096 (expected 36096, Δ0)
-  OK    mbedtls_free: 18176 (expected 18176, Δ0)
+  OK    mbedtls_peak: 37244 (expected 37244, Δ0)
+  OK    mbedtls_free: 19076 (expected 19076, Δ0)
   OK    lwip_mem_free: 7576 (expected 7576, Δ0)
-  OK    lwip_pbufs_free: 14 (expected 14, Δ0)
+  OK    lwip_pbufs_free: 13 (expected 13, Δ0)
   OK    stack_log: 832 (expected 832, Δ0)
   OK    stack_service: 3852 (expected 3852, Δ0)
   OK    stack_harness: 2848 (expected 2848, Δ0)