DS5003 Datasheet, PDF (18/24 Pages) Maxim Integrated Products – Secure Microprocessor Chip

Secure Microprocessor Chip
When the application software is executed, the
DS5003’s internal CPU operates as normal. Logical
addresses are calculated for op code fetch cycles and
also data read and write operations. The DS5003 can
perform address encryption on logical addresses as
they are generated internally during the normal course
of program execution. In a similar fashion, data is
manipulated by the CPU in its true representation.
However, data is also encrypted when it is written to the
external program/data SRAM, and is restored to its
original value when it is read back.
When an application program is stored in the previously
described format, it is virtually impossible to disassem-
ble op codes or to convert data back into its true repre-
sentation. Address encryption has the effect that the op
codes and data are not stored in the contiguous form in
which they were assembled, but rather in seemingly
random locations in memory. This effect makes it virtu-
ally impossible to determine the normal flow of the pro-
gram. As an added protection measure, the address
encryptor also generates dummy read-access cycles
whenever time is available during program execution.
Dummy Read Cycles
Like the DS5002FP, the DS5003 generates a dummy
read-access cycle to nonsequential addresses in exter-
nal SRAM memory whenever time is available during
program execution. This action further complicates the
task of determining the normal flow of program execu-
tion. During these pseudorandom dummy cycles, the
SRAM is read to all appearances, but the data is not
used internally. Through the use of a repeatable
exchange of dummy and true read cycles, it is impossi-
ble to distinguish a dummy cycle from a real one.
Encryption Algorithm
The DS5003 incorporates a proprietary hardware algo-
rithm that performs the scrambling of address and data
on the byte-wide bus to the SRAM. Improvements
include the following:
• 64-bit encryption key (protected by the security
lock function).
• Incorporation of DES-like operations to provide a
greater degree of nonlinearity.
• Customizable encryption.
Encryption Key
As previously described, the on-chip 64-bit encryption
key is the basis of both the address and data encryptor
circuits. When the loader is given certain commands,
the key is generated from an on-chip hardware ran-
dom-number generator. This action is performed just
prior to actually loading the code into the external
SRAM. This scheme prevents characterization of the
encryption algorithm by continuously loading new,
known keys. It also frees the user from the burden of
protecting the key selection process.
The random-number generator circuit uses the asyn-
chronous frequency differences of two internal ring
oscillators and the processor master clock (determined
by XTAL1 and XTAL2). As a result, a true random num-
ber is produced.
Vector RAM
A 48-byte vector RAM area is incorporated on-chip,
and is used to contain the reset and interrupt vector
code in the DS5003. It is included in the architecture to
help ensure the security of the application program.
If reset and interrupt vector locations were accessed
from the external nonvolatile program/data RAM during
the execution of the program, it would be possible to
determine the encrypted value of known addresses.
This could be done by forcing an interrupt or reset con-
dition and observing the resulting addresses on the
byte-wide address/data bus. For example, it is known
that when a hardware reset is applied, the logical pro-
gram address is forced to location 0000h and code is
executed starting from this location. It would then be
possible to determine the encrypted value (or physical
address) of the logical address value 0000h by observ-
ing the address presented to the external SRAM follow-
ing a hardware reset. Interrupt vector address
relationships could be determined in a similar fashion.
By using the on-chip vector RAM to contain the inter-
rupt and reset vectors, it is impossible to observe such
relationships. The vector RAM eliminates the unlikely
possibility that an application program could be deci-
phered by observing vector address relationships. Note
that the dummy accesses mentioned are conducted
while fetching from vector RAM.
The vector RAM is automatically loaded with the user’s
reset and interrupt vectors from the Intel hex file during
bootstrap loading.
Security Lock
Once the application program has been loaded into the
DS5003’s external and vector RAM, the security lock
can be enabled by issuing the Z command in the boot-
strap loader. While the security lock is set, no further
access to program/data information is possible by the
on-chip ROM. Access is prevented by both the boot-
strap loader firmware and the DS5003 encryptor cir-
cuits.
Access to the SRAM can only be regained by clearing
the security lock by the U command in the bootstrap
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