Access Control Privacy

Introducing Coconut

Coconut is a cryptographic signature scheme that produces privacy-enhanced credentials. It lets application programmers who are concerned with resource access control to think and code in a new way.

Most of the time, when we build system security, we think of who questions:

  • Has Alice identified herself (authentication)?
  • Is Alice allowed to take a specific action (authorisation)?

Coconut fundamentally changes these questions. Rather than asking who a user is, it allows application designers to ask different questions, mostly centred around questions of rights:

  • Does the entity taking this action have a right to do X?

This allows a different kind of security. Many of the computer systems we talk to every day don’t need to know who we are, they only need to know if we have a right to use the system. Coconut allows signing authorities and validators to work together to determine whether a given private key holder has a right to take an action. The credentials are generated cooperatively by decentralised, trustless systems.

Once the credentials are generated, they can be re-randomized: entirely new credentials, which no one has ever seen before, can be presented to service providers, and magically validated without being linkable back to the credential originally given out by validators.

These properties allow Coconut credentials to act as something like a decentralized and fully private version of OAuth credentials, or like cryptographic bearer tokens generated by decentralised systems. The tokens can be mutated so that they are not traceable, but still verified with the original permissions intact.

Users present cryptographic claims encoded inside the credentials to get secure access to resources despite the systems verifying credential usage not being able to know who they are.

Re-randomisation vs pseudonymity

We stand on the shoulders of giants. Ten years ago, Bitcoin showed the way forward by allowing people to control resource access without recourse to who questions. Rather, in Bitcoin and succeeding blockchains, a private key proves a right to use.

But as we can now see, private keys in blockchain systems act only as a minor barrier to finding out who is accessing resources. A Bitcoin or Ethereum address is effectively a long-lived pseudonym which is traceable through successive transactions.

Coconut allows you to build truly private access control systems rather than pseudonymous ones.

How does Coconut work?

Just like normal credentials, Nym’s Coconut credentials can be signed with a secret key and later verified by anybody with the correct public key. But Nym credentials have additional superpowers when compared to “normal” signature schemes like RSA or DSA.

Specifically, Coconut is a blinded, re-randomizable, selective disclosure threshold credential signature scheme. That’s quite a mouthful, so let’s break it down into its component parts.

Let’s say you have a message with the content Alice controls X in hand. In addition to the normal sign(message, secretKey) and verify(message, publicKey) functions present in other signature schemes, Coconut adds the following:

  1. Blind signatures - disguises message content so that the signer can’t see what they’re signing. This defends users against signers: the entity that signed can’t identify the user who created a given credential, since they’ve never seen the message they’re signing before it’s been blinded (turned into gobbledygook). Coconut uses zero-knowledge proofs so that the signer can sign confidently without seeing the unblinded content of the message.

  2. Re-randomizable signatures - take a signature, and generate a brand new signature that is valid for the same underlying message Alice controls X. The new bitstring in the re-randomized signature is equivalent to the original signature but not linkable to it. So a user can “show” a credential multiple times, and each time it appears to be a new credential, which is unlinkable to any previous “show”. But the underlying content of the re-randomized credential is the same (including for things like double-spend protection). This once again protects the user against the signer, because the signer can’t trace the signed message that they gave back to the user when it is presented. It also protects the user against the relying party that accepts the signed credential. The user can show re-randomized credentials repeatedly, and although the underlying message is the same in all cases, there’s no way of tracking them by watching the user present the same credential multiple times.

  3. Selective disclosure of attributes - allows someone with the public key to verify some, but not all, parts of a message. So you could for instance selectively reveal parts of a signed message to some people, but not to others. This is a very powerful property of Coconut, potentially leading to diverse applications: anonymous voting systems, selective revelation of medical data, anonymized KYC systems, etc.

  4. Threshold issuance - allows signature generation to be split up across multiple nodes and decentralised, so that either all signers need to sign (n of n where n is the number of signers) or only a threshold number of signers need to sign a message (t of n where t is the threshold value).

Taken together, these properties provide privacy for Peaps when it comes to generating and using signatures for cryptographic claims.

A slightly expanded view of Coconut is available in this blog post.

Using Coconut for blockchain transaction privacy

In the context of a blockchain currency system, Coconut allows Peaps to create a privacy-enhanced Coconut credential which provably represents an amount under control of a given entity. The credential can then be “spent” anonymously, as if it were the original value. Double-spending protections apply to the credential, so it can only be spent once. Nym Validators can then unlock the value so it can be redeemed by the party holding the anonymized credential.

Although there’s still work to be done to integrate it against various blockchains, in principle Coconut can anonymise blockchain transactions in any system which provides multi-sig. Bitcoin and Ethereum are obvious first targets here. We’re also looking into Cosmos integration at the moment.

Coconut is simple and flexible, and can ensure privacy for more than coin transfers; it can provide privacy for more complex smart contracts as well.

Finally, it should be mentioned that Coconut can be applied to both blockchain and non-blockchain systems - it’s a general purpose technology.