feat: send over TLS instead of plain TCP

An mbedTLS stream wrapping the lwIP TCP stream, and the collector moves to 6514.
Server authentication only: the device verifies the collector against the trust
anchor it already holds and presents nothing of its own.

  Flash        +13,076 B    (+688 on the previous stage)
  RAM          +35,664 B      (+28,280)
  mbedTLS peak +14,724 B  (36,056 absolute)
  Log stack       +712 B     (unchanged)
  Service       +3,800 B       (+2,808)

Flash is the number worth reading. Adding TLS to this device costs well under a
kilobyte, not the ~150 KB mbedTLS occupies, because a device that already speaks
mTLS was carrying mbedTLS long before SolidSyslog arrived. What is measured here is
the code that drives TLS, not TLS itself. It is also smaller than it would have been
as a single hop from UDP: the TCP stream underneath was already linked and paid for.

The RAM accounts for itself exactly, and only 632 bytes of it is the library:

  mbedTLS pool, 32 -> 53 KiB   +21,504
  service seam, 2 -> 8 KiB      +6,144
  stream and session objects       +632
                               =+28,280

Both of those resizes were forced by the device, in that order, and both failed
loudly first. The pool was sized for the broker session alone, so the handshake
could not allocate: "MbedTlsStream category 0x0402", eighty-four times, and nothing
reached the collector. Given room, the handshake then overflowed the service seam:
"FATAL: stack overflow in task service", which takes the device down before it can
report, so neither figure can be read from the run that fails.

Both are then sized from a run that completes, by the rules the baseline already
uses. The pool is the measured peak times 1.5 rounded up to the next KiB — 36,092
gives 53 KiB, and the margin is fragmentation headroom rather than spare capacity,
because buffer_alloc hands out contiguous space. The seam is twice the measured
high-water of 3,852, and configMINIMAL_STACK_SIZE * 15 falls 24 bytes short of that,
so it takes 16. A peak that moves ~100 bytes run to run can put the first rule
either side of a KiB boundary; 53 or 54 both hold it.

The service stack carries the handshake, which is why it nearly quadruples while the
log stack does not move at all. A task that calls Log is unaffected by the transport
underneath it.

The TLS stream takes the trust anchor and DRBG as handles at create time, not paths
or PEM, so SimulatedExistingApp_StartCrypto() has to run before the scheduler. The
drain window covers a connect and handshake rather than a datagram, sized against
the library's 5 s handshake budget.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: DavidCozens

CMakeLists.txt

@@ -39,7 +39,7 @@ set(LWIP_CONTRIB_FREERTOS_DIR "${LWIP_DIR}/contrib/ports/freertos")
 # link target — only the header-configured packs below do.
 # https://docs.cososo.co.uk/solid-syslog/getting-started/#path-a--cmake-consumer
 # Pinned to a commit until there is a release tag to pin to.
-set(SOLIDSYSLOG_PLATFORMS "LwipRaw;Atomics;FreeRtos;FatFs" CACHE STRING "" FORCE)
+set(SOLIDSYSLOG_PLATFORMS "LwipRaw;Atomics;FreeRtos;FatFs;MbedTls" CACHE STRING "" FORCE)
 
 # Compile-time limits. Every tunable is #ifndef-guarded, so this file only needs
 # the ones this device wants changed.
@@ -171,7 +171,7 @@ target_include_directories(baseline PRIVATE
 # library, or context struct sizes diverge between consumer and library.
 target_compile_definitions(baseline PRIVATE MBEDTLS_USER_CONFIG_FILE=${MBEDTLS_USER_CONFIG_HEADER})
 
-target_link_libraries(baseline PRIVATE mbedtls mbedx509 mbedcrypto SolidSyslog SolidSyslog::LwipRaw SolidSyslog::FreeRtos SolidSyslog::FatFs)
+target_link_libraries(baseline PRIVATE mbedtls mbedx509 mbedcrypto SolidSyslog SolidSyslog::LwipRaw SolidSyslog::FreeRtos SolidSyslog::FatFs SolidSyslog::MbedTls)
 
 target_link_options(baseline PRIVATE
     -mcpu=cortex-m3 -mthumb

README.md

@@ -10,15 +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 — Origin address
+## This stage — TLS
 
-`origin` gains the `ip` PARAM, read from the same interface address `HOSTNAME` reports. A relay or
-NAT between the device and the collector rewrites what the collector observes; this is what the
-device says about itself, and it survives the hop.
+Records are encrypted in transit, and the collector is authenticated before anything is sent to it.
+The sender wraps its TCP stream in mbedTLS, moves to 6514, and checks the collector against the
+trust anchor the device already holds.
+
+Until now the audit trail crossed the network in cleartext — readable, and alterable, by anything
+on the path.
 
