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HCS412I Datasheet(PDF) 3 Page - Microchip Technology |
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HCS412I Datasheet(HTML) 3 Page - Microchip Technology |
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3 / 44 page  © 2002 Microchip Technology Inc. Preliminary DS41099C-page 3 HCS412 1.1 Encoder Overview The HCS412 code hopping transcoder is designed specifically for passive entry systems; primarily vehicle access. The transcoder portion of a passive entry sys- tem is integrated into a transmitter, carried by the user and operated to gain access to a vehicle or restricted area. The HCS412 is meant to be a cost-effective yet secure solution to such systems, requiring very few external components (Figure 2-6). 1.1.1 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. 1.1.2 HCS412 SECURITY The HCS412, on the other hand, employs the KEELOQ 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 HCS412 is based on the patented KEELOQ 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 information (before coding) differs by only one bit from that of the previous transmission, statisti- cally greater than 50 percent of the next transmission’s encrypted bits will change. 1.1.3 HCS412 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 crypt 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 3.2. FIGURE 1-1: BUILDING THE TRANSMITTED CODE WORD (ENCODER) 1.2 Identify Friend or Foe (IFF) Overview Validation of a token first involves an authentication device sending a random challenge to the token. The token then replies with a calculated response that is a function of the received challenge and the stored crypt key. The authentication device, transponder reader, performs the same calculation and compares it to the token’s response. If they match, the token is identified as valid and the transponder reader can take appropri- ate action. The HCS412’s 32-bit IFF response is generated using one of two possible encryption algorithms and one of two possible crypt keys; four combinations total. The authenticating device precedes the challenge with a five bit command word dictating which algorithm and 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 and an inductive challenge, RF response for longer range (~1.5m) passive entry appli- cations. Button Press Information EEPROM Array 32 Bits of Encrypted Data Serial Number Transmitted Information Crypt Key Sync Counter Serial Number KEELOQ Encryption Algorithm |
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