In this second part of our discussion on Credential Roaming, we are going to discuss about its relation to key archival as well as offer some practical alternatives to credential portability.
As we saw in the previous blog post, the capability of “transporting” private keys in Active Directory is one of the basic pillars of the process’s ability to roam credentials. However, this should in no way be seen as a good way of backing them up for future restores. The procedure that should be used for key backup is the Windows PKI process of Key Archival and there is no way of replacing this with credential roaming’s private key “upload” process. In fact, even though private keys do reside inside the AD database, there is no known or supported way of extracting them in any form, in case of disaster recovery procedures. Moreover, if the user deletes his certificate along with his private key from one of his profiles, this deletion will propagate into all other profiles where the certificate has roamed to.
Even though it has already been mentioned in the previous blog post, it is worth noting that an easy and quick workaround for credential roaming is the use of roaming user profiles, which (of course) involves a lot more administrative costs and changes in relation to the activation of credential roaming in ΡΚΙ. In accordance to this, the concurrent use of user profile roaming and credential roaming is not supported.
Another workaround (although not as automated as Credential Roaming) is the storage of certificates and private keys in a smart card. This solution does make the user’s credentials available in every PC that he logs on to, however it assumes smart card hardware availability (this can also be a USB-based token) and also assumes a planned or existing smart card infrastructure, which does come with high deployment and administrative costs. However, a carefully planned and executed smart card deployment can furthermore be used for other applications, like VPN remote access, local computer and RDP user login etc. In general, and independently of the fact that a smart card deployment is a good alternative to credential roaming, the use of smart cards is recommended in all medium- & high-security PKI deployments, as many security officers are not happy with the storage of private leys inside an operating system (inside non-dedicated devices, in general).
Credential roaming is the ability of a Windows Server PKI infrastructure to support roaming certificates from a Windows computer to another Windows computer, according to where a user is logged on. The way it works is as follows: when a user requests a certificate, its local operating system generates a private and public key and, using a secure channel between the computer and the CA, it uploads the private key and the signed request to the CA. The private key is securely stored in Active Directory and can later be downloaded to any PC that a domain user logs on to.
All this is fine and dandy and, based on specs, it is the perfect solution for users that tend to use a multitude of computers for their business use. However, setting up the infrastructure to support it is not as easy as one would have thought. In this post, we will take a look at some of the disadvantages and difficulties of credential roaming. In the post to follow, we will make some more general observations as well as offer some alternatives to credential roaming. So, to cut a long story short, credential roaming:
- Demands a complicate (but not very time-consuming) procedure of installation and use, as well as many scenarios to be tested to make sure it works as expected.
- Extends the physical size of the Active Directory database, according to the issued certificates to be roamed. If the service is extended to a few hundred or thousands of certificates, we may have an AD database which will grow to a few hundred MBs larger. Accordingly, the AD backup/restore time will be bigger, the AD database will become more fragmented etc. Microsoft Support has been involved in many support cases where a sudden increase of the Active Directory database has been attributed to the activation of credential roaming.
- Demands Windows XP SP3 or Windows XP SP2 with a specific update, Windows Vista , 7 or 8. It does not support other operating systems (for example, mobile OSs).
- Needs difficult and time-consuming troubleshooting techniques for Windows XP (Vista/7 use CAPI2 logging, so it’s easier there).
- Increases the organization’s attack vector. In case we don’t use enhanced security measures at the PCs that will use private key actions (i.e., lock workstation, Bitlocker protection, users’ security training etc.), using multiple points where certificates are accessible and used, we multiply the possibility of someone extracting the private key(s).
- Poses problems in network EFS encryption. Under normal circumstances, credential roaming does not work as expected, due to the nature of the logon that it supports (credential roaming: local logon, network shares: network logon). The specific limitation is described in KB907247 (in Credential roaming will not be used when using EFS to encrypt files on a file server). EFS (and EFS using credential roaming) has not been designed to work with network shares, but only for local encryption. Some workarounds exist, such as using Roaming profiles instead of credential roaming (KB837359), use Offline Files and local encryption of the cache, use of web folders (in Remote EFS Operations on File Shares and Web Folders).