A single-use seal is a cryptographic primitive, proposed by Peter Todd in 2016, that is a uniquely identifiable object which can be closed over (‘sealed to’) a message exactly once, producing a publicly verifiable proof that the seal was closed over that specific message and no other. The canonical implementation uses a Bitcoin UTXO as the seal: because the consensus rules permit each UTXO to be spent only once, the transaction that spends it can commit to exactly one message, giving a tamper-evident, non-equivocable ‘open once’ guarantee. Single-use seals, combined with proof of publication, form the foundation of client-side validation and underpin Bitcoin smart-contract systems such as RGB and primitives such as Block Trails.
Overview
- The defining property is single use: a seal can be opened (closed-over) at most once, so it cannot equivocate between two conflicting messages. This is exactly the guarantee Bitcoin already provides for UTXOs — the Bitcoin Network consensus enforces that each output is spent once — which is why a UTXO makes a natural seal medium.
- To seal a message to a UTXO, the holder commits to the message in the transaction that spends the UTXO (for example via a Taproot tweak or an
OP_RETURN). Anyone watching the chain can later verify the seal was closed over precisely that commitment.
Why it matters
- Single-use seals let a smart-contract system move validation off-chain while keeping uniqueness on-chain. The blockchain only orders and de-duplicates seal closures; the semantic correctness of each state is checked by the recipient. This is the architecture behind RGB Protocol, Block Trails, and RGB and Client Side Validation, and it is what makes those systems private and scalable relative to global on-chain execution.
Components
- Seal definition — an unambiguous identifier of the object to be sealed (e.g. a specific UTXO outpoint).
- Closing operation — the act that binds a message to the seal (spending the UTXO with a commitment).
- Witness / proof — the on-chain evidence that the seal was closed over the message, verifiable by any client.
- Medium — the Proof of Publication layer (Bitcoin, or another append-only medium) guaranteeing the closure is visible and ordered.