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Azure Storage - Client Side Encryption

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A few days ago, client-side encryption for Azure Storage was announced. In this post we will take a look over this feature.

First of all, you should know that the encryption/decryption takes place on client side. This means, that the content will be already encryption when it will arrive on Azure. This encryption technique is called Envelop Technique. It is very useful when you want to add another security layer over your data.
Out of the box, there are client library for .NET (including Windows Phone). For other languages, like Java is not yet supported, but because the encryption algorithm is a well know one, you may be able to implement it on other platforms also.

The encryption algorithm that is used by client library is AES (Advanced Encryption Standard). It is important to know that the encryption keys are generated by the client library. And is NEVER stored in Azure Storage. The encryption key should be stored in a different location. This library is full integration with Key Vault Service, that allow us to persist the keys in a safe location.

The encryption mechanism is not complicated, but is very useful. You don't need to encrypt by yourself, using different libraries and orchestrate all the activities. From a developer perspective, the life is more simple and the chances to introduce a bug are reduced.

There are some small things that we should consider when using this library.
First thing is related to blobs. All reads and writes operations needs to be made on the entire blob content. This means that we should not upload encrypted content using PutBlock. All the time, we should use DownloadToXXX, BlobReadStreamXXX, UploadFromXXX and OpenWriteXXX.
This is necessary because each time when you upload content, a new IV (Initialization Vector) is generated. This information is stored in blob metadata and will be used during decryption.
A similar situation is on Azure Tables. We have full support for action like Insert and Replace, but Merging is not supported.
The interesting thing is when we are using Azure Queue and we want to encrypt content. In this situation, the encryption information (IV and a random CEK) are added to each message. This means that we can work separately with each message. Take into account that for each message, the message body will contain also the IV and CEK. Because of this the size of the message and queue will be bigger.

In the below diagram, we can see where the encryption metadata are stored (IV, CEK).


If you want to enforce to encryption all the content that is stored in Azure Storage, you can set the 'RequireEncryption' to TRUE. In this way, you will enforce the client to encrypt all the content that is send. If you want to take a look over the implementation you can see the source code on GitHub - https://github.com/Azure/azure-storage-net/blob/master/Lib/ClassLibraryCommon/Blob/BlobEncryptionPolicy.cs.
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