Fixed Code (Fixed / Learning Code)
In fixed code systems the transmitter broadcasts exactly the same packet on every press. On common learning encoders the packet typically consists of a 20-bit identity and a 4-bit command field. It contains no cryptographic protection; when the transmission on air is recorded and replayed, the receiver counts it as a valid command.
Fixed code became widespread in garage door, barrier and low-risk remote control applications because it is simple and cheap. Its working logic is this: the identity field says which transmitter is talking, the command field says which key was pressed, and these two fields never change.
- Strength: the encoder and decoder are very simple, learning is done with a single key, and cost is low.
- Weakness: because the packet is the same every time, it is open to a replay attack.
- Limit: even though the identity field is wide, this provides not confidentiality but only discrimination.
For this reason fixed code alone is not considered sufficient in risk-bearing applications such as moving loads, lifting and hydraulic levers; rolling code, encryption and session authentication are expected. The term fixed code may come up as a feature you read during product selection; it is important to distinguish that the expression encoded does not mean encrypted.
Context in which this term is used: Receiver-transmitter pairing procedure