TLS disable ECDSA for MQTT to ensure we don't break fingerprints after #22649 (#22656)

CHANGELOG.md

@@ -11,6 +11,7 @@ All notable changes to this project will be documented in this file.
 
 ### Changed
 - ESP32 disable PSRAM check (and on restart some relay toggles) with `#define DISABLE_PSRAMCHECK` (#21266)
+- TLS disable ECDSA for MQTT to ensure we don't break fingerprints after #22649
 
 ### Fixed
 

lib/lib_ssl/tls_mini/src/WiFiClientSecureLightBearSSL.cpp

@@ -195,6 +195,7 @@ void WiFiClientSecure_light::_clear() {
   _recvapp_buf = nullptr;
   _recvapp_len = 0;
   _insecure = false;  // set to true when calling setPubKeyFingerprint()
+  _rsa_only = true;   // for now we disable ECDSA by default
   _fingerprint_any = true; // by default accept all fingerprints
   _fingerprint1 = nullptr;
   _fingerprint2 = nullptr;
@@ -813,7 +814,10 @@ extern "C" {
 
       // The tag string doesn't really matter, but it should differ depending on
       // key type. For ECDSA it's a fixed string.
-      sha1_update_len(&shactx, "ecdsa-sha2-nistp256", 19); // tag
+      sha1_update_len(&shactx, "ecdsa", 5); // tag
+      int32_t curve = eckey.curve;
+      sha1_update_len(&shactx, &curve, 4);   // curve id as int32
+      sha1_update_len(&shactx, "curve", 5); // tag2
       sha1_update_len(&shactx, eckey.q, eckey.qlen);       // exponent
     }
   #endif
@@ -888,16 +892,27 @@ extern "C" {
     BR_TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
     BR_TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
   };
+  static const uint16_t suites_RSA_ONLY[] = {
+    BR_TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
+  };
 #endif
 
   // Default initializion for our SSL clients
-  static void br_ssl_client_base_init(br_ssl_client_context *cc) {
+  static void br_ssl_client_base_init(br_ssl_client_context *cc, bool _rsa_only) {
     br_ssl_client_zero(cc);
     // forbid SSL renegotiation, as we free the Private Key after handshake
     br_ssl_engine_add_flags(&cc->eng, BR_OPT_NO_RENEGOTIATION);
 
     br_ssl_engine_set_versions(&cc->eng, BR_TLS12, BR_TLS12);
+#ifdef ESP8266
     br_ssl_engine_set_suites(&cc->eng, suites, (sizeof suites) / (sizeof suites[0]));
+#else
+    if (_rsa_only) {
+      br_ssl_engine_set_suites(&cc->eng, suites_RSA_ONLY, (sizeof suites_RSA_ONLY) / (sizeof suites_RSA_ONLY[0]));
+    } else {
+      br_ssl_engine_set_suites(&cc->eng, suites, (sizeof suites) / (sizeof suites[0]));
+    }
+#endif
     br_ssl_client_set_default_rsapub(cc);
     br_ssl_engine_set_default_rsavrfy(&cc->eng);
 
@@ -945,7 +960,7 @@ bool WiFiClientSecure_light::_connectSSL(const char* hostName) {
     _ctx_present = true;
     _eng = &_sc->eng; // Allocation/deallocation taken care of by the _sc shared_ptr
 
-    br_ssl_client_base_init(_sc.get());
+    br_ssl_client_base_init(_sc.get(), _rsa_only);
     if (_alpn_names && _alpn_num > 0) {
       br_ssl_engine_set_protocol_names(_eng, _alpn_names, _alpn_num);
     }

lib/lib_ssl/tls_mini/src/WiFiClientSecureLightBearSSL.h

@@ -85,6 +85,10 @@ class WiFiClientSecure_light : public WiFiClient {
       _fingerprint2 = f2;
       _fingerprint_any = f_any;
       _insecure = true;
+      _rsa_only = true;     // if fingerprint, we limit to RSA only
+    }
+    void setRSAOnly(bool rsa_only) {
+      _rsa_only = rsa_only;
     }
     const uint8_t * getRecvPubKeyFingerprint(void) {
       return _recv_fingerprint;
@@ -150,6 +154,7 @@ class WiFiClientSecure_light : public WiFiClient {
 
     bool _fingerprint_any;            // accept all fingerprints
     bool _insecure;                   // force fingerprint
+    bool _rsa_only;                   // restrict to RSA only key exchange (no ECDSA - enabled to force RSA fingerprints)
     const uint8_t *_fingerprint1;          // fingerprint1 to be checked against
     const uint8_t *_fingerprint2;          // fingerprint2 to be checked against
     uint8_t _recv_fingerprint[20];   // fingerprint received

lib/libesp32/HttpClientLight/src/HttpClientLight.cpp

@@ -89,6 +89,7 @@ public:
     {
         BearSSL::WiFiClientSecure_light& wcs = static_cast<BearSSL::WiFiClientSecure_light&>(client);
         wcs.setPubKeyFingerprint(_fingerprint_any, _fingerprint_any, true); // allow all fingerprints
+        wcs.setRSAOnly(false);          // although we use fingerprint, we allow ECDSA
         return true;
     }