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<p class="MsoNormal">Hi Folks,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I followed up to make sure we have the behavior for Windows understood. WVT (WinVerifyTrust) will do revocation checking for the TSA cert and if timestamped with that TSA, and it will consider the signature as invalid even if the signing
cert is still valid at the time of checking. Corey’s point about the broad usage leads to larger impact in the revocation scenario does play a large factor and why I would like to see the TSA entity certificate max validity come down to 15 months, and we remove
the rekey requirement. <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal">Ian<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b>From:</b> Cscwg-public <cscwg-public-bounces@cabforum.org>
<b>On Behalf Of </b>Corey Bonnell via Cscwg-public<br>
<b>Sent:</b> Thursday, January 6, 2022 11:40 AM<br>
<b>To:</b> Bruce Morton <bruce.morton@entrust.com>; cscwg-public@cabforum.org<br>
<b>Subject:</b> [EXTERNAL] Re: [Cscwg-public] Follow-up on Time-stamp Authority Items<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Hi Bruce,<o:p></o:p></p>
<p class="MsoNormal">Comments inline.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> I did reach out to Oracle about revocation and time-stamping. The answer was that a signature is not trusted after the Code Signing certificate has expired or has been revoked. As such, even if the signature was time-stamped it would
not be trusted if the Code Signing certificate is revoked or expired.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">This is great to know, thank you for confirming with Oracle and sharing with the group.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> QUESTION - Why do we need to change the TSA private key with 15 months if the private key is protected the same way as a CA key (i.e., FIPS 140-2 Level 3 and NetSec)?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">To the best of my knowledge, Authenticode does not perform any revocation checking for the TSA certificate chain (even for the end-entity). This means that if the Private Key corresponding to a TSA were compromised, then it would be possible
to forge timestamp tokens to any date within the validity period of the TSA certificate. If such a compromise were to occur, I imagine the only mitigation is for RP software (i.e., Windows) to blocklist the TSA Public Key, thus rendering all timestamp tokens
signed by that key invalid. If that TSA Key Pair were in use for a long time (more than 15 months), then more signed application code would be negatively affected than would be if the TSA key were regularly rotated. Thus, mandating key rotation limits the
ecosystem impact of such a key compromise.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Ian, does this match your understanding?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal">Corey<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b>From:</b> Cscwg-public <<a href="mailto:cscwg-public-bounces@cabforum.org">cscwg-public-bounces@cabforum.org</a>>
<b>On Behalf Of </b>Bruce Morton via Cscwg-public<br>
<b>Sent:</b> Friday, December 17, 2021 1:21 PM<br>
<b>To:</b> <a href="mailto:cscwg-public@cabforum.org">cscwg-public@cabforum.org</a><br>
<b>Subject:</b> [Cscwg-public] Follow-up on Time-stamp Authority Items<o:p></o:p></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">TSA Certificate Validity Period<o:p></o:p></p>
<ul style="margin-top:0in" type="disc">
<li class="MsoListParagraph" style="margin-left:0in;mso-list:l1 level1 lfo3">I did reach out to Oracle about revocation and time-stamping. The answer was that a signature is not trusted after the Code Signing certificate has expired or has been revoked. As
such, even if the signature was time-stamped it would not be trusted if the Code Signing certificate is revoked or expired.<o:p></o:p></li></ul>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">TSA Rekey every 15 months<o:p></o:p></p>
<ul style="margin-top:0in" type="disc">
<li class="MsoListParagraph" style="margin-left:0in;mso-list:l1 level1 lfo3">CSBR 9.4 states, “The Timestamp Authority MUST use a new Timestamp Certificate with a new private key no later than every 15 months to minimize the impact to users in the event that
a Timestamp Certificate's private key is compromised. The validity for a Timestamp Certificate must not exceed 135 months. The Timestamp Certificate MUST meet the "Minimum Cryptographic Algorithm and Key Size Requirements" in Appendix A for the communicated
time period.”<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0in;mso-list:l1 level1 lfo3">CSBR 16.1 (2) states, “A Timestamp Authority MUST protect its signing key using a process that is at least to FIPS 140-2 Level 3, Common Criteria EAL 4+ (ALC_FLR.2), or higher. The
CA MUST protect its signing operations in accordance with the CA/Browser Forum’s Network Security Guidelines. Any changes to its signing process MUST be an auditable event.”<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0in;mso-list:l1 level1 lfo3">QUESTION - Why do we need to change the TSA private key with 15 months if the private key is protected the same way as a CA key (i.e., FIPS 140-2 Level 3 and NetSec)?<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0in;mso-list:l1 level1 lfo3">The discussion on the call is that a TSA certificate is a leaf certificate, but since the key is managed the same as a Subordinate CA it seems to be more like a CA certificate.<o:p></o:p></li></ul>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Open for comments.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks, Bruce.<o:p></o:p></p>
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