Saturday, April 27, 2013

Using Git for Making Law

The other day I was reading the transcript of the Constitution of the United States, and I noticed annotations in the body whenever a paragraph is changed by an amendment. In a sense, legal text has developed a process of "patching" similar to how programmers manage source code of a software system. A "patch" in the revision control sense is a precise editing instruction for insertion and deletion of text, and that allows a reviewer to easily tell exactly what has been changed.

There are many revision control systems, but I'm only going to write about this one called Git because that's the one I know more about. Git is a particularly interesting revision control system in that it enjoys the following properties.
  • The repository is distributed. A complete revision history is replicated among people who choose to clone a repository. This includes all objects that are required to reconstruct a given snapshot some point in the history.
  • Revision is structured as a tree-like lineage of the SHA1 cryptographic digest which serves the purpose of integrity checking. That means any modification to the history will result in a different lineage because the digest will be different. Forging SHA1 digest is a computationally expensive process that requires brute-forcing at least \( 2^{61} \) possibilities to find a collision.
  • Each revision, called a "commit", identifies authorship of the change. Furthermore, a commit can be signed using asymmetric cryptography. This allows a person other than the author of the changes to endorse a change. The endorsement is also computationally hard to forge and can be easily verified by a computer.
It turns out that Joel C. Salomon had attempted to represent the US constitution as a Git repository as an exercise, after a proposal on Hacker News. I think this exercise would become even more interesting if he had incorporated cryptographic signing to his approach.

When using asymmetric cryptography, a pair of keys are made that are mathematically associated, the public key and the secret (private) key. For encryption, the public key is used to encrypt, and the secret key is used to decrypt. For signing, the secret key is used to sign, and the public key is used to verify the signature. The public key is what everyone else has to trust, while the secret key most be kept safely guarded or the trust would be breached.

In effect, using Git, we can develop a totally democratic legal system where:
  • Anyone can clone the text of law and make modifications to their own drafts. They can also see how other drafts are formulated over time and who wrote what section.
  • The complete lineage of legal system development will be of interest to legal historians.
  • Only revisions that are signed by a trusted public key will become legally effective.
Of course, this brings the question of how to establish trust. Normal individuals can use a Web of Trust system where a group will endorse each other's public key for trustworthiness. People could organize a key-signing party where they will meet each other in person and endorse the public keys of the people they met. For the purpose of public affair, it's probably easier to organize trust by certificate authority. However, regardless of how trust is established, the public key is only as trustworthy as the manner how the secret key is guarded.

As a corollary, if a secret key is stored on a computer, then the public key is only as trustworthy as the computer that stores the secret key. We all know how computers are notoriously difficult to secure. It then makes sense to have a limited-purpose computer to store secret keys, and reduce the problem to physical security of that computer. Although secret keys can also be encrypted with a passphrase, brute-forcing the passphrase is still much easier than decrypting a message or forging a signature without the secret key.

And then I found out about smart cards that are essentially the limited-purpose computer. Git uses GnuPG for revision signing, and there are two main card types that can be used with GnuPG. One is the commercially available PKCS#15 cards supported by OpenSC (via gnupg-pkcs11), and the other is OpenPGP card developed by g10code. Gooze gives certain free software developers a free Feitian PKI smart card which is a PKCS#15 card. Free Software Foundation Europe gives every fellowship member an Fellowship Smartcard which is a branded OpenPGP card. You can also buy OpenPGP cards from Kernel Concepts in Germany.

These cryptography smart cards are interesting because they can generate the public and secret key pair on the card, and the secret key never leaves the card. The signing and decryption happens on the card itself. The card is protected by a passphrase similar to how secret key is protected. In addition, the card will only allow a limit number of tries before locking itself out, so it improves the physical security.

However, if the secret key never leaves the smart card, then losing the smart card will lose the secret key which is a big deal because the web of trust has to be established from scratch again. To deal with key management problems, some people create several key pairs, a master key pair used for signing only, and several subkeys, one for encryption, one for signing, and one for authentication. Trust is established for the master key, which is used to endorse the subkeys.

Note that FSFE does not recommend using the smart card for the master key, but only for subkeys. I'm not sure what their rationale is, but I think it should be the other way around.

First create a master key pair on the primary smart card. Use it to endorse subkeys. The subkeys can be stored on a relatively insecure computer for daily use, or on a secondary smart card that you carry around. Keep the master key smart card in a safe place like a bank vault. Only use the master key when revoking an existing subkey, endorsing new subkeys, or endorsing other people's master public key.

The technology to revolutionize the editorial process of law making is readily available and in practical use. Git can be used to track revision lineage, and asymmetric cryptography can be used to given authority on a particular revision. Practical means to establish and secure trust is available in the form of a smart card, and there are effective means of key management that minimizes the chance of losing a key. It is finally time for law making which existed since the beginning of written history to enter the informational age!
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