Rainbow Electronics AT88SA100S - Manual (Page 16)

16
AT88SA100S [Preliminary]
8558A–SMEM–03/09
4.4. GenPersonalizationKey
This command generates a decryption digest that will be used by the subsequent command (LoadSram) to decrypt the
key value that is to be written into the SRAM. This command must be run immediately prior to LoadSram within the
same watchdog cycle.
This command loads a transport key from an internal secure storage location and then uses that key along with an
input seed to generate a decryption digest using SHA-256. Neither the transport key nor the decryption digest can be
read from the chip. Upon completion, an internal bit is set indicating that the decryption digest has been generated and
is ready to use by LoadSram. This bit is cleared (and the digest lost) when the watchdog timer expires, the chip goes to
sleep or the power is cycled.
Table 15. Input Parameters
Name Size Notes
Opcode GenPers 1 0x20
Param1 Zero 1 Must be 0x00
Param2 KeyID 2 Identification number of the personalization key to be loaded
Data Seed 16 Seed for digest generation. The least significant bit of the last
byte is ignored.
Table 16. Output Parameter
Name Size Notes
Success 1
Upon successful execution, a value of 0 will be returned by the AT88SA100S
chip.
The SHA-256 message body used to create the decryption digest which is internally stored in the chip consists of the
following 512 bits:
256 bits Stored Key[KeyID]
64 bits All 1’s
127 bits Input seed
1 bit ‘1’ pad
64 bits length of message in bits, fixed at 512