DS1963S

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Read Authenticated Page [A5h]

This command, which is applicable to pages 0 to 15 only, combines reading a full or partial memory page

with the computation of a SHA-1 message authentication code. After the master has issued the command

code and specified a valid target address it will receive the page data beginning at the target address

through the end of the data page, the value of the write-cycle counter of the page, the value of the write

cycle of the secret associated with the page and the inverted CRC of the command code, target address,

page data and counter values. Immediately after the CRC is received the SHA engine begins the

computation of the message authentication code over the secret associated with the selected page, all 32

data bytes of the selected page, the page write cycle counter, page number, the device’s ROM registration

number and the 3-byte “challenge” that is taken from the scratchpad locations 20 through 22. The result

of the SHA computation is then placed in the scratchpad from location 8 through 27 for the master to

read. While the SHA computation takes place the master will read all 1’s. As the computation is finished

the pattern will change to alternating 0’s and 1’s. The master must read at least 8 bits of this alternating

pattern. Otherwise the device might not properly respond to a subsequent Reset Pulse. Typically the

master will next take all the page data, etc., compute the message authentication code on its own (see the

Compute SHA command, “Validate Data Page” function), and compare it to the data in the scratchpad to

determine whether the DS1963S knows the correct secret associated with the data page.

Compute SHA [33h]

The Compute SHA command provides the environment for six functions that employ the SHA engine to

generate message authentication codes in different ways. The seventh way to run the SHA engine is

through the Read Authenticated Page command, which has been described above to some extent. The full

details of all SHA computations are found in this section. Table 1 gives an overview of these functions.

SHA Functions Overview Table 1

Command or Function Name

Roaming Button Coprocessor Button Applicability

Read Authenticated Page

yes

N/A

Pages 0 to 15

Validate Data Page Function

N/A

yes

Pages 0 to 15

Sign Data Page Function

N/A

yes

Pages 0 and 8 only

Compute Challenge Function

yes

N/A

Not with pages 0 and 8

Authenticate Host Function

yes

N/A

Not with pages 0 and 8

Compute first Secret Function

yes

yes

Pages 0 to 15

Compute next Secret Function

yes

yes

Pages 0 to 15

A DS1963S device can be used in a system in two ways: a) as mobile data carrier that is associated to a

person that carries it, and b) as a coprocessor and data safe for a host computer or “bus master”. Either

application requires a secret to be installed in the DS1963S device. The functions needed to install secrets

in one or more steps are called Compute First Secret and Compute Next Secret. A DS1963S that works as

a coprocessor has to accomplish two functions: a) verify whether a roaming device belongs to the system

(i. e., whether it knows the secret), and b) generate or check a signature that protects data from mani-

pulation. These functions are accomplished by the functions Validate Data Page and Sign Data Page.

The main SHA function of a roaming device is Read Authenticated Page, which provides the coprocessor

device data and message authentication code needed for the Validate Data Page function. The two re-

maining SHA functions that a roaming device may have to execute are Compute Challenge and Authenti-

cate Host. These functions are not used in applications such as vending machines. However, they are es-

sential for user- and host authentication, which will set the MATCH flag of the roaming device. Since the

MATCH flag is part of the SHA computation of Read Authenticated Page, Validate Data Page, and Sign

Data Page, the resulting message authentication code depends on and therefore reveals whether the host