DS2703 Dallas Semiconductor, DS2703 Datasheet - Page 7

• Manufacturer Part Number: DS2703
• Description: SHA-1 Battery Pack Authentication IC
• Manufacturer: Dallas Semiconductor

SHA Computation
The variables A, B, C, D, E and constants H0, H1, H2, H3, and H4 are initialized as follows:
A
B
C
D
E
The final values of variables A, B, C, D, and E are generated by looping through the following set of computations
for t = 0 to 79 (discarding any carry-out). Finally, the H0-H4 constants are added to the A-E variables respectively,
which are then concatenated to form the 160-bit MAC, ABCDE.
for ( t = 0 to 79 )
{
}
160-bit MAC := (A+H0) | (B+H1) | (C+H2) | (D+H3) | (E+H4)
DS2703 AUTHENTICATION COMMANDS
WRITE CHALLENGE [0Ch]. This command writes 64 bits in the message block. The LSB of the 64-bit data can
begin immediately after the MSB of the command has been completed. If more than 8 bytes are written, the final
value in the challenge register will be indeterminate. The Compute MAC and Compute Next Secret (with or without
ROM ID) function commands clear the challenge value. Therefore the Write Challenge command must be issued
prior to every Compute MAC or Compute Next Secret command for reliable results.
COMPUTE MAC WITHOUT ROM ID [36h]. This command initiates a SHA-1 computation on the 512 bit block
comprised of words W0 - W15. The 64-bit secret and the 64-bit challenge are loaded in the message block. The
DS2703 takes up to 100us after receiving this command to begin computing the MAC. This gives the host ample
time to connect the DQ pin to a low impedance node prior to the high current demand computation. The DQ pin
must not fall below V
data communications (i.e. terminate the low source impedance mode). After the DQ pin has returned to normal
impedance, the host must write eight write zero time slots and then issue 160 read time slots to get the MAC. The
32-bit registers A, B, C, D, and E are used during every cycle of the hash algorithm and their final values at
calculation cycle t=79 are added to the values H0-H4 and stored in registers A-E. The new word ABCDE is now the
MAC. After issuing the command and waiting a minimum of t
allows the use of a master secret and message digest response independent of the ROM ID.
COMPUTE MAC WITH ROM ID [35h]
This command is structured the same as the Compute MAC without ROM ID, except that the ROM ID is loaded to
the message block. Including the ROM ID unique to each DS2703 in the MAC computation allows the use of a
unique secret in each token and a master secret in the host device. See application note “White Paper 4”, available
at http://www.maxim-ic.com, for more information.
SHA-1 related commands used while authenticating a battery or peripheral device are summarized in Table for
convenience. Four additional commands for clearing, computing and locking of the Secret are described in detail in
the following section.
:=
:=
:=
:=
:=
TMP
E
D
C
B
A
67452301h
EFCDAB89h
98BADCFEh
10325476h
C3D2E1F0h
:=
:=
:=
:=
:=
:=
PULLUP_MIN
D
C
S
A
TMP
S
30
5
(A) + F
(B)
during the computation period, t
t
(B,C,D) + W
H0
H1
H2
H3
H4
t
+ K
:=
:=
:=
:=
:=
t
+ E
7 of 20
67452301h
EFCDAB89h
98BADCFEh
10325476h
C3D2E1F0h
COMP
COMP
, the host reads the 20-byte
. The host must release the DQ pin for 1-Wire
MAC.
This command