Security enforcement in a system with a multiplicity of end units

A method for authenticating a source of a signal in a system comprising a multiplicity of electrical units, the method comprising:

• receiving by a receiver a signal comprising a command;
• retrieving a characteristic of the signal;
• associating the characteristic with an alleged transmitter of the signal;
• determining a transmitter of the signal;
• decoding the command from the signal;
• and
• subject to the transmitter being the same as the alleged transmitter, and that the transmitter of the signal is authorized to send the command to the receiver, performing the command,
• wherein the characteristic is selected from the group consisting of:
• an average rise time of a multiplicity of clock pulses of the transmitter of the signal;
• an average fall time of the multiplicity of clock pulses of the transmitter of the signal;
• and a jitter of the multiplicity of clock pulses of the transmitter of the signal.

A method for authenticating a source of a signal in a system comprising a multiplicity of electrical units connected along a transmission line, the method comprising:

• receiving a transmission;
• pushing current along the transmission line;
• measuring impedance along the transmission line in response to the current;
• determining a transmitter by comparing the impedance to at least one stored impedance;
• extracting an alleged transmitter from the signal;
• decoding a command from the signal;
• and
• subject to the transmitter being the same as the alleged transmitter, and to the transmitter is authorized to transmit the command to the receiver, performing the command.

A system comprising:

• a multiplicity of electrical units, each electrical unit configured to send or receive messages, wherein at least one of the electrical units comprises a processor adapted to:
• receive by a receiver a signal comprising a characteristic and a command;
• retrieve the characteristic of the signal;
• associate the characteristic with an alleged transmitter of the signal;
• determine a transmitter of the signal;
• decode the command from the signal;
• and
• subject to the transmitter being the same as the alleged transmitter, and that the transmitter of the signal is authorized to transmit the command to the receiver, perform the command,
• wherein the characteristic is a digital characteristic,
• wherein the characteristic comprises a unique key sequence, such that digital codes of any unique key sequences within the system are orthogonal to each other, and
• wherein associating the characteristic with the transmitter of the signal comprises performing internal multiplication of the signal with one or more unique key sequences, each unique key sequence associated with one transmitter, wherein the transmitter is associated with the unique key sequence of at least one unique key sequences for which the internal multiplication is non-zero.