DS5003 Datasheet, PDF (19/24 Pages) Maxim Integrated Products – Secure Microprocessor Chip

Secure Microprocessor Chip
loader. This action triggers several events that defeat
tampering. First, the encryption key is instantaneously
erased. Without the encryption key, the DS5003 can no
longer decrypt the contents of the SRAM. Therefore, the
application software can no longer be correctly execut-
ed, nor can it be read back in its true form by the boot-
strap loader. Second, the vector RAM area is also
instantaneously erased, so that the reset and vector
information is lost. Third, the bootstrap loader firmware
sequentially erases the encrypted SRAM area. Lastly,
the loader creates and loads a new random key.
The security lock bit is constructed using a multiple-bit
latch that is interlaced for self-destruction in the event
of tampering. The lock is designed to set up a “domino
effect” such that erasure of the bit results in an unstop-
pable sequence of events that clears critical data
including encryption key and vector RAM. In addition,
this bit is protected from probing by the top-coating
feature.
Self-Destruct Input (SDI)
The self-destruct input (SDI) pin is an active-high input
that is used to reset the security lock in response to a
variety of user-defined external events. The SDI input is
intended to be used with external tamper-detection cir-
cuitry. It can be activated with or without operating
power applied to the V_CC pin. Activation of the SDI pin
instantly resets the security lock and causes the same
sequence of events previously described for this
action. In addition, power is momentarily removed from
the byte-wide bus interface including the V_CC pin,
resulting in the loss of data in external SRAM.
Top-Layer Coating
The DS5003M is provided with a special top-layer coat-
ing that is designed to prevent a probe attack. This
coating is implemented with second-layer metal added
through special processing of the microcontroller die.
This additional layer is not a simple sheet of metal, but
rather a complex layout that is interwoven with power
and ground, which are in turn connected to logic for the
encryption key and the security lock. As a result, any
attempt to remove the layer or probe through it results
in the erasure of the security lock and/or the loss of
encryption key bits.
Bootstrap Loading
Initial loading of application software into the DS5003 is
performed by firmware within the on-chip bootstrap
loader communicating with a PC by the on-chip serial
port. Table 1 summarizes the commands accepted by
the bootstrap loader.
When the bootstrap loader is invoked, portions of the
256-byte scratchpad RAM area are automatically over-
written with zeros and then used for variable storage for
the bootstrap firmware. Also, a set of 8 bytes is gener-
ated using the random-number generator circuitry and
saved as a potential word for the 64-bit encryption key.
Any read or write operation to the DS5003’s external
program/data SRAM can only take place if the security
lock bit is in a cleared state. Therefore, the first step in
loading a program should be the clearing of the securi-
ty lock bit through the U command.
Table 1. Serial Bootstrap Loader
Commands
COMMAND
FUNCTION
C
Return CRC-16 of the program/data SRAM.
D
Dump RAM memory specified by MSL bit as
Intel hex format.
F
Fill program/data SRAM.
G
Get data from P0, P1, P2, and P3.
L
Load Intel hex file.
N
Set freshness seal—all program and data is
lost.
P
Put data into P0, P1, P2, and P3.
R
Read status of SFRs (MCON, RPCTL, MSL).
T
Trace (echo) incoming Intel hex code.
U
Clear security lock.
V
Verify program/data memory with incoming
Intel hex data.
W
Write special function registers (MCON,
RPCTL, MSL).
Z
Set security lock.
Execution of certain bootstrap loader commands result
in the loading of the newly generated 64-bit random
number into the encryption key word. These commands
are as follows:
Fill
F
Load L
Dump D
Verify V
CRC C
Execution of the Fill and Load commands load the
encrypted data into SRAM using encryption keys from
the newly generated key word. The subsequent execu-
tion of the Dump command within the same bootstrap
session causes the contents of the encrypted SRAM to
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