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Azure Key Vault | How Secrets and Keys are stored

I'm pretty sure that most of you heard about Azure Key Vault. If not I recommend to take a look over this page that describes in details how Azure Key Vault helps us as a safeguard for our application secrets and cryptographic keys (like certificates).

Scope
The main scope of this post is to take a look on how our secrets are stored. This is important because there are keys that cannot be recovered once generated or stored and we might end up without keys in the case we lose them.

What is HSM?
HSM is an acronym for Hardware Security Module. It is a physical device that can manage digital keys by providing cryptographic capabilities. HSM is playing the role of a safeguard by offering cryptographic capabilities directly by the hardware.

Is the tuple <keys, secrets> stored inside HSM?

No, there is no need to store this information in HSM. Secrets are stored outside the HSM, but they are encrypted using a key chain that terminates inside the HSM.
An analogy related to key chains and HSM is with our phones and finger marks. Imagine this key chain like passwords that you have stored for different banking accounts on your mobile phone. You can decide to encrypts password on your phone using you finger mark. You'll have your password encrypted on your phone storage and your finger will play the role of the HSM.

Where things are stored and how?
When we talk about a key vault system there are 3 things that are stored - secrets, software protected keys and HSM protected keys. Each of them are stored in the proper location based on the security importance.

  • Secrets - Stored as encrypted content using key chains. Content is decrypted only when is requested by an external source
  • Software protected keys - (so called RSA soft key) Stored encrypted and can be provided by clients or generated by Azure Key Vault.
  • HSM protected keys - They keys are not only encrypted but are protected by HSM Security Word. This system is part of Azure Key Vault and is one of the most secure places, used to store the most sensible information of Key Vault. 
Are key stored in HSM?
It is important to know that Azure Key Vault is not only HSM. The system is more complex than a bunch of  HSMs connected together. All Key Vaults systems have similar architecture and for all of them keys and secrets are not stored inside HSMs units. Encryption, decryption and storing 'master keys' are done inside HSMs, but consumer keys and secrets are stored outside the HSMs all the time encrypted in safe places. 

What is main difference between Premium and Standard tier?
From security perspective, the most important difference is on what kind of keys can be created. Only in premium tier your are allowed to create HSM protected keys, in comparison with standard tier where you can create only secrets and software protected keys.  


Can I retrieve back the keys and secrets?
(applicable for all Key Vault systems)
Once you stored something inside HSM, it is not possible to retrieve them anymore. For example if you import in HSM a certificate it is a one way road. You will be able to use it, but you will not be able to export it anymore. This is applicable only for HSM protected keys,
Software protected keys and secrets can be exported. The main scope of secrets is to store in a secure place and access them when needed.

The theory says that HSM keys (HSM protected keys) can be exported (backup's) but are encrypted all the time and can be decrypted only by the HSM once it is important back inside HSM. When they are exported they are encrypted and impossible to access. Don't forget that Azure Key Vault don't have an export API for HSM protected keys.  

Conclusion
Secrets and Keys are stored by Azure Key Vault all the time encrypted. Even if Secrets and Software protected keys are not stored in HSM, during storage they are encrypted using keys stored in HSM.

Comments

  1. Omer Levi HevroniJune 5, 2017 at 2:12 PM

    It is still unclear to me - does HSM protected keys are stored in HSM or not?

    ReplyDelete
    Replies
    1. Master keys yes, consumer keys no (based on my understanding). This is the standard of a KeyVault system also, not only Azure specific.

      Delete

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