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 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

In order 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 represent 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 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 used? In general, the principles of operation include 3 steps:

• Seal Definition.

• Seal Closing.

• Seal Verification.

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

Seal Definition

In Seal definition, Alice promise to Bob (either in private or in public) to create some message (in practice an 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 represents the committed 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 represent 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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