Added constant time string comparison to avoid possible time-based attacks. (#3836)

* Added constant time strings comparison to avoid possible time-based attacks

* Fixed data types

* Fixed indentation

* Moved string comnparison in constant time to String class; modified function body to assure constant time comparison despite compiler optimizations

* Removed wrong code

* Fixed error and prevented compiler optimization to delete u1 local variable

* Avoid timing attacks on string comparison

* Minor

* changed counter names, removed else

cores/esp8266/WString.cpp

@@ -477,6 +477,33 @@ unsigned char String::equalsIgnoreCase(const String &s2) const {
     return 1;
 }
 
+unsigned char String::equalsConstantTime(const String &s2) const {
+    // To avoid possible time-based attacks present function
+    // compares given strings in a constant time.
+    if(len != s2.len)
+        return 0;
+    //at this point lengths are the same
+    if(len == 0)
+        return 1;
+    //at this point lenghts are the same and non-zero
+    const char *p1 = buffer;
+    const char *p2 = s2.buffer;
+    unsigned int equalchars = 0;
+    unsigned int diffchars = 0;
+    while(*p1) {
+        if(*p1 == *p2)
+            ++equalchars;
+        else
+            ++diffchars;
+        ++p1;
+        ++p2;
+    }
+    //the following should force a constant time eval of the condition without a compiler "logical shortcut"
+    unsigned char equalcond = (equalchars == len);
+    unsigned char diffcond = (diffchars == 0);
+    return (equalcond & diffcond); //bitwise AND
+}
+
 unsigned char String::startsWith(const String &s2) const {
     if(len < s2.len)
         return 0;

cores/esp8266/WString.h

@@ -194,6 +194,7 @@ class String {
         unsigned char operator <=(const String &rhs) const;
         unsigned char operator >=(const String &rhs) const;
         unsigned char equalsIgnoreCase(const String &s) const;
+        unsigned char equalsConstantTime(const String &s) const;
         unsigned char startsWith(const String &prefix) const;
         unsigned char startsWith(const String &prefix, unsigned int offset) const;
         unsigned char endsWith(const String &suffix) const;

libraries/ArduinoOTA/ArduinoOTA.cpp

@@ -229,7 +229,7 @@ void ArduinoOTAClass::_onRx(){
     String result = _challengemd5.toString();
 
     ota_ip.addr = (uint32_t)_ota_ip;
-    if(result.equals(response)){
+    if(result.equalsConstantTime(response)) {
       _state = OTA_RUNUPDATE;
     } else {
       _udp_ota->append("Authentication Failed", 21);

libraries/ESP8266WebServer/src/ESP8266WebServer.cpp

@@ -119,7 +119,7 @@ bool ESP8266WebServer::authenticate(const char * username, const char * password
         return false;
       }
       sprintf(toencode, "%s:%s", username, password);
-      if(base64_encode_chars(toencode, toencodeLen, encoded) > 0 && authReq.equals(encoded)){
+      if(base64_encode_chars(toencode, toencodeLen, encoded) > 0 && authReq.equalsConstantTime(encoded)) {
         authReq = String();
         delete[] toencode;
         delete[] encoded;