git-remote-gcrypt

GNU Privacy Guard-encrypted git remote

Manual section:	1

Description

git-remote-gcrypt is a git remote helper to push and pull from repositories encrypted with GnuPG, using a custom format. This remote helper handles URIs prefixed with gcrypt::.

Supported backends are local, rsync:// and sftp://, where the repository is stored as a set of files, or instead any <giturl> where gcrypt will store the same representation in a git repository, bridged over arbitrary git transport. Prefer local or rsync:// if you can use one of those; see "Performance" below for discussion.

There is also an experimental rclone:// backend for early adoptors only (you have been warned).

The aim is to provide confidential, authenticated git storage and collaboration using typical untrusted file hosts or services.

Installation

• use your GNU/Linux distribution's package manager -- Debian, Ubuntu, Fedora, Arch and some smaller distros are known to have packages
• run the supplied install.sh script on other systems

Quickstart

Create an encrypted remote by pushing to it:

git remote add cryptremote gcrypt::rsync://example.com/repo
git push cryptremote master
> gcrypt: Setting up new repository
> gcrypt: Remote ID is :id:7VigUnLVYVtZx8oir34R
> [ more lines .. ]
> To gcrypt::[...]
> * [new branch]      master -> master

Configuration

The following git-config(1) variables are supported:

remote.<name>.gcrypt-participants

gcrypt.participants

Space-separated list of GPG key identifiers. The remote is encrypted to these participants and only signatures from these are accepted. gpg -k lists all public keys you know.

If this option is not set, we encrypt to your default key and accept any valid signature. This behavior can also be requested explicitly by setting participants to simple.

The gcrypt-participants setting on the remote takes precedence over the repository variable gcrypt.participants.

remote.<name>.gcrypt-publish-participants

gcrypt.publish-participants

By default, the gpg key ids of the participants are obscured by encrypting using gpg -R. Setting this option to true disables that security measure.

The problem with using gpg -R is that to decrypt, gpg tries each available secret key in turn until it finds a usable key. This can result in unnecessary passphrase prompts.

gcrypt.gpg-args

The contents of this setting are passed as arguments to gpg. E.g. --use-agent.

remote.<name>.gcrypt-signingkey

user.signingkey

(The latter from regular git configuration) The key to use for signing. You should set user.signingkey if your default signing key is not part of the participant list. You may use the per-remote version to sign different remotes using different keys.

remote.<name>.gcrypt-rsync-put-flags

gcrypt.rsync-put-flags

Flags to be passed to rsync when uploading to a remote using the rsync:// backend. If the flags are set to a specific remote, the global flags, if also set, will not be applied for that remote.

remote.<name>.gcrypt-require-explicit-force-push

gcrypt.require-explicit-force-push

A longstanding bug is that every git push effectively has a --force.

If this flag is set to true, git-remote-gcrypt will refuse to push, unless --force is passed, or refspecs are prefixed with +.

There is a potential solution here: https://bugs.debian.org/877464#32

Environment variables

GCRYPT_FULL_REPACK

When set (to anything), this environment variable forces a full repack when pushing.

Examples

How to set up a remote for two participants:

git remote add cryptremote gcrypt::rsync://example.com/repo
git config remote.cryptremote.gcrypt-participants "KEY1 KEY2"
git push cryptremote master

How to use a git backend:

# notice that the target git repo must already exist and its
# `next` branch will be overwritten!
git remote add gitcrypt gcrypt::git@example.com:repo#next
git push gitcrypt master

The URL fragment (#next here) indicates which backend branch is used.

Notes

Collaboration

The encryption of the manifest is updated for each push to match the participant configuration. Each pushing user must have the public keys of all collaborators and correct participant config.

Dependencies

rsync, curl and rclone for remotes rsync:, sftp: and rclone: respectively. The main executable requires a POSIX-compliant shell that supports local.

GNU Privacy Guard

Both GPG 1.4 and 2 are supported. You need a personal GPG key. GPG configuration applies to algorithm choices for public-key encryption, symmetric encryption, and signing. See man gpg for more information.

Remote ID

The Remote ID is not secret; it only ensures that two repositories signed by the same user can be distinguished. You will see a warning if the Remote ID changes, which should only happen if the remote was re-created.

