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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.

Single-use-seal definition

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 w.
  • Verify(l,w,m) → bool — Verify that the seal l was closed over message m.

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 p.


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.

Trait structure

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 AsyncSealMedium.

Sample implementation

Examples of implementations can be found in bp::seals.

Further reading


Error returned by SealMedium and 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 returning 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 SealMediumError.

Single-use-seal status returned by SealMedium::get_seal_status and AsyncSealMedium::get_seal_status functions.


Trait for proof-of-publication medium on which the seals are defined and which can be used for convenience operations related to seals:

Asynchronous version of the SealMedium trait.

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.