nocoiner

NOTICE: The previous version (0.0.1) of this API used the Galois/Counter Mode encryption.
This authenticated encryption algorithm is not a committing encryption (that's, a
lockable box). The main reason is the weakness of the internal GCM hash against Collision
Attacks (a hash with 256-bits or 512-bits images would be likely resistant, but GCM uses
128-bits of output/digest images). For more details, please refer to the historical issue here.
The new API, therefore, breaks if you try to reveal/open commitments generated by the previous API.

About

This project implements Commitment Schemes using the
Encrypt-then-MAC approach of authenticated encryption. Because this kind of
encryption algorithm provides both Message Confidentiality and Integrity, it fits
perfectly the Hiding and Binding properties of Commitment Schemes.
Confidentiality protects the message against passive attacks while integrity
protects it from active attacks.

The hiding property states that it is impossible to discover the secret with the
commitment data left alone, that is, the commitment receiver can't know the
secret until the commitment sender reveals that through her opening key.

The binding property, on the other hand, ensures invariants on the commitment
sender side. It disallows the sender to change the secret by using a different
opening key. While the sender can refuse to reveal her secret, she can't cheat
on the game. There's a variant of commitment schemes called Timed Commitments
where the receiver can brute-force the commitment in the case of the sender
aborting the game by refusing to send the opening key, tho. Another variant
called Fuzzy Commitments accepts some noise during opening phase.

Commitment Schemes are one of the many Secure Multiparty Computation
protocols/primitives, Secret Sharing is other famous cryptographic
primitive in such field.

Installation

For the stable release, just type:

$ opam install nocoiner

To install/test the unstable version on this repository (assuming you're
inside the project's root directory):

$ make install # 'make uninstall' reverts the changes

Testing

$ make test

Usage

As library (assuming you have linked the package nocoiner below):

let secret = "I have nothing to hide."
let (c, o) = Nocoiner.commit secret

assert (secret = Nocoiner.reveal ~commitment:c ~opening:o)

Here, the Nocoiner.commit operation is non-deterministic and the
Nocoiner.reveal is deterministic. The Nocoiner.reveal operation may throw
the following exceptions:

• Nocoiner.Reasons.InvalidCommitment, if the parsing of commitment fails.

• Nocoiner.Reasons.InvalidOpening, if the opening key contains invalid data.

• Nocoiner.Reasons.BindingFailure, if both commitment & opening are unrelated.

As the command-line interface (ignore all the $ below while typing):

$ echo "Something not really secret..." > secret.txt
$ cat secret.txt | nocoiner commit \
  --commitment-file=commitment-box.txt \
  --opening-file=opening-key.txt
$ nocoiner reveal \
  --commitment-file=commitment-box.txt \
  --opening-file=opening-key.txt > secret-output.txt
$ cat secret-output.txt

The complete API reference is available here. Coverage reports are
generated too, please refer to the respective page.

Disclaimer

This library was not fully tested against side-channel attacks. Keep in mind
that the use cases of this library is for Secure Multiparty games such as online
Gambling and Auctions. With other use cases, the security of this cryptographic
primitive can be deemed as flawed.

Note that players can abort in the middle of a Commit-and-Reveal game, so you
should as well deal with that on your code logic. The random encryption key
and input vector only ensure the uniqueness locally, it's also possible to
happen collisions of both random data on a distributed setting (it's due the
sources of entropy being remote and different - so commitments and openings
would be identical, think on that even if this probability is small). ~~In such
case, you can either take a fingerprint of the host machine and a timestamp
nonce into account, in the same sense of Elliott's CUID library~~ (we already
cover that issue of distributed collisions by using a fingerprint of hashed
process context).