Performance

Using an arbitrary <giturl> or an sftp:// URI requires uploading the entire repository history with each push. This means that pushes of your repository become slower over time, as your git history becomes longer, and it can easily get to the point that continued usage of git-remote-gcrypt is impractical.

Thus, you should use these backends only when you know that your repository will not ever grow very large, not just that it's not large now. This means that these backends are inappropriate for most repositories, and likely suitable only for unusual cases, such as small credential stores. Even then, use rsync:// if you can. Note, however, that rsync:// won't work with a repository hosting service like Gitolite, GitHub or GitLab.

rsync URIs

The URI format for the rsync backend is rsync://user@host/path, which translates to the rsync location user@host:/path, accessed over ssh. Note that the path is absolute, not relative to the home directory. An earlier non-standard URI format is also supported: rsync://user@host:path, which translates to the rsync location user@host:path

rclone backend

In addition to adding the rclone backend as a remote with URI like gcrypt::rclone://remote:subdir, you must add the remote to the rclone configuration too. This is typically done by executing rclone config. See rclone(1).

The rclone backend is considered experimental and is for early adoptors only. You have been warned.

Repository format

EncSign(X): Sign and Encrypt to GPG key holder

Encrypt(K,X): Encrypt using symmetric-key algorithm

Hash(X): SHA-2/256

B: branch list

L: list of the hash (Hi) and key (Ki) for each packfile

R: Remote ID

To write the repository:

Store each packfile P as Encrypt(Ki, P) → P' in filename Hi

where Ki is a new random string and Hash(P') → Hi

Store EncSign(B || L || R) in the manifest

To read the repository:

Get manifest, decrypt and verify using GPG keyring → (B, L, R)

Warn if R does not match previously seen Remote ID

for each Hi, Ki in L:

Get file Hi from the server → P'

Verify Hash(P') matches Hi

Decrypt P' using Ki → P then open P with git

Manifest file

Example manifest file (with ellipsis for brevity):

$ gpg -d 91bd0c092128cf2e60e1a608c31e92caf1f9c1595f83f2890ef17c0e4881aa0a
542051c7cd152644e4995bda63cc3ddffd635958 refs/heads/next
3c9e76484c7596eff70b21cbe58408b2774bedad refs/heads/master
pack :SHA256:f2ad50316...cd4ba67092dc4 z8YoAnFpMlW...3PkI2mND49P1qm
pack :SHA256:a6e17bb4c...426492f379584 82+k2cbiUn7...dgXfyX6wXGpvVa
keep :SHA256:f2ad50316...cd4ba67092dc4 1
repo :id:OYiSleGirtLubEVqJpFF

Each item extends until newline, and matches one of the following:

<sha-1> <gitref>

Git object id and its ref

pack :<hashtype>:<hash> <key>

Packfile hash (Hi) and corresponding symmetric key (Ki).

keep :<hashtype>:<hash> <generation>

Packfile hash and its repack generation

repo <id>

The remote id

extn <name> ...

Extension field, preserved but unused.

Detecting gcrypt repos

To detect if a git url is a gcrypt repo, use: git-remote-gcrypt --check url Exit status is 0 if the repo exists and can be decrypted, 1 if the repo uses gcrypt but could not be decrypted, and 100 if the repo is not encrypted with gcrypt (or could not be accessed).

Note that this has to fetch the repo contents into the local git repository, the same as is done when using a gcrypt repo.

Known issues

Every git push effectively has --force. Be sure to pull before pushing.

git-remote-gcrypt can decide to repack the remote without warning, which means that your push can suddenly take significantly longer than you were expecting, as your whole history has to be reuploaded. This push might fail over a poor link.

git-remote-gcrypt might report a repository as "not found" when the repository does in fact exist, but git-remote-gcrypt is having authentication, port, or network connectivity issues.

See also

git-remote-helpers(1), gpg(1)

Credits

The original author of git-remote-gcrypt was GitHub user bluss.

The de facto maintainer in 2013 and 2014 was Joey Hess.

The current maintainer, since 2016, is Sean Whitton <spwhitton@spwhitton.name>.

License

This document and git-remote-gcrypt are licensed under identical terms, GPL-3 (or 2+); see the git-remote-gcrypt file.