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Hello S-1-1-0, PowerShell CryptoGuy (aka @Crypt32) is here again. Today I want to discuss about X.509 Name Constraints certificate extension. It is not widely used, but sometimes it is necessary. As extension name depicts, it is used to provide constraints or restrictions to certificate subject and subject alternative names (SAN) extension.
Name Constraints extension is defined and described in RFC 5280 §18.104.22.168. Extension presence in an end-entity certificate does not have any effect and is applied only to CA certificates that issue certificates to end entities. Once defined, the extension applies restrictions on any certificates that appear below that CA in the tree. Name Constraints may appear further in the certification path to set more restrictive constraints. It is not possible to set less restrictive constraints at lower levels. This prevents low-level (in the certification path meaning) CAs to violate restrictions applied at higher levels.
Figure 1 - sample certificate chain
Here we see a 3-tier PKI hierarchy with applied Name Constraints extension at 2nd level (below root). This is indicated by a yellow triangle. Name Constraints restrictions are applied to all directly and indirectly issued certificates. CA-2 doesn’t define Name Constraints extension in its own certificate, but restrictions still apply to certificates issued by CA-2 indirectly.
Recently I was asked about how to read Enrollment Agent Rights and Certificate Manager Restrictions in ADCS. At first, I would like to make a little introduction about the subject.
With Active Directory Certificate Services (ADCS) you can designate one or more enrollment agents to enroll on behalf of other users. One of the most common scenarios is smart card provisioning. Suppose, you purchased smart cards and plan to issue them to employees. You will designate one or more highly trusted persons who will:
Enrollment Agent Restrictions cover the last point in the list. Restrictions define three major parts:
Almost everyday we hear about SHA1 deprecation policy. Many commercial CAs now sign end-entity certificates with SHA2 (actually, SHA256) and. Some of them upgrade issuing CAs to SHA2. Many security administrators move their private CAs and certificates to SHA2 signatures. Unfortunately, not all do this migration correctly. Companies just configure their CAs to sign certificates with SHA256. Is this enough? Actually, not.
Recently, a friend of mine asked a question about key exchange in SSL without encrypting the key. His question came after examining a Key Usage certificate extension setting in the certificate template. There are two options: Allow key exchange only with key encryption and Allow key exchange without key encryption. How it can be possible that the key is not encrypted??? Here is an image of the respective setting: