ST33TPHF20SPI datasheet(22/47 Pages) STMICROELECTRONICS

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FIPS140-2 SECURITY POLICY

Page 22 of 47

NON-PROPRIETARY DOCUMENT

Executing the expected policy commands sequence in case of policy authorization

session (defined policy must follow recommendations listed in §1.7.3).

Do the comparison between reference value and computed value. If both match,

command execution is authorized.

More details on HMAC and policy sessions can be found in §19 of [TPM2.0 Part1 r1.16].

2.2.2

Authorization strength

As minimum value of authorization or policy values might be 160-bit random values (based on

unbiased distribution of ‘0’ and ‘1’), the probability for an attacker to guess the authorization

data is:

2160

= 6.84 ∗ 10−49

This value is then higher than the minimum of 1*10-6 required by [FIPS140-2].

The number of attempts per minute that an attacker can make is limited by the DAM (Dictionary

Attack Mechanism). DAM consists in counting the number of failed authentication. When this

counter reaches a pre-defined threshold, a lockout period is started. During this period, no

authorized command execution is allowed and a specific error is returned in TPM response

until period expires. Next table indicates the threshold values and the lockout durations:

Table 16: DAM lockout durations

Failed authentication counter

>31

Lockout period (in seconds)

7200

This table indicates that an attacker can do a maximum (during the first minute) of 32 trials per

minute before DAM being active. As a result the probability per minute that a random attempt

will lead to a successful authorization matches FIPS requirements. Value is equal to:

32 ∗

2160

= 2.19 ∗ 10−47

This value is then higher than the minimum of 1*10-5 required by [FIPS140-2].

NB: commands handling (reception, processing and response sending) is negligible compared

to the lockout periods and not taken into account in the above computation.

NB2: DAM parameters might be changed by using TPM2_DictionnaryAttackParameters

command. However to operate in a FIPS approved mode, they shall not be changed in order

not to decrease the authorization strength computed above.

2.2.3

Authorization protection

By following recommendations to operate in FIPS mode of operation, authorization data

associated to objects, NV indexes or hierarchies are never output from TPM in plaintext form

and thus are protected from unauthorized disclosure.

Authorization can be changed via the following services:



TPM2_ObjectChangeAuth



TPM2_HierarchyChangeAuth



TPM2_NV_ChangeAuth

As indicated in Table 18, roles that imply authentication are associated with these services

meaning that authentication are protected against unauthorized modification and substitution.

TPM authorization mechanism (HMAC or policy digest comparison) does not provide any

information about authentication data or policy sequence. Authentication indicates pass

(command executed) or fail (command not executed) and does not provide feedback that

could weaken the strength of authentication.

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