 <!-- STAGE-COST:START (generated by scripts/gen-cost-table.py — do not edit by hand) -->
 
-**Cost above baseline: Flash +12,388 B, RAM +7,384 B.**
+**Cost above baseline: Flash +13,076 B, RAM +35,664 B.**
 
 <!-- STAGE-COST:END -->
 
@@ -47,6 +50,7 @@ committed as [`run-report.md`](run-report.md), and rewritten by every stage.
 | Larger cap | headroom for the grown record, so full-width counters cannot push it into truncation | +11,980 | +9,440 |
 | Smaller ring | most of the cap rise given back, now the store rather than the ring holds a backlog | +11,980 | +7,384 |
 | Origin address | the device's own address in the record, which a relay or NAT between it and the collector cannot rewrite | +12,388 | +7,384 |
+| TLS | a collector the device authenticates, and records no longer readable on the wire | +13,076 | +35,664 |
 
 *Deltas are bytes above the baseline, which is itself Flash 350,308 B, RAM 111,192 B.*
 

app/AppConfig.h

@@ -14,7 +14,7 @@
  * it. Whatever deepens a seam grows it here and is charged for it. The reported
  * figure is high-water usage, which does not depend on the allocation. */
 #define LOG_TASK_STACK_WORDS (configMINIMAL_STACK_SIZE * 4U)
-#define SERVICE_TASK_STACK_WORDS (configMINIMAL_STACK_SIZE * 4U)
+#define SERVICE_TASK_STACK_WORDS (configMINIMAL_STACK_SIZE * 16U)
 #define LOG_TASK_PRIORITY (tskIDLE_PRIORITY + 1U)
 #define SERVICE_TASK_PRIORITY (tskIDLE_PRIORITY + 1U)
 
@@ -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 (32 * 1024)
+#define SIMULATED_APP_MBEDTLS_HEAP_BYTES (53 * 1024)
 
 #endif /* APP_CONFIG_H */

app/main.c

@@ -70,7 +70,8 @@ static void HarnessTask(void* parameters)
      * before the figures are taken. What arrived is the collector's word. */
     bool logged = LogTask_EmitOnce(5000U);
     (void) printf("[device]   first record logged: %s\n", logged ? "yes" : "FAILED");
-    vTaskDelay(pdMS_TO_TICKS(500U));
+    /* Long enough for the TLS negotiation, not just the send. */
+    vTaskDelay(pdMS_TO_TICKS(3000U));
 
     (void) Measure_Report();
 

app/syslog/Syslog.c

@@ -1,14 +1,16 @@
 /* See Syslog.h.
  *
- * A TCP stream over lwIP behind a circular buffer: Log enqueues and returns, and
- * the service task drains and sends. The mutex is what makes those two sides
+ * A TLS stream over lwIP TCP behind a circular buffer: Log enqueues and returns,
+ * and the service task drains and sends. The mutex is what makes those two sides
  * safe on different tasks.
  *
  * Unlike a header field, an SD PARAM has no NILVALUE: an unset one is omitted
  * entirely rather than written as "-". */
 
 #include "Syslog.h"
 
+#include "DeviceCertStore.h"
+
 #include "SolidSyslogBlockStore.h"
 #include "SolidSyslogCircularBuffer.h"
 #include "SolidSyslogConfig.h"
@@ -23,6 +25,7 @@
 #include "SolidSyslogLwipRawMarshal.h"
 #include "SolidSyslogLwipRawResolver.h"
 #include "SolidSyslogLwipRawTcpStream.h"
+#include "SolidSyslogMbedTlsStream.h"
 #include "SolidSyslogMetaSd.h"
 #include "SolidSyslogOriginSd.h"
 #include "SolidSyslogSdValue.h"
@@ -47,7 +50,7 @@
  * 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) 5601U)
+#define SYSLOG_COLLECTOR_PORT ((uint16_t) 6514U)
 
 /* 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
@@ -134,11 +137,20 @@ void Syslog_Start(void)
 
     struct SolidSyslogLwipRawTcpStreamConfig tcpConfig = {.Sleep = SyslogSleep};
 
