RavenDB uses an algorithm called XChaCha20Poly1305 with 256 bits key to encrypt the data. But as we have learned, just using a key is not good enough, we need to use a nonce as well. This is easy when you need to encrypt a message in one go, but the way encryption in RavenDB works, we need to encrypt pieces of the data (randomly, depending on the way users are accessing the system).
In order to do that, RavenDB encrypt each page (usually 8KB in size) independently of each other. We actually use a different key for each page, derived from the master key, but I’ll touch on that in a different post. Here, I want to talk about nonce.
Encryption today is not just about hiding data, it is also about being able to detect if the value has been tampered with, typically called authenticated encryption (AEAD). The algorithm we use requires 16 bytes for the message authentication code (MAC). The problem is that we need to store that MAC somewhere, and the nonce as well. And that value cannot be in the page itself, since that is encrypted.
Luckily for us, we have the page header, a 64 bytes that are reserved at the beginning of each page. And we planned things accordingly to ensure that RavenDB will use only 32 bytes out of the header, giving us 32 bytes free for the encryption to use. The problem is that the XChaCha20Poly1305 algorithm uses a 16 bytes MAC and a 24 bytes nonce. And that is a bit too much to fit in 32 bytes, as you can imagine. Here is the kind of space allocation we have:
Increasing the size of the page header will have repercussions throughout our system, very late in the game, so we didn’t want to do that. Instead, we cheated. The 16 bytes of the nonce are generated using a cryptographic random number generator, but we pass a pointer to the page header 8 bytes before the nonce, so the encryption algorithm also takes the last 8 bytes of the page header itself into account in the nonce. We are guaranteed at least 128 bits of strong randomness there, and the page header itself will change from time to time, obviously, but we rely on the nonce random bytes to ensure uniqueness.
In this manner, we are able to fit the encryption requirements into our existing file structure and have strong encryption without uprooting everything.