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HCS473SLSQTP Datasheet(PDF) 6 Page - Microchip Technology |
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HCS473SLSQTP Datasheet(HTML) 6 Page - Microchip Technology |
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6 / 68 page  HCS473 DS40035C-page 4 Preliminary 2002 Microchip Technology Inc. • Transponder Reader (Reader, for short) - A device that authenticates a transponder using bi- directional communication. • Transport code - An access code, ‘password’ known only by the manufacturer, allowing write access to certain secure device memory areas (Section 3.2.3.2). 1.2 Encoder Overview The HCS473 code hopping transcoder is designed specifically for passive entry systems; particularly vehi- cle access. The transcoder portion of a passive entry system is integrated into a fob, carried by the user and operated to gain access to a vehicle or restricted area. The HCS473 is meant to be a cost-effective yet secure solution to such systems, requiring very few external components (Figure 2-1). 1.2.1 LOW-END SYSTEM SECURITY RISKS Most low-end keyless entry transmitters are given a fixed identification code that is transmitted every time a button is pushed. The number of unique identification codes in a low-end system is usually a relatively small number. These shortcomings provide an opportunity for a sophisticated thief to create a device that ‘grabs’ a transmission and retransmits it later, or a device that quickly ‘scans’ all possible identification codes until the correct one is found. 1.2.2 HCS473 SECURITY The HCS473, on the other hand, employs the K EELOQ code hopping technology coupled with a transmission length of 69 bits to virtually eliminate the use of code ‘grabbing’ or code ‘scanning’. The high security level of the HCS473 is based on the patented K EELOQ technol- ogy. A block cipher based on a block length of 32 bits and a key length of 64 bits is used. The algorithm obscures the information in such a way that even if the transmission’s pre-encrypted information differs by only one bit from that of the previous transmission, sta- tistically greater than 50 percent of the transmission’s encrypted result will change. 1.2.3 HCS473 HOPPING CODE The 16-bit synchronization counter is the basis behind the transmitted code word changing for each transmis- sion; it increments each time a button is pressed. Once the device detects a button press, it reads the button inputs and updates the synchronization counter. The synchronization counter and crypto key are input to the encryption algorithm and the output is 32 bits of encrypted information. This encrypted data will change with every button press, its value appearing externally to ‘randomly hop around’, hence it is referred to as the hopping portion of the code word. The 32-bit hopping code is combined with the button information and serial number to form the code word transmitted to the receiver. The code word format is explained in greater detail in Section 3.1.2. 1.3 Identify Friend or Foe (IFF) Overview Validation of a transponder first involves an authenti- cating device sending a random challenge to the device. The transponder then replies with a calculated response that is a function of the received challenge and its stored crypto key. The authenticating device, transponder reader, performs the same calculation and compares it to the transponder’s response. If they match, the transponder is identified as valid and the transponder reader can take appropriate action. The HCS473’s IFF response is generated using one of two possible crypto keys. The authenticating device precedes the challenge with a three bit field dictating which key to use in calculating the response. The bi-directional communication path required for IFF is typically inductive for short range (<10cm) transpon- der applications with an inductive challenge and induc- tive response. Longer range (~1.5m) passive entry applications still transmit using the LF inductive path but the response is transmitted RF. |
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