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X.509 Name Constraints certificate extension – all you should know

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.

Brief Description

Name Constraints extension is defined and described in RFC 5280 §4.2.1.10. 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.

PKI Hierarchy

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.


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How to read ADCS Enrollment Agent/Certificate Manager rights in PowerShell

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.

Enrollment Agents

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:

  • instruct employees about smart card usage policies;
  • register smart card serial number/other data in the accounting system (some certificate lifecycle management system);
  • prepare smart card for use (print labels and so on);
  • install certificate for another employee.

Enrollment Agent Restrictions cover the last point in the list. Restrictions define three major parts:


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SHA1 to SHA2 transition: do it properly!

About the problem

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.


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SSL key exchange with and without encryption

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:

Key Usage extension configuration in certificate template


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Test web server SSL/TLS protocol support with PowerShell

Recently I was tasked to configure SSL/TLS protocols and cipher suites for internal web servers via Group Policy. At first, we collected a list of web server and web client applications to determine the weakest possible SSL/TLS protocols. Once the list was complete, we deployed sample policy in test OU and finally applied them to the rest domain.

Now I was tasked to scan web servers to determine if they match new security policy. In order to minimize my effort in testing, I wrote a simple PowerShell script that accepts a list of web URLs and tests each host with a list of SSL protocols: SSLv2, SSLv3, TLS 1.0, TLS 1.1 and TLS 1.2. Here is a sample code:


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