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encryption.md

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Syncrypt Encryption

In this document you will find a detailed description of how Syncrypt encrypts your files, interacts with the server and handles your cryptographic keys. Remember that you don't have to take our word for it. You can verify the described scheme in the code of our GPLv3-licensed client.

You should understand the basics of symmetric as well as public key cryptography.

Vaults

Syncrypt employs both symmetric and asymmetric encryption to secure your files. Asymmetric encryption on the level of the vault means that you hold the private key to the vault and the server will never see it. That key pair will be used to encrypt file metadata and vault metadata.

Currently, the vault metadata protected by the vault key solely consists of

  • the vault name.

By default, Syncrypt uses 4096 bit RSA keys.

File encryption

For the actual encryption of the contents of a file Syncrypt uses AES, a symmetric encryption algorithm. The AES key is part of the file metadata and is protected by the vault key.

What is sent to the server when a file is uploaded?

  • SHA-256 Hash of the file path
  • Message Authentication Code (MAC), see Authenticated Encryption below
  • File metadata (which is also protected by the vault key):
    • File name and path
    • AES key
  • Encrypted file contents

Prior to the encryption, the file content is compressed using snappy.

Authenticated Encryption

To ensure data integrity, Syncrypt uses a Message Authentication Code (MAC) for each file revision. This code is basically the SHA-256 hash of the plaintext file contents and the file's AES key. In Authenticated Encryption, this is known as Encrypt-and-MAC. We might change the AE protocol to Encrypt-then-MAC in the future, because it has been proven more robust.

Sharing files with new users or devices

In addition to the vault and file keys, Syncrypt also has the concept of user keys. These keys identify a user (or a device). In order to share a vault with a new user or a new device, the above mentioned vault keys need to be transmitted to the new device. This can be done either

  • manually by exporting and importing the keys or
  • by letting Syncrypt encrypt those keys with the public key of the target user or device.

In the latter case, Syncrypt will transmit those encrypted keys over the Syncrypt server. Note that this step is fully optional and secured by the same strong public key cryptography that secures your vault in the first place.

Example

In this section you can find a description of all files that are created by the Syncrypt client and what they are used for.

Lets say you've created a vault and also pushed some files. To see all files that Syncrypt uses for this vault, simply type

find .vault

in the vault's directory. It should give you a list of files like this:

.vault
.vault/metadata
.vault/metadata/ac
.vault/metadata/ac/7599eb8787664eb1c45b915539f7d3eb2dd796eecf24b919ce9b5b80d930ed
.vault/metadata/36
.vault/metadata/36/65d65394f4f58a56a256ad6dd8621c68118d90fe56a19387e251c19cec2d2e
.vault/metadata/7d
.vault/metadata/7d/72b25977fd4b48208b8242906a307d477d7d5b26b655cd2c80fb6f12cf7e5f
.vault/config
.vault/id_rsa
.vault/id_rsa.pub

If you are familiar with SSH keys, you should recognize the key pair id_rsa and id_rsa.pub. These files are the private and public key to this vault, respectively. The config file contains such information as the vault's name, ID and files to ignore.

In the folder .vault/metadata the metadata of all files is stored. As mentioned above, we will identify a file by the SHA-256 hash of its file name. This hash is also the filename in the metadata folder. In this metadata, you can find the AES key and the actual filename of the file.