diff --git a/src/encString.cpp b/src/encString.cpp
index 3fd97c7..3f629c7 100644
--- a/src/encString.cpp
+++ b/src/encString.cpp
@@ -3,6 +3,23 @@
 #include <libb64/cdecode.h>
 #include <cutils.hpp>
 
+bool isBase64(const char *p)
+{
+    char c;
+
+    if (strlen(p) % 4 != 0)
+        return false;
+    do {
+        c = *p;
+        if (isalnum(c))   // alphanumberics are OK
+            continue;
+        if (c == '/' || c == '+' || c == '=')   // + or / or = is OK.
+            continue;
+        return false;
+    } while (*++p != 0);
+    return true;
+}
+
 void strEncode(String &str)
 {
 
@@ -41,17 +58,20 @@ void strDecode(String &str)
     uint32_t cid = ESP_getChipId();
     uint32_t *pkey = (uint32_t *)&key[0];
     uint8_t data[str.length()];
-    unsigned int clen = base64_decode_chars(src, str.length(), (char *)data);
     String result = F("");
 
-    if (0 != clen) {
-        *pkey = cid;
-        for (i = 0; i < clen; i++) {
-            b = data[i];
-            b ^= key[enciphidx];
-            ++enciphidx;
-            enciphidx %= sizeof(key);
-            result.concat(char(b));
+    if (isBase64(str.c_str())) {
+        unsigned int clen = base64_decode_chars(src, str.length(), (char *)data);
+
+        if (0 != clen) {
+            *pkey = cid;
+            for (i = 0; i < clen; i++) {
+                b = data[i];
+                b ^= key[enciphidx];
+                ++enciphidx;
+                enciphidx %= sizeof(key);
+                result.concat(char(b));
+            }
         }
     }
     str = result;
diff --git a/src/encString.hpp b/src/encString.hpp
index bd1fbc9..f1835fa 100644
--- a/src/encString.hpp
+++ b/src/encString.hpp
@@ -16,4 +16,13 @@ void strEncode(String &str);
  */
 void strDecode(String &str);
 
+/**
+ * @brief Overeni, ze je retezec Base64
+ * 
+ * @param p Ukazatel na retezec
+ * @return true Retezec je kodovany pomoci Base64
+ * @return false Retezec neni kodovany pomoci Base64
+ */
+bool isBase64(const char *p);
+
 #endif