Set of traits that allow to implement Peter’s Todd single-use seal paradigm. Information in this file partially contains extracts from Peter’s works listed in “Further reading” section.
Analogous to the real-world, physical, single-use-seals used to secure shipping containers, a single-use-seal primitive is a unique object that can be closed over a message exactly once. In short, a single-use-seal is an abstract mechanism to prevent double-spends.
A single-use-seal implementation supports two fundamental operations:
Close(l,m) → w— Close seal l over message m, producing a witness
Verify(l,w,m) → bool— Verify that the seal l was closed over message
A single-use-seal implementation is secure if it is impossible for an attacker to cause the Verify function to return true for two distinct messages m1, m2, when applied to the same seal (it is acceptable, although non-ideal, for there to exist multiple witnesses for the same seal/message pair).
Practical single-use-seal implementations will also obviously require some way of generating new single-use-seals:
Gen(p)→l— Generate a new seal basing on some seal definition data
Single-use-seal: a commitment to commit to some (potentially unknown) message. The first commitment (i.e. single-use-seal) must be a well-defined (i.e. fully specified and unequally identifiable in some space, like in time/place or within a given formal informational system). Closing of a single-use-seal over message: a fulfilment of the first commitment: creation of the actual commitment to some message in a form unequally defined by the seal. Witness: data produced with closing of a single use seal which are required and sufficient for an independent party to verify that the seal was indeed closed over a given message (i.e. the commitment to the message had being created according to the seal definition).
NB: It’s important to note, that while its possible to deterministically define was a given seal closed it yet may be not possible to find out if the seal is open; i.e. seal status may be either “closed over message” or “unknown”. Some specific implementations of single-use-seals may define procedure to deterministically prove that a given seal is not closed (i.e. opened), however this is not a part of the specification and we should not rely on the existence of such possibility in all cases.
The module defines trait
SingleUseSeal that can be used for
implementation of single-use-seals with methods for seal close and
verification. A type implementing this trait operates only with messages
(which is represented by any type that implements
AsRef<[u8]>,i.e. can be
represented as a sequence of bytes) and witnesses (which is represented by
an associated type
SingleUseSeal::Witness). At the same time,
SingleUseSeal can’t define seals by itself — and also knows nothing
about whether the seal is in fact closed: this requires a “seal medium”: a
proof of publication medium on which the seals are defined.
The module provides two options of implementing sch medium: synchronous
SealMedium and asynchronous
Examples of implementations can be found in
- Peter Todd. Preventing Consensus Fraud with Commitments and Single-Use-Seals. https://petertodd.org/2016/commitments-and-single-use-seals.
- Peter Todd. Scalable Semi-Trustless Asset Transfer via Single-Use-Seals and Proof-of-Publication. 1. Single-Use-Seal Definition. https://petertodd.org/2017/scalable-single-use-seal-asset-transfer
Error returned by
AsyncSealMedium functions related
to work with publication id (
SealMedium::PublicationId). Required since
not all implementation of
SealMedium may define publication identifier,
and the traits provide default implementation for these functions always
SealMediumError::PublicationIdNotSupported. If the
implementation would like to provide custom implementation, it may embed
standard error related to
SealMedium operations within
SealMediumError::MediumAccessError case; the type of MediumAccessError
is defined through generic argument to
Trait for proof-of-publication medium on which the seals are defined and which can be used for convenience operations related to seals:
Single-use-seal trait: implement for a data structure that will hold a single-use-seal definition and will contain a business logic for closing seal over some message and verification of the seal against the message and witness.