transcrypt

A script to configure transparent encryption of sensitive files stored in a Git repository. Files that you choose will be automatically encrypted when you commit them, and automatically decrypted when you check them out. The process will degrade gracefully, so even people without your encryption password can safely commit changes to the repository's non-encrypted files.

transcrypt protects your data when it's pushed to remotes that you may not directly control (e.g., GitHub, Dropbox clones, etc.), while still allowing you to work normally on your local working copy. You can conveniently store things like passwords and private keys within your repository and not have to share them with your entire team or complicate your workflow.

Overview

transcrypt is in the same vein as existing projects like git-crypt and git-encrypt, which follow Git's documentation regarding the use of clean/smudge filters for encryption. In comparison to those other projects, transcrypt makes substantial improvements in the areas of usability and safety.

• transcrypt is just a Bash script and does not require compilation
• transcrypt uses OpenSSL's symmetric cipher routines rather than implementing its own crypto
• transcrypt does not have to remain installed after the initial repository configuration
• transcrypt generates a unique salt for each encrypted file
• transcrypt uses safety checks to avoid clobbering or duplicating configuration data
• transcrypt facilitates setting up additional clones as well as rekeying
• transcrypt adds an alias git ls-crypt to list all encrypted files

Salt Generation

The decryption -> encryption process on an unchanged file must be deterministic for everything to work transparently. To do that, the same salt must be used each time we encrypt the same file. Rather than use a static salt common to all files, transcrypt first has OpenSSL generate an HMAC-SHA256 cryptographic hash-based message authentication code for each decrypted file (keyed with a combination of the filename and transcrypt password), and then uses the last 16 bytes of that HMAC for the file's unique salt. When the content of the file changes, so does the salt. Since an HMAC has been proven to be a PRF, this method of salt selection does not leak information about the original contents, but is still deterministic.

Usage

The requirements to run transcrypt are minimal:

• Bash
• Git
• OpenSSL
• column and hexdump commands (on Ubuntu/Debian install bsdmainutils)
• xxd command if using OpenSSL version 3 (on Ubuntu/Debian is included with vim)

...and optionally:

• GnuPG - for secure configuration import/export

You also need access to the transcrypt script itself. You can add it directly to your repository, or just put it somewhere in your $PATH:

$ git clone https://github.com/elasticdog/transcrypt.git
$ cd transcrypt/
$ sudo ln -s ${PWD}/transcrypt /usr/local/bin/transcrypt

Installation via Packages

A number of packages are available for installing transcrypt directly on your system via its native package manager. Some of these packages also include man page documentation as well as shell auto-completion scripts.

• Arch Linux
• Heroku (via Buildpacks)
• NixOS
• OS X (via Homebrew)

...see the INSTALL document for more details.

Initialize an Unconfigured Repository

transcrypt will interactively prompt you for the required information, all you have to do run the script within a Git repository:

$ cd <path-to-your-repo>/
$ transcrypt

If you already know the values you want to use, you can specify them directly using the command line options. Run transcrypt --help for more details.

Designate a File to be Encrypted

Once a repository has been configured with transcrypt, you can designate for files to be encrypted by applying the "crypt" filter, diff, and merge to a pattern in the top-level .gitattributes config. If that pattern matches a file in your repository, the file will be transparently encrypted once you stage and commit it:

$ cd <path-to-your-repo>/
$ echo 'sensitive_file  filter=crypt diff=crypt merge=crypt' >> .gitattributes
$ git add .gitattributes sensitive_file
$ git commit -m 'Add encrypted version of a sensitive file'

The .gitattributes file should be committed and tracked along with everything else in your repository so clones will be aware of what is encrypted. Make sure you don't accidentally add a pattern that would encrypt this file :-)

For your reference, if you find the above description confusing, you'll find that this repository has been configured following these exact steps.

Listing the Currently Encrypted Files

For convenience, transcrypt also adds a Git alias to allow you to list all of the currently encrypted files in a repository:

$ git ls-crypt
sensitive_file

Alternatively, you can use the --list command line option:

$ transcrypt --list
sensitive_file

You can also use this to verify your .gitattributes patterns when designating new files to be encrypted, as the alias will list pattern matches as long as everything has been staged (via git add).

After committing things, but before you push to a remote repository, you can validate that files are encrypted as expected by viewing them in their raw form:

$ git show HEAD:<path-to-file> --no-textconv

The <path-to-file> in the above command must be relative to the top-level of the repository. Alternatively, you can use the --show-raw command line option and provide a path relative to your current directory:

$ transcrypt --show-raw sensitive_file

Initialize a Clone of a Configured Repository

If you have just cloned a repository containing files that are encrypted, you'll want to configure transcrypt with the same cipher and password as the origin repository. The owner of the origin repository can dump the credentials for you by running the --display command line option:

$ transcrypt --display
The current repository was configured using transcrypt v0.2.0
and has the following configuration:

  CONTEXT:  default
  CIPHER:   aes-256-cbc
  PASSWORD: correct horse battery staple

Copy and paste the following command to initialize a cloned repository:

  transcrypt -c aes-256-cbc -p 'correct horse battery staple'

Once transcrypt has stored the matching credentials, it will force a checkout of any exising encrypted files in order to decrypt them.

Rekeying

Periodically, you may want to change the encryption cipher or password used to encrypt the files in your repository. You can do that easily with transcrypt's rekey option:

$ transcrypt --rekey

As a warning, rekeying will remove your ability to see historical diffs of the encrypted files in plain text. Changes made with the new key will still be visible, and you can always see the historical diffs in encrypted form by disabling the text conversion filters:

$ git log --patch --no-textconv

After rekeying, all clones of your repository should flush their transcrypt credentials, fetch and merge the new encrypted files via Git, and then re-configure transcrypt with the new credentials.

