DS2705: SHA-1 Authentication Master

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MAC Comparison

After the SHA-1 computation is completed by the remote token, the DS2705 and remote SHA-1 token both contain

a MAC result based on the secret key. The results are compared by the DS2705 on a bit by bit basis as the MAC

data is read in from the remote token. Note that the secret is never transmitted on the bus and thus cannot be

captured by observing bus traffic.

Multiple Authentication Attempts

The DS2705 is configurable for multiple authentication attempts or re-tries to avoid reporting authentication failure

in the event of contact bounce or a noisy communication channel. When configured for more than one retry, the

status outputs are kept at the previous state until one attempt succeeds or all attempts fail. It is always

recommended to configure the DS2705 for at least one retry.

Signaling Authentication Results

Authentication results are signaled on the open drain

PASS and FAIL output pins. During an authentication attempt,

both outputs remain at their previous state. After authentication is complete, the pass or fail status is reported until

the display is cleared by one of the following conditions:

§

CHAL pin returning to inactive logic level.

§

Battery token removal detected when no 1-Wire Presence Pulse is returned in response to a 1-Wire

Reset.

Table 4. PASS/FAIL Outputs

CONDITION

FOM BIT

PASS OUTPUT

FAIL OUTPUT

Token Not Present

x

Hi-Z

Hi-Z

Authentication in

Progress

x

No Change

No Change

Complete: Pass

x

LOW

Hi-Z

0

Hi-Z

LOW

Complete: Fail

1

Hi-Z

Pulse

PROGRAMMING AND CONFIGURING

The DS2705 requires a configuration step prior to deployment to program the 64-bit challenge, 160-bit response

and to set up desired configuration options. Configuration is performed in slave mode using the SDQ and VPP

pins. The Challenge-and-Response pair, and option data are programmed in on-chip EEPROM that requires an

externally supplied programming voltage. After programming and verifying the EEPROM data, setting of the Lock

bits is recommended to prevent future modification. SDQ and VPP have internal pull downs which prevent the pins

from floating during normal operation.