Secrets Management and Chef 2014-08-08
Everyone has secrets. Database passwords, API credentials, recovery questions. These secrets need to be stored somewhere, and then made available to servers that use them.
This post has now been expanded into a conference talk, with updated information and more detail. You can check it out over on my Talks page.
When working with secrets we have a few needs above and beyond that of “normal” configuration data. As with any security-relevant system, the overarching rule must be the Principle of Least Privilege. This means that if a server doesn’t require a specific secret, it should not have access to it. We also generally want some level of access logging to analyze any future problems. We are sometimes willing to give up some ground, usually in the form of version control. Accessing the old value of a password is only needed in “oops” situations, so it isn’t always a hard requirement for the storage system.
Online vs. offline
The first decision to make is if you are looking for online (also called hot) storage or offline (also called cold). Online system are used for data that is needed by servers non-interactively. This is the bulk of secrets, things like database passwords are needed every time you spin up a new webapp server, so they need to be retrieved without specific human intervention.
Offline systems are for secrets you don’t access often, but do need to store somewhere for future reference. For example, the master password on an AWS account should never be needed during day-to-day operations, but you do need to keep it written down somewhere safe. Offline systems are generally more secure in an absolute sense, but require human interaction to access data, sometimes from more than one person.
I covered some of these issues in my article on data bags. Just in the context of secrets, data bags don’t really offer great support for either of our required features. Least Privilege can be accomplished but only with Enterprise Chef’s ACL system, and that is a difficult beast to manage to say the least. Access logs do exist, but there is nothing to easily search/manage them. If you use Hosted Chef, the access logs are not directly accessible at all.
I’ll spare you many more reasons they are unsuitable but overall I recommend not using data bags for secrets storage.
Encrypted data bags
Encrypted data bags use a shared secret and symmetric encryption of the data bag values. The current version (v2) uses AES-256-CBC with an additional SHA256 HMAC. The next version (v3) will use AES-256-GCM.
This offers a bit of a trade, you can achieve Least Privilege by ensuring that only those that are granted access will have the decryption key for a particular secret. The downside of this is now you need to manage and distribute the decryption keys. While this isn’t impossible, the keys are secrets themselves so this is a bit of a recursive problem. On the positive side, because the data is encrypted at rest it can be checked in to version control.
Additionally the primary APIs for working with encrypted data bags only allow one decryption key per server, which usually results in an all-or-nothing approach and thus violating Least Privilege. Because encrypted data bags use the same APIs for access, the same issues with audit logging carry over.
The same general issues apply to Ansible’s Vault system.
Chef-vault builds on encrypted data bags. Rather than a single shared decryption key, chef-vault creates a separate copy of the data bag item for each node that is granted access, using the existing RSA public/private key pair normally used for Chef API authentication. This means you no longer have to worry about distributing the decryption keys, but it also moves chef-vault into the gray area between online and offline storage systems.
Once a server is granted access to a secret, it can continue to access it in an online manner. However granting or revoking new servers requires human interaction. This means chef-vault is incompatible with auto-scaling or self-healing systems. It also inherits the same issues with audit logging as all other data-bag driven approaches.
If you are okay with the limitations on auto-scaling, chef-vault is a solid option for storing secrets. Make sure to check out the accompanying cookbook for some handy DSL extensions.
The Citadel cookbook uses a different approach. Rather than control access via encryption, it uses a Trusted Third Party to mediate access, specifically AWS IAM. It makes use of the IAM Role feature of EC2 to provide AWS API credentials to the server. Combined with a private S3 bucket and IAM access policies bound to a role and you can very tightly control access to secrets. Access logs and versioning are both available through S3, as is at-rest encryption though this is most likely a red herring for security.
The big downside of this is it requires you to be entirely AWS-based. It also comes with a fair amount of complexity on the IAM configuration side, though this can be somewhat handled through tools like CloudFormation. If you are already committed to using AWS, it is my current recommendation for secrets storage.
Trousseau has a lot of similarities to chef-vault. It follows the same pattern of encrypting the secrets separately for each server that will have access to them, but it uses GPG instead of Chef’s encrypted data bag system. This makes it easier to interface with non-Chef tools, but it doesn’t have the same slick DSL extensions for use within Chef. It also requires an external synchronization server of some kind, currently it supports S3 and SCP as mechanisms to get the encrypted data to the server before Trousseau can process it. Due to the complexities of the synchronization, I would consider Trousseau to be a mostly offline storage system.
I don’t actually know of anyone using Trousseau with Chef, so this is mentioned largely for completeness.
Barbican is a young project being developed by Rackspace for OpenStack. Its goal is to handle infrastructure-level secrets storage for OpenStack, such as Cinder encryption keys. I don’t think it is yet at a point where it could be used smoothly for secrets storage with Chef, but I am hopeful for the future. It could eventually allow something like how Citadel works, but against a local Barbican server instead of AWS.
Red October is an N-of-M storage system developed by CloudFlare. It is primarily aimed at offline storage, but does provide a remote API for online use. Its defining feature is the N-of-M encryption, meaning that a given secret can be encrypted so that any N out of the total M people can access it. Let’s say you have 5 engineers, you could set some secrets to be 1-of-5 so they are accessible by anyone, while more important secrets could be 3-of-5 to ensure a majority of the team authorizes the access. For very high-value secrets this helps ensure a single laptop compromise doesn’t put you at risk.
Unfortunately tooling around it is incredibly minimal, and it offers little logging. If you are looking for a solid offline storage tool for business-critical secrets, definitely give Red October a look.
ZooKeeper, etcd, consul
Some might be tempted to store their secrets alongside their other run-time configuration data in ZooKeeper, etcd, or consul. ZooKeeper does have an ACL system to restrict access, but I’ve not seen many cases of it being used well due to the high level of complexity. etcd and consul both lack authentication and authorization controls, but they are being worked on.
If you are willing to bite off the complexity that is ZooKeeper ACLs, it can be a good option. You will need to consider the ZooKeeper hosts a Trusted Third Party for the most part, so be prepared to harden those machines more than usual.
One potential solution for this mess is to move more services toward asymmetric keys for authentication instead of shared (symmetric) secrets. This is already supported in both PostgreSQL and MySQL. This trades secrets management for identity management, which is still a hard problem but does have some nicer characteristics. Notably the private key for a given server never has to leave the machine, and the public key doesn’t need to be kept secret from anyone. The hard part becomes knowing which keys to authorize for which resources and managing signatures. This generally requires another Trusted Third Party to handle role/identity information, like a chef-server or the AWS API.
The tooling isn’t there today, but this does offer a path out of the current miasma.
If you are 100% on AWS, use Citadel.
If you will never use any kind of auto-scaling, use Chef-vault.
If you need to store rarely used but high-value secrets, use Red October.
If none of these apply, you are likely between a rock and a hard place.
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