Single-use Seals and Proof of Publication

Single-use Seals are cryptographic primitives proposed by Peter Todd in ~2016. They are a kind of cryptographic commitment that resembles the application of a physical seal to a container. They can be used to prove a sequence of events to a party, thereby limiting the risk that such a sequence may be altered after it has been established. This implies that such commitment schemes are a more sophisticated form of both simple commitments (i.e. digest/hash) and timestamping.

Physical single-use seals: once closed their protected content cannot be altered

To work properly, Single-use Seals require a Proof-of-Publication Medium: it may be a medium with global consensus (such as blockchains), not necessarily decentralized, which has the ability to be difficult to forge or replicate once issued and made public. A newspaper represents a widespread example of this concept.

The Proof-of-Publication Medium will be used:

• To prove that every member p in an audience P has received a certain message m.

• To prove that the message m has not been published.

• To prove that some member q is in the audience P.

With these properties, we can give a more formal definition:

Single-Use-Seal is a formal promise to commit to a (yet) unknown message in the future, once and only once, such that the fact of commitment is demonstrably known to all members of a certain audience.

With this definition and the general properties above, we can compare the properties of the various cryptographic primitives along with Single-use Seals:

Property	Simple commitment (digest/hash)	Timestamp	Single-Use-Seals
Commitment publication does not reveal the message	Yes	Yes	Yes
Proof of the commitment time / message existence before certain date	Not Possible	Possible	Possible
Prove that no alternative commitment can exist	Not Possible	Not Possible	Possible

So how can we practically construct a disposable seal and what operations have to be used? In general, the principles of operation include 3 steps:

• Seal Definition.

• Seal Closing.

• Seal Verification.

For the following operation examples, we will use the well-known computer science characters, Alice and Bob.

Seal Definition

In Seal Definition, Alice promises to Bob (either in private or in public) to create some message (in practice a hash of some data):

• At a well-defined point in time and space.

• Using an agreed publication medium.

Seal Closing

When Alice publishes the message following all the rules stated in the Seal definition, in addition, she produces also a witness, which is the proof that the seal has indeed been closed.

By closing a message with a single-use seal, such message cannot be altered. In it's digital form, inscribed in Layer 1, the seal cannot be opened any more.

Seal Verification

Once closed the seal, being "single-use", cannot be opened nor closed again. The only thing Bob can do is to check whether the seal has actually been closed around the message commitment, using as inputs: the seal, the witness, and the message (which is a commitment to some data).

In Computer Science Language the whole procedure can be summed up as follows:

seal <- Define()                         # Done by Alice, accepted by Bob.

witness <- Close(seal, message)          # Close a seal over a message, done by Alice.

bool <- Verify(seal, witness, message)   # Verify that the seal was closed, done by Bob.

The combination of Single-Use-Seals and Client-Side-Validation enables a distributed system that does not require global consensus (i.e. a blockchain) to store all the data that matters to some counterparts, providing a high level of scalability and privacy. However, this is not enough to make the system work. Because the definition of a single-use seal is done on the client side and does not need to be included in the global consensus medium, a party can’t prove that the definition of the seal ever took place even if one is a member of the audience observing the publication medium.

We therefore need a “chain” of Single-Use-Seals, where the closure of the previous seal incorporates the definition of subsequent seal(s): this is what RGB does together with Bitcoin:

• Messages represent the commitment to client-side validated data.

• Seal definitions are bitcoin UTXO.

• The commitment is a hash entered within a Bitcoin transaction.

• The seal closure can be a UTXO that is spent or an address to which a transaction credits some bitcoins.

• The chain of connected transaction spends represents the Proof-of-Publication.

In the next chapters, we will explore in detail how RGB implements the concept of Single-Use-Seal by storing the commitments of its operation in the Bitcoin blockchain.

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