DS5003 Datasheet, PDF (17/24 Pages) Maxim Integrated Products – Secure Microprocessor Chip

Secure Microprocessor Chip
Security Circuitry
Figure 9 shows the on-chip functions associated with
the DS5003’s software security feature. Encryption logic
consists of an address encryptor and a data encryptor.
Although each encryptor uses its own algorithm for
encrypting data, both depend on the 64-bit key word
that is contained in the encryption key registers. Both
the encryptors operate during loading of the application
software and also during its execution.
The address encryptor translates each logical address,
i.e., the normal sequence of addresses that are gener-
ated in the logical flow of program execution, into an
encrypted address (or physical address) at which the
byte is actually stored. Each time a logical address is
generated, either during program loading or during
program execution, the address encryptor circuitry
uses the value of the 64-bit key word and of the
address itself to form the physical address, which are
presented on the address lines of the SRAM. The
encryption algorithm is such that there is one and only
one physical address for every possible logical
address. The address encryptor operates over the
entire memory range, which is configured during boot-
strap loading for access on the byte-wide bus.
As bootstrap loading of the application software is per-
formed, the data encryptor logic transforms the op
code, operand, or data byte at any given memory loca-
tion into an encrypted representation. As each byte is
read back to the CPU during program execution, the
internal data encryptor restores it to its original value.
When a byte is written to the external nonvolatile pro-
gram/data SRAM during program execution, that byte
is stored in encrypted form as well. The data encryption
logic uses the value of the 64-bit key, the logical
address to which the data is being written, and the
value of the data itself to form the encrypted data,
which is written to the nonvolatile program/data SRAM.
The encryption algorithm is repeatable, such that for a
given data value, encryption key value, and logical
address the encrypted byte is always the same.
However, there are many possible encrypted data val-
ues for each possible true-data value due to the algo-
rithm’s dependency on the values of the logical
address and encryption key.
PROGRAM
COUNTER
DATA
POINTER
SECURE INTERNAL ADDRESS BUS
ADDRESS
ENCRYPTOR
ENCRYPTED BYTE-WIDE
ADDRESS BUS
16
BOOTSTRAP
LOADER
RANDOM-
NUMBER
GENERATOR
SECURITY
LOCK
64-BIT ENCRYPTION KEY
EXTERNAL
BYTE-WIDE
RAM
SECURE INTERNAL DATA BUS
DATA
ENCRYPTOR
ENCRYPTED BYTE-WIDE
DATA BUS
8
SDI
(SELF-DESTRUCT
INPUT)
Figure 9. Security Circuitry
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