sig

The simple code signature toolchain for git repos.

Features

• Attach any number of signatures to any given git ref
• Verify git history contains a minimum threshold of unique commit siguatures
• Verify signatures belong to a defined GPG alias group
• Verify code changes made since last time minimum valid signatures were present
• Allow user to manually verify new keys and add to alias groups on the fly
• Prompt user to install or upgrade any required tools as needed
• Signs aginst git agnostic "tree hash" so signatures survive rebases
  • So long as the directory contents at a given ref do not change

Install

1. Clone

   git clone git@gitlab.com/pchq/sig.git sig
   

2. Review source code and signatures manually

   Using sig to verify the signatures of sig itself is not recommended.

   Consider using the following one liner which is much faster to review:

   while read -r line; do \
     gpg --verify \
       <(printf "$line" | sed 's/.*pgp://g'| openssl base64 -d -A) \
       <(printf "$line" | sed 's/pgp:.*/pgp/g'); \
   done < <(git notes --ref=signatures show)
   

3. Copy to $PATH

   cp sig ~/.local/bin/
   

Usage

• sig verify [-g,--group=] [-t,--threshold=] [-r,--ref= ] [-d,--diff=]
  • Verify m-of-n signatures by given group are present for a given git ref.
• sig add
  • Add signature to this git ref
• sig fetch [-g,--group=]
  • Fetch key by fingerprint. Optionally add to group.
• sig help
  • Show help text.
• sig version
  • Show version information.

Methods

Git

This method verifies the current HEAD was signed exactly as-is by one or more keys.

This counts the commit signature, and any number of signed tags pointing at this ref.

Assumptions

• Single sig mode: Repo contents controlled by signer
• Multi-sig mode: Repo contents verified by multiple signers
• Multi-sig group mode: Repo contents approved by specified individuals
• Hashing scheme for respective backend is not broken: (sha256)

Examples

Verify at least one signature is present with a known key

sig verify

Verify 2 unique signatures from known keys

sig verify --threshold 2

Verify 3 unique signatures from specified signing group

sig verify --threshold 3 --group myteam

Show diff between HEAD and last ref with 2 verified unique signatures

sig verify --threshold 2 --diff

Add signature

sig add

Frequently Asked Questions

Why Bash?

Because it is easy to quickly verify at any time, has wide OS compatibility and the majority of the needed operations are calling other programs already on most systems like gpg and openssl.

If this were in another language it would be harder to audit on the fly, would require the user to have a specific language toolchain installed, and it would still mostly just be a bunch of shell executions to call system binaries anyway.

Why PGP?

In spite of many popular claims to the contrary, PGP is still the most well supported protocol for distribution, verification, and signing for keys held by individual humans. It is also the only protocol with wide HSM support allowing you to keep keys out of system memory and require physical approval for each operation. E.G a trezor, ledger, yubikey, etc.

Admittedly the GnuPG codebase itself is a buggy dated mess, but PGP as a spec is still Pretty Good for many use cases. A recent modern rewrite by a number of former GnuPG team members is near complete and set to give PGP a long and stable future.

See: https://sequoia-pgp.org/

Why not "notary" ?

Notary is very well designed and well supports many HSMs.

It may be worth supporting as an alternate method in the future if m-of-n multisig is ever implemented as a part of the TUF specification which has been on their TODO list for a few years now.

It has the very desirable feature of conditionally expiring signatures which no other solution has at the time of this writing, which comes from it being purpose built for software signing concerns.

See: The Update Framework

Why not straight "openssl" ?

Openssl has HSM support via OpenSC that is fairly well supported via PKSC#11.

Contributions suggesting this an alterantive backend to OpenPGP are welcome, however they would have to also come with methods for key discovery and pinned key groups via configuration files of some kind.

PGP gives us these features almost for free.

Why not "signify", "age", or "crev" ?

These alternatives have poor if any support for HSM workflows and thus put private keys at too much risk of theft or loss to recommend for general use at this time.

That said, verifying folders/repos that use these methods is certianly of value and contributions to support doing this on systems where those tools are available are welcome.