git-remote-gcrypt

GNU Privacy Guard-encrypted git remote

Manual section:	1

Description

Remote helper programs are invoked by git to handle network transport. This helper handles gcrypt:: URLs that will access a remote repository encrypted with GPG, using our custom format.

Supported locations 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.

The aim is to provide confidential, authenticated git storage and collaboration using typical untrusted file hosts or services. PLEASE help us evaluate how well we meet this design goal!

Quickstart

Install git-remote-gcrypt by running the supplied install.sh script.

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 unncessary passphrase prompts.

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.

gcrypt.gpg-args

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

Environment Variables

GCRYPT_FULL_REPACK: This environment variable forces full repack when pushing.

Note to users of GnuPG version 2

When a repository was pushed with gcrypt.publish-participants false (i.e. the recipient is anonymous), and you are using GnuPG version 2, you will need to include --try-all-secrets or --try-secret-key ID in gcrypt.gpg-args.

GnuPG version 1 always tries all available secret keys when it encounters a message with an anonymous recipient.

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 and curl for remotes rsync: and sftp: 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.

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 if 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.

License

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

arpad9/git-remote-gcrypt