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We have a openssl/kerberos/openssh user-authentication which require a private and public keypair.

I need to log in and fetch some data from a database. But I'm not allowed to store the data in the database in plain form. And the private key used to decrypt it needs to be stored offsite.

We do not want to use transparrent data encryption like in oracle,db2,mysql etc but create our own.

Instead of adding another encryption layer can I resue the keypair I already have.

This is to comply with PCIDSS requirements for storing sensitive data. https://www.pcisecuritystandards.org/security_standards/index.php

Some best practice advices really appreciated.

Thanks

MortenB
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2 Answers2

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Generally it is discouraged to use the same key pair for authentication and encryption. That is why a GnuPG key contains actually 2 private keys for instance. From the cryptographer's point of view you'll need to store another key pair for encryption or use a symmetric key for data encryption.

Do you read and write database contents with always the same client? If so, you can use a symmeric encryption and store the key on the client.

Daniel Böhmer
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PCI is huge and getting it wrong could destroy a business, so finding an expert might be best.

That said, here's my thoughts: If all data just needs to be encrypted so that only an off-site party can open it, use a good public key cryptosystem to save the data. (Read the documentation enough to make sure that the library encrypts the data with a session key, and encrypts the session key with the public key of the remote party. You don't ever want to encrypt data with a public key, only session keys.)

If all data needs to be stored with a per-user key, you can do that too, but Kerberos was designed specifically to avoid public key cryptosystems -- they went to a huge amount of effort to build something almost similar using only symmetric ciphers. Clever, but this might mean you can't even use the keys you were hoping to use (if the requirement really is that only an offsite-stored-key can decrypt the stored ciphertext).

Hope this helps.

sarnold
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