encryption with protected hmac key

noisemeter-device/noisemeter-device.ino

@@ -284,8 +284,8 @@ void saveNetworkCreds(WebServer& httpServer) {
   // Confirm that the form was actually submitted.
   if (httpServer.hasArg("ssid") && httpServer.hasArg("psk")) {
     const auto id = String(buildDeviceId());
-    const auto ssid = Secret(id).encrypt(httpServer.arg("ssid"));
-    const auto psk = Secret(id).encrypt(httpServer.arg("psk"));
+    const auto ssid = Secret::encrypt(id, httpServer.arg("ssid"));
+    const auto psk = Secret::encrypt(id, httpServer.arg("psk"));
 
     // Confirm that the given credentials will fit in the allocated EEPROM space.
     if (!ssid.isEmpty() && Creds.canStore(ssid) && Creds.canStore(psk)) {
@@ -323,7 +323,7 @@ int tryWifiConnection()
 
   WiFi.mode(WIFI_STA);
   const auto id = String(buildDeviceId());
-  const auto stat = WiFi.begin(Secret(id).decrypt(ssid).c_str(), Secret(id).decrypt(psk).c_str());
+  const auto stat = WiFi.begin(Secret::decrypt(id, ssid).c_str(), Secret::decrypt(id, psk).c_str());
   if (stat == WL_CONNECT_FAILED)
     return -1;
 

noisemeter-device/secret-store.cpp

@@ -0,0 +1,51 @@
+#include "secret-store.h"
+
+#include <esp_hmac.h>
+#include <mbedtls/aes.h>
+
+constexpr static unsigned BITS = 256; // do not change
+
+namespace Secret {
+
+String encrypt(String key, String in)
+{
+    mbedtls_aes_context aes;
+    mbedtls_aes_init(&aes);
+
+    const auto kb = key.c_str();
+    const auto kl = key.length();
+    {
+        uint8_t hmac[BITS / 8];
+        esp_hmac_calculate(HMAC_KEY0, kb, kl, hmac);
+        mbedtls_aes_setkey_enc(&aes, hmac, BITS);
+    }
+
+    char out[in.length()];
+    mbedtls_aes_crypt_ecb(&aes, MBEDTLS_AES_ENCRYPT,
+        reinterpret_cast<const uint8_t *>(in.c_str()),
+        reinterpret_cast<uint8_t *>(out));
+    return out;
+}
+
+String decrypt(String key, String in)
+{
+    mbedtls_aes_context aes;
+    mbedtls_aes_init(&aes);
+
+    const auto kb = key.c_str();
+    const auto kl = key.length();
+    {
+        uint8_t hmac[BITS / 8];
+        esp_hmac_calculate(HMAC_KEY0, kb, kl, hmac);
+        mbedtls_aes_setkey_dec(&aes, hmac, BITS);
+    }
+
+    char out[in.length()];
+    mbedtls_aes_crypt_ecb(&aes, MBEDTLS_AES_DECRYPT,
+        reinterpret_cast<const uint8_t *>(in.c_str()),
+        reinterpret_cast<uint8_t *>(out));
+    return out;
+}
+
+} // namespace Secret
+

noisemeter-device/secret-store.h

@@ -1,42 +1,13 @@
-#include <esp_hmac.h>
-#include <mbedtls/aes.h>
+#ifndef SECRET_STORE_H
+#define SECRET_STORE_H
 
-class Secret
-{
-  constexpr static int BITS = 256; // do not change
-  mbedtls_aes_context aes;
-  uint8_t hmac[BITS / 8];
-
-  bool generateKey(String key) {
-    const auto result = esp_hmac_calculate(HMAC_KEY4, key.c_str(), key.length(), hmac);
-    return result == ESP_OK;
-  }
-
-  String process(String in, int mode) {
-    uint8_t out[64] = {0};
-    mbedtls_aes_crypt_ecb(&aes, mode, (const uint8_t *)in.c_str(), out);
-    return String((char *)out);
-  }
-
-public:
-  Secret(String key) {
-    mbedtls_aes_init(&aes);
-    generateKey(key);
-  }
+#include <WString.h>
 
-  ~Secret() {
-    mbedtls_aes_free(&aes);
-  }
-
-  String encrypt(String in) {
-    mbedtls_aes_setkey_enc(&aes, hmac, BITS);
-    return process(in, MBEDTLS_AES_ENCRYPT);
-  }
-
-  String decrypt(String in) {
-    mbedtls_aes_setkey_dec(&aes, hmac, BITS);
-    return process(in, MBEDTLS_AES_DECRYPT);
-  }
-};
+namespace Secret
+{
+    String encrypt(String key, String in);
+    String decrypt(String key, String in);
+}
 
+#endif // SECRET_STORE_H