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Certificate Trust Lists (authroot.stl)

signify.authenticode

Microsoft has its own root CA verification program. More information about this root program can be found on Microsoft's website.

Trust of certificates is distributed to Windows computers using Certificate Trust Lists. They used to be updated through Windows Update, though they are now updated independently. Certificate Trust Lists are not very well documented, but a 2009 whitepaper does a decent job of defining the used structures.

For interpreting the various attributes, we had to do some more extended research. The following lists the deprecation policies in place and how we have found that they are mapped to attributes in the CTL.

CTL deprecation policies

Removal

Removal of a root from the CTL. All certificates that chain to the root are no longer trusted.

In this case, the entry will be removed from the CTL; we do not need to perform further checks.

EKU Removal

Removal of a specific EKU from a root certificate. All End entity certificates that chain to this root can no longer utilize the removed EKU, independent of whether or not the digital signature was timestamped.

The EKU is removed from the set of allowed EKU's in CertificateTrustSubject.extended_key_usages

or added to the set of disallowed EKU's in CertificateTrustSubject.disallowed_extended_key_usages.

Disallow

This feature involves adding the certificate to the Disallow CTL. This feature effectively revokes the certificate. Users cannot manually install the root and continue to have trust.

Disallowed certificates are put in a separate authroot.stl file, and removed from the normal CTL. We do not

verify disallowed certificates.

Disable

All certificates that chain to a disabled root will no longer be trusted with a very important exception; digital signatures with a timestamp prior to the disable date will continue to validate successfully.

Empirical evidence has shown that in this case, CertificateTrustSubject.disallowed_filetime will be set.

In the case that only an EKU is disabled, it is removed from the set of allowed EKU's in CertificateTrustSubject.extended_key_usages or added to the set of disallowed EKU's in CertificateTrustSubject.disallowed_extended_key_usages

NotBefore

Allows granular disabling of a root certificate or specific EKU capability of a root certificate. Certificates issued AFTER the NotBefore date will no longer be trusted, however certificates issued BEFORE to the NotBefore date will continue to be trusted. Digital signatures with a timestamp set before the NotBefore date will continue to successfully validate.

In this case, the CertificateTrustSubject.not_before_filetime will be set. In the case that this applies

to a single EKU, CertificateTrustSubject.not_before_extended_key_usages will be set as well.

The CertificateTrustList object can be used in combination with a signify.x509.CertificateStore to make sure that the certificates in the store are valid according to the additional conditions in the CTL. Use TRUSTED_CERTIFICATE_STORE for a certificate store with associated CTL.

CertificateTrustList

CertificateTrustSubject

TRUSTED_CERTIFICATE_STORE

A signify.x509.CertificateStore with associated CertificateTrustList.

TRUSTED_CERTIFICATE_STORE_NO_CTL

A signify.x509.CertificateStore without an associated CertificateTrustList.

Signify uses a separate project (mscerts) to ensure an up-to-date certificate bundle. This project is maintained by the same authors as Signify.