One of the common problems in working with encryption is getting caught up with the intimate details of things like encryption algorithms, key lengths, cipher modes, and other minutiae. Not that these details aren’t important – depending on what you’re doing they might be critical – but in the larger scheme of things these aren’t the aspects most likely to trip up your implementation. Before we get into different key management strategies, let’s take a moment to look at crypto systems at the macro level. We will stick to data encryption for this paper, but these principles apply to other types of cryptosystems as well.
Note: For simplicity I will often use “encryption” instead of “cryptographic operation” through this series. If you’re a crypto geek, don’t get too hung up… I know the difference – it’s for readability.
The three components of a data encryption system
Three major components define the overall structure of an encryption system:
- The data: The object or objects to encrypt. It might seem silly to break this out, but the security and complexity of the system are influenced by the nature of the payload, as well as by where it is located or collected.
- The encryption engine: The component that handles the actual encryption (and decryption) operations.
- The key manager: The component that handles key and passes them to the encryption engine.
In a basic encryption system all three components are likely located on the same system. Take personal full disk encryption (the default you might use on your home Windows PC or Mac) – the encryption key, data, and engine are all kept and run on the same hardware. Lose that hardware and you lose the key and data – and the engine, but that isn’t normally relevant.
But once we get into SMB and the enterprise we tend to split out the components for security, management, reliability, and compliance.
Building a data encryption system
Where you place these components define the structure, security, and manageability of your encryption system:
Full Disk Encryption
Our full disk encryption example above isn’t the sort of approach you would want to take for an organization of any size greater than 1. All major FDE systems do a good job of protecting the key if the device is lost, so we aren’t worried about security too much from that perspective, but managing the key on the local system means the system is much less manageable and reliable than if all the FDE keys are stored together.
Enterprise-class FDE manages the keys centrally – even if they are also stored locally – to enable a host of more advanced functions; including better recovery options, audit and compliance, and the ability to manage hundreds of thousands of systems.
Let’s consider another example: database encryption. By default, all database management systems (DBMS) that support encryption do so with the data, the key, and the encryption engine all within the DBMS. But you can mix and match those components to satisfy different requirements.
The most common alternative is to pull the key out of the DBMS and store it in an external key manager. This can protect the key from compromise of the DBMS itself, and increases separation of duties and security. It also reduces the likelihood of lost keys and enables extensive management capabilities – including easier key rotation, expiration, and auditing.
But the key could be exposed to someone on the DBMS host itself because it must be stored in memory at before it can be used to encrypt or decrypt. One way to protect against this is to pull both the encryption engine and key out of the DBMS. This could be handled through an external proxy, but more often custom code is developed to send the data to an external encryption server or appliance. Of course this adds complexity and latency…
Cloud computing has given rise to a couple additional scenarios.
- To protect an Infrastructure as a Service (IaaS) storage volume running at an external cloud provider you can place the encryption engine in a running instance, store the data in a separate volume, and use an external key manager which could be a hardware appliance connected through VPN and managed in your own data center.
- To protect enterprise files in an object storage service like Amazon S3 or RackSpace Cloud Files, you can encrypt them on a local system before storing them in the cloud – managing keys either on the local system or with a centralized key manager. While some of these services support built-in encryption, they typically store and manage the key themselves, which means the provider has the (hopefully purely theoretical) ability to access your data. But if you control the key and the encryption engine the provider cannot read your files.
Backup and storage encryption
Many backup systems today include some sort of an encryption option, but the implementations typically offer only the most basic key management. Backup up in one location and restoring in another may be a difficult prospect if the key is stored only in the backup system.
Additionally, backup and storage systems themselves might place the encryption engine in any of a wide variety of locations – from individual disk and tape drives, to backup controllers, to server software, to inline proxies.
Some systems store the key with the data – sometimes in special hardware added to the tape or drive – while others place it with the engine, and still others keep it in an external key management server.
Between all this complexity and poor vendor implementations, I tend to see external key management used for backup and storage more than for just about any other data encryption usage.
Our last example is application encryption. One of the more secure ways to encrypt application data is to collect it in the application, send it to an encryption server or appliance, and then store the encrypted data in a separate database. The keys themselves might be on the encryption server, or could even be stored in yet another system. The separate key store offers greater security, simplifies management for multiple encryption appliances, and helps keep keys safe for data movement – backup, restore, and migration/synchronization to other data centers.
In each of these examples we see multiple options for where to place the components – with the corresponding tradeoffs in security, manageability, and other aspects.
This list isn’t comprehensive but should give you a taste of the different ways these bits and pieces can be mixed and matched for different data encryption systems.
Next we will build on this to define the four major key management strategies, followed by recommendations for how to pick the right options to suit on our needs.