git-annex/doc/encryption.mdwn

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git-annex mostly does not use encryption. Anyone with access to a git
repository can see all the filenames in it, its history, and can access
any annexed file contents.

Encryption is needed when using [[special_remotes]] like Amazon S3, where
file content is sent to an untrusted party who does not have access to the
git repository.

Such an encrypted remote uses strong ([[symmetric|design/encryption]] or
asymmetric) encryption on the contents of files, as well as HMAC hashing
of the filenames. The size of the encrypted files, and access patterns
of the data, should be the only clues to what is stored in such a
remote.

You should decide whether to use encryption with a special remote before
any data is stored in it. So, `git annex initremote` requires you
to specify "encryption=none" when first setting up a remote in order
to disable encryption. To use encryption, you run 
run `git-annex initremote` in one of these ways:

* `git annex initremote newremote type=... encryption=hybrid keyid=KEYID ...`
* `git annex initremote newremote type=... encryption=shared`
* `git annex initremote newremote type=... encryption=pubkey keyid=KEYID ...`

## hybrid encryption keys

The [[hybrid_key_design|design/encryption]] allows additional
encryption keys to be added on to a special remote later. Due to this
flexability, it is the default and recommended encryption scheme.
 
	git annex initremote newremote type=... [encryption=hybrid] keyid=KEYID ...

Here the KEYID(s) are passed to `gpg` to find encryption keys. 
Typically, you will say "keyid=2512E3C7" to use a specific gpg key.
Or, you might say "keyid=joey@kitenet.net" to search for matching keys.

To add a new key and allow it to access all the content that is stored
in the encrypted special remote, just run `git annex
enableremote` specifying the new encryption key:

	git annex enableremote myremote keyid+=788A3F4C

While a key can later be removed from the list, note that
that will **not** necessarily prevent the owner of the key
from accessing data on the remote (which is by design impossible to prevent,
short of deleting the remote). In fact the only sound use of `keyid-=` is
probably to replace a revoked key:

	git annex enableremote myremote keyid-=2512E3C7 keyid+=788A3F4C

See also [[encryption_design|design/encryption]] for other security
risks associated with encryption.

## shared encryption key

Alternatively, you can configure git-annex to use a shared cipher to
encrypt data stored in a remote. This shared cipher is stored,
**unencrypted** in the git repository. So it's shared among every
clone of the git repository. 
	
	git annex initremote newremote type=... encryption=shared

The advantage is you don't need to set up gpg keys. The disadvantage is
that this is **insecure** unless you trust every clone of the git
repository with access to the encrypted data stored in the special remote.

## regular public key encryption

This alternative simply encrypts the files in the special remotes to one or
more public keys. It might be considered more secure due to its simplicity
and since it's exactly the way everyone else uses gpg.

	git annex initremote newremote type=.... encryption=pubkey keyid=KEYID ...

A disavantage is that is not easy to later add additional public keys
to the special remote. While the `enableremote` parameters `keyid+=` and
`keyid-=` can be used, they have **no effect** on files that are already
present on the remote. Probably the only use for these parameters is 
to replace a revoked key:

	git annex enableremote myremote keyid-=2512E3C7 keyid+=788A3F4C

But even in this case, since the files are not re-encrypted, the revoked
key has to be kept around to be able to decrypt those files.
(Of course, if the reason for revocation is
that the key has been compromised, it is **insecure** to leave files
encrypted using that old key, and the user should re-encrypt everything.)

(Because filenames are MAC'ed, a cipher still needs to be
generated (and encrypted to the given key IDs).)

## MAC algorithm

The default MAC algorithm to be applied on the filenames is HMACSHA1. A
stronger one, for instance HMACSHA512, one can be chosen upon creation
of the special remote with the option `mac=HMACSHA512`. The available
MAC algorithms are HMACSHA1, HMACSHA224, HMACSHA256, HMACSHA384, and
HMACSHA512. Note that it is not possible to change algorithm for a
non-empty remote.