[Cscwg-public] Timestamp Certificate and SubCA updates

Mon Mar 11 08:32:59 UTC 2024

Thanks Dimitris, I’ve reviewed and accepted the suggestions. 

> witnessed by members of two different Trusted Roles (not by two Trusted Role Members, i.e. you can't use two persons of the same Trusted Role). 

TBH, I’m not sure why it couldn’t be two persons of the same Trusted Role? 

> In general, a "key destruction" ceremony includes the deletion of all copies of the key, including copies that reside in backups. If we require a "key destruction" ceremony, the "restore key" case is nonsensical. We probably need to work on this some more so that we all have the same understanding and expectations. 
> It's ok to keep the keys in backups but if you happen to restore them in an HSM, you must not use them to sign anything. If a CA/TSA can also "destroy" the key, meaning that all copies of that private key can be unequivocally/securely deleted (i.e. without a way to recover the key), including any instance of the key as part of a backup, the better!

Agreed in general regarding the Key Destruction ceremony. However having to also destroy the backup of the key, and do this again for any next key every 18 months, can be a lengthy procedure, specially if backups are stored securely and offline in different places around the world. That’s why for this case we specifically call out that backups don’t need to be destroyed. 

But your point on an HSM restoring an entire partition and that violating the requirement, is valid. 

For reference, the current proposed language is: 
The CA MAY maintain existing backup sets containing the Private Key corresponding to a Timestamp Certificate. The CA MUST NOT restore the Private Key corresponding to a Timestamp Certificate contained within the backup if the Timestamp Certificate was issued more than 15 months prior to restoration of the backup. 

What about we once more use the NSR language and state: 
The CA MAY maintain existing backup sets containing the Private Key corresponding to a Timestamp Certificate. The CA SHOULD NOT restore the Private Key corresponding to a Timestamp Certificate contained within the backup if the Timestamp Certificate was issued more than 15 months prior to restoration of the backup. If the CA does restore such a Private Key, the CA SHALL only restore the Private Key in a suitable HSM while it’s maintained in a High Security Zone and in an offline state or air-gapped from all other networks and perform a new key destruction ceremony prior to the HSM being brought to an online state. 

Thoughts? 

As a side-note, I wonder if there’s a task for the NSWG (or Definitions WG once it’s setup) to define terms for online and offline HSMs 

From: Cscwg-public <cscwg-public-bounces at cabforum.org> on behalf of Dimitris Zacharopoulos (HARICA) via Cscwg-public <cscwg-public at cabforum.org>
Date: Sunday, 10 March 2024 at 10:30
To: cscwg-public at cabforum.org <cscwg-public at cabforum.org>
Subject: Re: [Cscwg-public] Timestamp Certificate and SubCA updates 

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Hi Martijn,

Two suggestions submitted on GitHub.

Regarding the prohibition of restoring a private key of a Timestamp Certificate, I'm not sure how universal this can be because some HSMs restore an entire slot/partition, which might contain Private Keys associated with obsolete Timestamp Certificates. As the ballot is written, such an action would be a violation.

In general, a "key destruction" ceremony includes the deletion of all copies of the key, including copies that reside in backups. If we require a "key destruction" ceremony, the "restore key" case is nonsensical. We probably need to work on this some more so that we all have the same understanding and expectations.

Let me restate the intent of this requirement as discussed all this time, and please correct me if I'm wrong.

IMO, the goal is to put the keys associated with Timestamp Certificates out of use, 15 months after the notBefore of the Timestamp Certificate. 

In order to achieve some level of assurance for this action, the proposal is to delete the keys from the HSM 18 months after the notBefore of the Timestamp Certificate, in an audited way, witnessed by members of two different Trusted Roles (not by two Trusted Role Members, i.e. you can't use two persons of the same Trusted Role). 

It's ok to keep the keys in backups but if you happen to restore them in an HSM, you must not use them to sign anything. If a CA/TSA can also "destroy" the key, meaning that all copies of that private key can be unequivocally/securely deleted (i.e. without a way to recover the key), including any instance of the key as part of a backup, the better!

Thoughts?

Dimitris.

On 6/3/2024 2:07 μ.μ., Martijn Katerbarg via Cscwg-public wrote: 

All, 

As discussed last week, I’d send out the draft language for this ballot once more before starting the discussion period. The latest version can be found in https://github.com/cabforum/code-signing/pull/34 <https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fcabforum%2Fcode-signing%2Fpull%2F34&data=05%7C02%7Cmartijn.katerbarg%40sectigo.com%7C5e8ff97a46964cd97d6f08dc40e4b919%7C0e9c48946caa465d96604b6968b49fb7%7C0%7C0%7C638456598304253222%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C0%7C%7C%7C&sdata=jt5w8Oh%2BJS%2FZ%2FFyU%2BGCERgmT7hq6frpyLwiKUhEVRFw%3D&reserved=0> 

I’ve made changes this morning to add 3 effective dates, these are: 

* For the removal of private keys associated with timestamp certificates, effective June 1st, 2024, CAs will need to properly log the removal of said key. 

* While I expect CAs to already properly log this for audit purposes even now, there may be exceptions for when this has not been done, for example a private key or timestamp certificate that was signed maybe 20 years ago. This language is added to avoid any confusion on from what point there needs to be an audit trail 

* Effective April 15, 2025, private keys associated with SubCAs containing the “Time Stamping” EKU will need to be placed in offline HSMs. 

* I believe a roughly one year effective date is appropriate here, since CAs may need to move keys from one HSM to another. 

* For private keys associated with timestamp certificates that were issued for greater than 15 months, CAs will need to remove the private keys 18 months after certificate issuance, starting April 15, 2025. 

* Likewise, I feel like anything involving HSM process changes, should have a longer effective date, and it makes sense to align this with the effective date above. 

I’ll start a ballot on this early next week, unless there is concern with the above. 

Regards,

Martijn 

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