Implementing a public key infrastructure (PKI) is a recurring task for me. More and more customers tend to implement a PKI in their environment. Mostly not to increase security, rather then to get rid of browser warnings because of self-signed certificates, to secure intra-org email communication with S/MIME, or to sign Microsoft Office macros.
What is a 2-tier PKI?
Why is a multi-tier PKI hierarchy a good idea? Such a hierarchy typically consits of a root Certificate Authority (CA), and an issuing CA. Sometimes you see a 3-tier hierarchy, in which a root CA, a sub CA and an issuing CA are tied together in a chain of trust.
A root CA issues, stores and signs the digital certificates for sub CA. A sub CA issues, stores and signs the digital certificates for issuing CA. Only an issuing CA issues, stores and signs the digital certificates for users and devices.
In a 2-tier hierarchy, a root CA issues the certificate for an issuing CA.
In case of security breach, in which the issuing CA might become compromised, only the CA certificate for the issuing CA needs to be revoked. But what of the root CA becomes compromised? Because of this, a root CA is typically installed on a secured, and powered-off (offline) VM or computer. It will only be powered-on to publish new Certificate Revocation Lists (CRL), or to sign/ renew a new sub or issuing CA certificate.
Think about the processes! Creating a PKI is more than provisioning a couple of VMs. You need to think about processes to
- sign, and
Be aware of what a digital certificate is. You, or your CA, confirms the identity of a party by handing out a digital certificate. Make sure that no one can issue certificates without a proof of his identity.
Think about lifetimes of certificates! Customers tend to create root CA certificates with lifetimes of 10, 20 or even 40 years. Think about the typical lifetime of a VM or server, which is necessary to run an offline root CA. Typically the server OS has a lifetime of 10 to 12 years. This should determine the lifetime of a root CA certificate. IMHO 10 years is a good compromise.
For a sub or issuing CA, a lifespan of 5 years is a good compromise. Using the same lifetime as for a root CA is not a good idea, because an issued certificate can’t be longer valid than the lifetime of the CA certificate of the issuing CA.
A lifespan of 1 to 3 years for thinks like computer or web server certificates is okay. If a certificate is used for S/MIME or code signing, you should go for a lifetime of 1 year.
But to be honest: At the end of the day, YOU decide how long your certificates will be valid.
Publish CRLs and make them accessable! You can’t know if a certificate is revoked by a CA. But you can use a CRL to check if a certificate is revoked. Because of this, the CA must publish CRLs regulary. Use split DNS to use the same URL for internal and external requests. Make sure that the CRL is available for external users.
This applies not only to certificates for users or computers, but also for sub and issuing CAs. So there must be a CRL from each of your CAs!
I recommend to publish CRLs to a webserver and make this webserver reachable over HTTP. An issued certificate includes the URL or path to the CRL of the CA, that has issued the certificate.
Make sure that the CRL has a meaningful validity period. Of an offline root CA, which issues only a few certificates of its lifetime, this can be 1 year or more. For an issuing CA, the validity period should only a few days.
Publish AIA (Authority Information Access) information and make them accessable! AIA is an certificate extension that is used to offer two types of information :
- How to get the certificate of the issuing or upper CAs, and
- who is the OCSP responder from where revocation of this certificate can be checked
I tend to use the same place for the AIA as for the CDP. Make sure that you configure the AIA extension before you issue the first certificates, especially configure the AIA and CDP extension before you issue intermediate and issuing CA certificates.
Use a secure hash algorithm and key length! Please stop using SHA1! I recommend at least SHA256 and 4096 bit key length. Depending on the used CPUs, SHA512 can be faster than SHA256.
Create a CApolicy.inf! The CApolicy.inf is located uder C:\Windows and will will be used during the creation of the CA certificate. I often use this CApolicy.inf files.
For the root CA:
[Certsrv_Server] RenewalKeyLength = 4096 RenewalValidityPeriod = Years RenewalValidityPeriodUnits = 10 AlternateSignatureAlgorithm = 0 CNGHashAlgorithm = SHA512 CRLPeriod = years CRLPeriodUnits = 1
For the issuing CA:
[Certsrv_Server] RenewalKeyLength = 4096 RenewalValidityPeriod = Years RenewalValidityPeriodUnits = 5 AlternateSignatureAlgorithm = 0 CNGHashAlgorithm = SHA512 CRLPeriod = Days CRLPeriodUnits = 7 CRLDeltaPeriod = Hours CRLDeltaPeriodUnits = 12 [CRLDistributionPoint] URL = http://crl.domain.tld/crld/RootCA.crl
I do not claim that this is blog post covers all necessary aspects of such an complex thing like an PKI. But I hope that I have mentioned some of the important parts. And at least: I have a reference from which I can copy and paste the CApolicy.inf files. :D
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