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I would like to provision services from a Software as a Service (SaaS) provider. This is multitenant environment (multiple companies will be using a single instance of the application, with a single database, running on the same hardware and same OS).

Our company has a requirement to encrypt our data at rest (e.g. file stores, databases, backups).

Most of the guidance I have read states that the key should be kept with the customer or a third party and not with the cloud provider.

If I have the key; then every user in my company will need the key to access the file store once logged in? How would this work practically? Will the key be binded with my a users Username and Password?

Other option is to store the key in a configuration file on the cloud platform and managed by the cloud provider. But this creates another problem of securing the config file and the risk of having our key on a shared platform.

Where can the cloud provider store the key securely so that only I and my employees can use it?

What is best practice in terms of key management when having encrypted data on the cloud?

Best practice from my research states the following:

• Store the key with the customer or 3rd party

• Only authorised persons to access the key.

• Do not store the key on the same platform as the data.

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One approach to doing this is using end-to-end encryption, where the provider never has access to the keys. A system that uses proxy re-encryption (PRE) can allow each user to have her own keys, but admins can manage a group that represents the users for your company, so it is not necessary to re-encrypt all the files the company can access every time a new user is added or a user is removed.

A paper that describes this in detail is available here (this directs through my company's website to the ACM digital library - sorry for the extra hop, but it avoids the need to have ACM Digital Library access to get the paper). In summary, proxy re-encryption is a scheme originally designed to allow the delegation of encryption from one person to another. PRE schemes are based on public key cryptography, and they allow someone (the delegator) who has her own private key plus the public key of another person (the delegatee) to compute a re-encryption key. This key will allow ciphertexts encrypted to the delegator to be re-encrypted or transformed so that they can be decrypted using the delegatee's private key. This process does not allow the proxy or individual doing the transformation to decrypt the ciphertext.

If the delegator is an abstract entity that represents your company, and system administrators control access to that entity's private key, they can generate re-encryption keys to different users within the company. Revoking access is achieved by removing the re-encryption key between the company and an individual user.

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