+    /* 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,
+    };
+
     /* No EndpointVersion — this collector never moves, so the sender resolves
      * once and pins it. */
     struct SolidSyslogStreamSenderConfig senderConfig = {
         .Resolver = SolidSyslogLwipRawResolver_Create(),
-        .Stream = SolidSyslogLwipRawTcpStream_Create(&tcpConfig),
+        .Stream = SolidSyslogMbedTlsStream_Create(&tlsConfig),
         .Address = SolidSyslogLwipRawAddress_Create(),
         .Endpoint = CollectorEndpoint,
     };

measurements/stages.tsv

@@ -25,3 +25,4 @@ origin	Origin	the device named in the record itself, not inferred from the sourc
 cap-rise	Larger cap	headroom for the grown record, so full-width counters cannot push it into truncation
 buffer-halve	Smaller ring	most of the cap rise given back, now the store rather than the ring holds a backlog
 origin-ip	Origin address	the device's own address in the record, which a relay or NAT between it and the collector cannot rewrite
+tls	TLS	a collector the device authenticates, and records no longer readable on the wire

measurements/tls.csv

@@ -0,0 +1,13 @@
+# tls 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,362736
+flash_data,648
+static_bss,146208
+heap_used,4440
+mbedtls_peak,36056
+mbedtls_free,18216
+lwip_mem_free,7576
+lwip_pbufs_free,13
+stack_log,832
+stack_service,3852
+stack_harness,2848

run-report.md

@@ -1,4 +1,4 @@
-# solid-syslog-example — run (origin-ip)
+# solid-syslog-example — run (tls)
 
 ## Device (self-measured)
 
@@ -10,16 +10,16 @@
 [device]   first record logged: yes
 [report] --- SolidSyslog cost above baseline (simulated existing application) ---
 [report] key,current,baseline,used_above_baseline
-[report] flash_text,362056,349992,12064
-[report] flash_data,640,316,324
-[report] static_bss,117936,110876,7060
+[report] flash_text,362736,349992,12744
+[report] flash_data,648,316,332
+[report] static_bss,146208,110876,35332
 [report] heap_used,4440,4440,0
-[report] mbedtls_peak,21280,21332,-52
-[report] mbedtls_free,11488,11436,52
+[report] mbedtls_peak,36056,21332,14724
+[report] mbedtls_free,18216,11436,6780
 [report] lwip_mem_free,7576,7576,0
 [report] lwip_pbufs_free,13,14,-1
 [report] stack_log,832,120,712
-[report] stack_service,1044,52,992
+[report] stack_service,3852,52,3800
 [report] stack_harness,2848,2840,8
 [report] --- end ---
 [device] ready
@@ -29,7 +29,7 @@
 
 ```text
    text	   data	    bss	    dec	    hex	filename
- 362048	    648	 117936	 480632	  75578	/w/build/baseline-cross/baseline.elf
+ 362728	    656	 146208	 509592	  7c698	/w/build/baseline-cross/baseline.elf
 ```
 
 ## Listeners (proved before the device ran)
@@ -47,23 +47,23 @@
 ## Collector (syslog-ng) received
 
 ```text
-wire   <134>1 2026-07-29T10:10:44.420000Z 10.0.2.15 solid-syslog-example - BOOT [meta sequenceId="1" sysUpTime="242"][timeQuality tzKnown="1" isSynced="0"][origin software="solid-syslog-example" swVersion="0.1.0" enterpriseId="32473" ip="10.0.2.15"] device started
-parsed PRIORITY=134 TIMESTAMP=2026-07-29T10:10:44+00:00 HOSTNAME=10.0.2.15 APP_NAME=solid-syslog-example PROCID= MSGID=BOOT STRUCTURED_DATA=[meta sequenceId="1" sysUpTime="242"][timeQuality tzKnown="1" isSynced="0"][origin software="solid-syslog-example" swVersion="0.1.0" enterpriseId="32473" ip="10.0.2.15"] MSG=device started
+wire   <134>1 2026-07-29T10:34:01.430000Z 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"] device started
+parsed PRIORITY=134 TIMESTAMP=2026-07-29T10:34:01+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"] MSG=device started
 ```
 
-## Self-check (vs measurements/origin-ip.csv)
+## Self-check (vs measurements/tls.csv)
 
 ```text
-  OK    flash_text: 362056 (expected 362056, Δ0)
-  OK    flash_data: 640 (expected 640, Δ0)
-  OK    static_bss: 117936 (expected 117936, Δ0)
+  OK    flash_text: 362736 (expected 362736, Δ0)
+  OK    flash_data: 648 (expected 648, Δ0)
+  OK    static_bss: 146208 (expected 146208, Δ0)
   OK    heap_used: 4440 (expected 4440, Δ0)
-  OK    mbedtls_peak: 21280 (expected 21280, Δ0)
-  OK    mbedtls_free: 11488 (expected 11488, Δ0)
+  OK    mbedtls_peak: 36056 (expected 36056, Δ0)
+  OK    mbedtls_free: 18216 (expected 18216, Δ0)
   OK    lwip_mem_free: 7576 (expected 7576, Δ0)
   OK    lwip_pbufs_free: 13 (expected 13, Δ0)
   OK    stack_log: 832 (expected 832, Δ0)
-  OK    stack_service: 1044 (expected 1044, Δ0)
+  OK    stack_service: 3852 (expected 3852, Δ0)
   OK    stack_harness: 2848 (expected 2848, Δ0)
 ```