SCS152

DS40150B-page 10

Preliminary

© 1997 Microchip Technology Inc.

4.0

SIGNATURE FUNCTION

The signature function combines a system-supplied

value with the contents of the memory map to form an

8-bit value. Because UKEY0 is not known and the way

in which these values are combined, it is very difficult to

duplicate.

The signature function can, therefore, be used for a

variety of security related functions, including user/card

authentication and protecting against communication

tampering.

4.1

Signature Procedure

The following procedure controls the device to compute

a signature of the EEPROM contents:

1.

Issue a RESET command.

2.

Issue 259 RDBIT commands.

3.

Pulse SCI high twice.

4.

Issue 16 PUTBIT commands to enter the first 16

bits of the challenge.

5.

Issue 128 RDBIT commands.

Note: The bits read are the first 128 bits in the

EEPROM without any output mapping;

these bits must be used in checking the

result of the signature function.

6.

Issue 16 PUTBIT commands to enter the sec-

ond 16 bits of the challenge.

7.

Wait for the signature computation to complete.

8.

Issue 8 RDBIT command to read the result of

the signature computation.

9.

Updating the token counter.

4.2

Subtraction Procedure

The following procedure can be used to subtract a

number of tokens:

1.

Read the token counter value.

2.

Convert the token counter value into an octal for-

mat.

3.

Convert the number of tokens to subtract into an

octal format.

4.

Do the subtraction using ‘long subtraction’ mak-

ing note of the borrows.

Note: If the subtraction generated a borrow from

digit 4, the token counter is smaller than the

number subtracted.

5.

To update each digit in the order they are read

out (i.e., D4, then D3, then D2, etc.):

a)

Calculate by how much this digit must be

decremented. If,

during

subtraction,

a

borrow was generated from this digit to the

next higher digit, the decrement amount is

eight minus this digit’s final value, else it is

its initial value minus its final value.

b)

Read to the first ‘1’ bit in the field.

c)

If the following digit generated a borrow from

this digit, issue a BITPROG command, fol-

lowed by an ERASE command. (See

Section 5.1 on fail safe counter updating.)

d)

Decrement this digit as many times as cal-

culated in Step 5a, taking into account that

Step 5c may already have decremented this

digit. Each decrement is done by reading

the next bit in the field and issuing

a BITPROG command.

EXAMPLE 4-1:

SUBTRACT EXAMPLE

‘long subtraction’ to do the calculation the result is

Note the borrows from digits D2 and D1.

Digits 4 and 3 do not change. The sequence of com-

mands to update digit 2 is: read the second bit in the

field to field D2, BITPROG, and ERASE. The ERASE

command is required because of the borrow.

Updating digit ‘1’ requires the following sequence of

commands: read the first bit to field D1, BITPROG,

ERASE, RDBIT, and BITPROG. Once again, the

ERASE command is required because of the borrow.

Updating digit ‘0’ requires the sequence of commands:

read the first bit to field D0 and BITPROG.

EXAMPLE 4-2:

DECREMENTING 320:

Updating this token counter goes through the following

Note:

The final BITPROG actually decrements

the value.

30713

8

00024

8

30667

8

-----------------

00500

8

00001

8

00477

8

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