$ transcrypt --flush-credentials
$ git fetch origin
$ git merge origin/main
$ transcrypt -c aes-256-cbc -p 'the-new-password'

Command Line Options

Completion scripts for both Bash and Zsh are included in the contrib/ directory.

transcrypt [option...]

  -c, --cipher=CIPHER
         the symmetric cipher to utilize for encryption;
         defaults to aes-256-cbc

  -p, --password=PASSWORD
         the password to derive the key from;
         defaults to 30 random base64 characters

  --set-openssl-path=PATH_TO_OPENSSL
         use OpenSSL at this path; defaults to 'openssl' in $PATH

  -y, --yes
         assume yes and accept defaults for non-specified options

  -d, --display
         display the current repository's cipher and password

  -r, --rekey
         re-encrypt all encrypted files using new credentials

  -f, --flush-credentials
         remove the locally cached encryption credentials and  re-encrypt
         any files that had been previously decrypted

  -F, --force
         ignore whether the git directory is clean, proceed with the
         possibility that uncommitted changes are overwritten

  -u, --uninstall
         remove  all  transcrypt  configuration  from  the repository and
         leave files in the current working copy decrypted

   --upgrade
         uninstall and re-install transcrypt configuration in the repository
         to apply the newest scripts and .gitattributes configuration

  -l, --list
         list all of the transparently encrypted files in the repository,
         relative to the top-level directory

  -s, --show-raw=FILE
         show  the  raw file as stored in the git commit object; use this
         to check if files are encrypted as expected

  -e, --export-gpg=RECIPIENT
         export  the  repository's cipher and password to a file encrypted
         for a gpg recipient

  -i, --import-gpg=FILE
         import the password and cipher from a gpg encrypted file

  -C, --context=CONTEXT_NAME
         name for a context  with a different passphrase  and cipher from
         the  'default' context;   use this  advanced option  to  encrypt
         different files with different passphrases

  --list-contexts
         list all contexts configured in the  repository,  and warn about
         incompletely configured contexts

  -v, --version
         print the version information

  -h, --help
         view this help message

Caveats

Overhead

The method of using filters to selectively encrypt/decrypt files does add some overhead to Git by regularly forking OpenSSL processes and removing Git's ability to efficiently cache file changes. That said, it's not too different from tracking binary files, and when used as intended, transcrypt should not noticeably impact performance. There are much better options if your goal is to encrypt the entire repository.

Localhost

Note that the configuration and encryption information is stored in plain text within the repository's .git/config file. This prevents them from being transferred to remote clones, but they are not protected from inquisitive users on your local machine.

For safety, you may prefer to only have the credentials stored when actually updating encrypted files, and then flush them with --flush-credentials once you're done (make sure you have the credentials backed up elsewhere!). This will also revert any decrypted files back to their encrypted form in your local working copy.

Cipher Selection

Last up, regarding the default cipher choice of aes-256-cbc...there aren't any fantastic alternatives without pulling in outside dependencies. Ideally, we would use an authenticated cipher mode like id-aes256-GCM by default, but there are a couple of issues:

1. I'd like to support OS X out of the box, and unfortunately they are the lowest common denominator when it comes to OpenSSL. For whatever reason, they still include OpenSSL 0.9.8y rather than a newer release. Unfortunately, GCM-based ciphers weren't added until OpenSSL 1.0.1 (back in early 2012).

2. Even with newer versions of OpenSSL, the authenticated cipher modes don't work exactly right when utilizing the command line openssl enc.

I'm contemplating if transcrypt should append an HMAC to the aes-256-cbc ciphertext to provide authentication, or if we should live with the malleability issues as a known limitation. Essentially, malicious comitters without the transcrypt password could potentially manipulate the plaintext in limited ways (given that the attacker knows the original plaintext). Honestly, I'm not sure if the added complexity here would be worth it given transcrypt's use case.

Advanced

Contexts

Context names let you encrypt some files with different passwords for a different audience, such as super-users. The 'default' context applies unless you set a context name.

Add a context by reinitialising transcrypt with a context name then add a pattern with crypt-<CONTEXTNAME> attributes to .gitattributes. For example, to encrypt a file _top-secret in a "super" context:

# Initialise a new "super" context, and set a different password
$ transcrypt --context=super

# Add a pattern to .gitattributes with "crypt-super" values
$ echo >> .gitattributes \\
  'top-secret filter=crypt-super diff=crypt-super merge=crypt-super'

# Add and commit your top-secret and .gitattribute files
$ git add .gitattributes top-secret
$ git commit -m "Add top secret file for super-users only"

# List all contexts
$ transcrypt --list-contexts

# Display the cipher and password for the "super" context
$ transcrypt --context=super --display

License

transcrypt is provided under the terms of the MIT License.

Copyright © 2014-2020, Aaron Bull Schaefer.

Contributing

Linting and formatting

Please use:

• the shellcheck tool to check for subtle bash scripting errors in the transcrypt file, and apply the recommendations when possible. E.g: shellcheck transcrypt
• the shfmt tool to apply consistent formatting to the transcrypt file, e.g: shfmt -w transcrypt
• the Prettier tool to apply consistent formatting to the README.md file, e.g: prettier --write README.md

Tests

Tests are written using bats-core version of "Bash Automated Testing System" and stored in the tests/ directory.

To run the tests:

• install bats-core
• run all tests with: bats tests/
• run an individual test with e.g: bats tests/test_crypt.bats