fix: critical security issues from code review

- Thread safety: Add @Synchronized methods for ECDH buffer access
- Crypto: Use X509EncodedKeySpec instead of hard-coded EC curve params
- Sensitive data: Make aes_key/shared_secret private in CryptoECDH
- Credential storage: Migrate PSK/UUID to EncryptedSharedPreferences

src/client.cpp

@@ -260,7 +260,7 @@ static bool performEcdhHandshake(NimBLERemoteService *pSvc) {
         return false;
     }
 
-    memcpy(aesKey, ecdh.aes_key, AES_KEY_LEN);
+    memcpy(aesKey, ecdh.getAesKey(), AES_KEY_LEN);
     ecdh.free();
     ecdhReady = true;
     ESP_LOGI(TAG, "ECDH: handshake complete");

src/ecdh_helper.h

@@ -21,6 +21,9 @@ private:
     mbedtls_ctr_drbg_context ctr_drbg;
     bool initialized = false;
 
+    uint8_t shared_secret[32];
+    uint8_t aes_key[AES_KEY_LEN];
+
     CryptoECDH(const CryptoECDH&) = delete;
     CryptoECDH& operator=(const CryptoECDH&) = delete;
 
@@ -29,8 +32,9 @@ public:
 
     uint8_t public_key[ECDH_PUBKEY_LEN];
     size_t pub_len = 0;
-    uint8_t shared_secret[32];
-    uint8_t aes_key[AES_KEY_LEN];
+
+    const uint8_t* getAesKey() const { return aes_key; }
+    size_t getAesKeyLen() const { return AES_KEY_LEN; }
 
     bool init() {
         mbedtls_ecdh_init(&ecdh);

src/main.cpp

@@ -406,7 +406,7 @@ class EcdhCharacteristicCallbacks : public NimBLECharacteristicCallbacks {
             return;
         }
 
-        memcpy(clients[idx].aesKey, clients[idx].ecdh.aes_key, AES_KEY_LEN);
+        memcpy(clients[idx].aesKey, clients[idx].ecdh.getAesKey(), AES_KEY_LEN);
 
         uint8_t resp[ECDH_PKT_SIZE];
         memcpy(resp, clients[idx].ecdh.public_key, ECDH_PUBKEY_LEN);