ATSHA204A datasheet(86/93 Pages) MICROCHIP

86 / 93 page

DeriveKey always resets the UseFlag to 0xFF for the target key. This is the only mechanism to

reset the UseFlag bits.

Use of the DeriveKey command is optional. It is legal to be run only if WriteConfig<13> is set for

this slot. In some situations, it may be advantageous to simply have a key that can be used eight

times, in which case the other crypto commands clear the bits in UseFlag one at a time until all are

cleared and at which time the key is disabled.

13.3.5

Limited-use Key

If Slot<15>.LimitedUse is set, usage of key number 15 is limited through a different mechanism than the

single-use limitation described above, which applies only to slots 0 through 7.

Prior to any use of key 15 by a cryptographic command, the following takes place:

•

If all bytes in LastKeyUse are 0x00, return error.

•

Starting at bit 7 of the first byte of LastKeyUse (byte 68 in the Configuration zone), clear to zero the

first bit that is currently a one. If byte 68 is 0x00, check bit seven of byte 69 and so on up through

byte 83. Only a single bit is cleared each time prior to using key 15.

There is no reset mechanism for this limitation; after 128 uses (or the number of one bits set in

LastKeyUse on personalization), key 15 is permanently disabled. This capability is not susceptible to

power interruptions. Even if the power is interrupted during execution of the command, only a single bit in

LastKeyUse is unknown; all other bits in LastKeyUse are unchanged and the key remains unchanged.

If fewer than 128 uses are desired for key 15, then some of the bytes within this array should not be

initialized to 0xFF. As with UseFlag, the only legal values for bytes within this field (besides 0xFF) are

0x7F, 0x3F, 0x1F, 0x0F, 0x07, 0x03, 0x01, or 0x00. The total number of bits set to one indicates the

number of uses.

Example: Limited Use count set to a value of 16.

0xFF, 0xFF, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,

0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00

The LimitedUse bit is ignored by the Read and Write commands and LastKeyUse does

not change as a result of their execution. The LimitedUse bit is ignored by the copy

function of the CheckMac command. The LimitedUse bit is honored for the parent key

in the DeriveKey command, but is ignored for the target key.

13.3.6

Password Checking

Many applications require a user to enter a password to enable features, decrypt stored data, or to

pursue some other purpose. Typically, the expected password has to be stored somewhere in memory

and therefore can become, subject to discovery. The ATSHA204A can securely store the expected

password and perform a number of useful operations on it. The password is never passed in the clear to

the device and it cannot be read from the device. It is hashed with a random number in the system

software before being passed to the device. The nonce in TempKey must always have been generated

using the internal RNG when a Transport Key is utilized.

The copy capability of the CheckMac command enables the following types of password checking

options:

CheckMac does an internal comparison with the expected password and returns a Boolean to the

system to indicate whether the password was correctly entered or not.

ATSHA204A

Reference and Application Notes

DS40002025A-page 86

ATSHA204A Datasheet(PDF) 86 Page - Microchip Technology

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