A static-loop-current attack against the Kirchhoff-law-Johnson-Noise (KLJN) secure key exchange system Mutaz Y. Melhem and Laszlo B. Kish * Department of Electrical and Computer Engineering, Texas A & M University, 3128 TAMU, College Station, TX, 77843, USA; E-Mails: yar111@tamu.edu(MYM); Laszlo.Kish@ece.tamu.edu (L.B.K.) * Correspondence: Laszlokish@tamu.edu; Tel.: +1-979-847-9071 Received: date; Accepted: date; Published: date Abstract: A new attack against the Kirchhoff-Law-Johnson-Noise (KLJN) key distribution system is explored. The attack is based on utilizing a parasitic dc-voltage-source in the loop. Relevant situations often exist in the low-frequency limit in practical systems, especially when the communication is over a distance, due to a ground loop and/or electromagnetic interference (EMI). Surprisingly, the usual current/voltage comparison based defense method that exposes active attacks or parasitic features (such as wire resistance allowing information leak) does not function here. The attack is successfully demonstrated. Proposed defense methods against it are shown. Keywords: KLJN key exchange; unconditional security; ground loop vulnerability; passive attack. 1. Introduction 1.1 On secure communications Communications systems, standards and technologies have been developed since ancient history. Today we have the internet, Internet-of-Things (IoT), the operating 4 th generation wireless networks (LTE), and the expected 5 th generation wireless networks. An important requirement of any communication paradigm between these devices is to accomplish the communication securely. That is, to protect the privacy and integrity of the users’ data that is transferred over the network. To achieve the security of the transferred data that can contain sensitive information (e.g., bank account credentials, social security number, etc.) it is of utmost importance to defend against attacks. These attacks might be launched by an eavesdropper (Eve) who has access to the information channel between the communicating parties A (Alice) and B (Bob). The attack is passive if it is an eavesdropping without disturbing the channel. The attack is active (invasive) if Eve disturbs or changes the channel, such as during the man-in-the-middle attack. In the present paper, we introduce a new passive attack against the KLJN secure key exchange scheme. 1.1.1 Secure key exchange