Take the 2-minute tour ×
Information Security Stack Exchange is a question and answer site for Information security professionals. It's 100% free, no registration required.

In my application, it is required to encrypt a user's data before inserting it into the mysql database. I am using AES algorithm to encrypt the data using a 256 bit hash (and is calling it Data Encryption Key, or DEK in short).

To make the system more secure, instead of storing DEK in plain text in the app server, it is stored in encrypted form (en_dek) in the app server. The key to encrypt the DEK is stored in a totally separate server and is called the Key Encryption Key (KEK). So everytime a data is to be encrypted / decrypted, first the KEK is used to decrypt the en_dek, which gives me the actual DEK, and then this DEK is used to encrypt / decrypt the user's data.

Now my question is:

[1] Should I send (en_dek + data) to the key server or should I fetch the KEK into the app server? Which one is more safe practice?

[2] Both these methods have a flaw that if an attacker is able to get access to either of these servers, he will get to know both KEK and DEK. How can we prevent this?

[3] When we talk about key rotation, which of the two keys (DEK and KEK) is rotated?

share|improve this question
    
Currently I'm in the same situation, Hope you have integrated or chosen some best approach in your application. can you please provide some information? How did you overcome this situation? At last have you used the HSM or Key server, which is best way(Key server or HSM)? –  chandru Mar 3 at 10:38
add comment

1 Answer

Should I send (en_dek + data) to the key server or should I fetch the KEK into the app server? Which one is more safe practice?

Sending the key en_dek to the key server to be handled there is pretty much how a hardware security module works at a high level. The advantage of this approach is that the key material never touches the application server. So any compromise of the application server can only decrypt the keys that are currently in memory, as well as potentially submit decryption requests to the key server. However, the attacker does not automatically get access to the master key itself, which is important.

Given you're already talking about sending the data to another system to be decrypted, I wonder if it might not be better simply to send the data, thereby hiding the key entirely from the application server?

In any scheme like this, the security of the key depends entirely on the security of the server or HSM on which it is stored. Usually, true HSMs are reasonably good at protecting keys unless you're very handy with electronics.

Both these methods have a flaw that if an attacker is able to get access to either of these servers, he will get to know both KEK and DEK. How can we prevent this?

You can't, except by using a dedicated HSM, which is essentially what you're discussing building. I would simplify the scheme and keep the keys on one server dedicated only to cryptographic operations, and keep that server locked down.

For more on these two points, see the OWASP rules on cryptographic storage.

When we talk about key rotation, which of the two keys (DEK and KEK) is rotated?

Ideally, both. Rotating the KEK should be easier, since DEKs are fixed size and there shouldn't be all that many of them. Rotating DEKs might be harder if you are encrypting lots of data.

From the OWASP page, which says it better than I can:

Rekeying refers to the process of decrypting data and then re-encrypting it with a new key. Periodically rekeying data helps protect it from undetected compromises of older keys. The appropriate rekeying period depends on the security of the keys. Data protected by keys secured in dedicated hardware security modules might only need rekeying every three years. Data protected by keys that are split and stored on two application servers might need rekeying every year.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.