Note: This was originally published to the Bitcoin Development mailing list.

While not a new concept proof-of-publication is receiving a significant amount of attention right now both as an idea, with regard to the embedded consensus systems that make use of it, and in regard to the sidechains model proposed by Blockstream that rejects it. Here we give a clear definition of proof-of-publication and its weaker predecessor timestamping, describe some usecases for it, and finally dispel some of the common myths about it.

## What is timestamping?

A cryptographic timestamp proves that message m existed prior to some time t.

This is the cryptographic equivalent of mailing yourself a patentable idea in a sealed envelope to establish the date at which the idea existed on paper.

Traditionally this has been done with one or more trusted third parties who attest to the fact that they saw m prior to the time t. More recently blockchains have been used for this purpose, particularly the Bitcoin blockchain, as block headers include a block time which is verified by the consensus algorithm.

## What is proof-of-publication?

Proof-of-publication is what solves the double-spend problem.

Cryptographic proof-of-publication actually refers to a few closely related proofs, and practical uses of it will generally make use of more than one proof.

### Proof-of-receipt

Prove that every member p in of audience P has received message m. A real world analogy is a legal notice being published in a major newspaper - we can assume any subscriber received the message and had a chance to read it.

### Proof-of-non-publication

Prove that message m has not been published. Extending the above real world analogy the court can easily determine that a legal notice was not published when it should have been by examining newspaper archives. (or equally, because the notice had not been published, some action a litigant had taken was permissable)

### Proof-of-membership

A proof-of-non-publication isn’t very useful if you can’t prove that some member q is in the audience P. In particular, if you are to evaluate a proof-of-membership, q is yourself, and you want assurance you are in that audience. In the case of our newspaper analogy because we know what today’s date is, and we trust the newspaper never to publish two different editions with the same date we can be certain we have searched all possible issues where the legal notice may have been published.

### Real-world proof-of-publication: The Torrens Title System

Land titles are a real-world example, dating back centuries, with remarkable simularities to the Bitcoin blockchain. Prior to the torrens system land was transferred between owners through a chain of valid title deeds going back to some “genesis” event establishing rightful ownership independently of prior history. As with the blockchain the title deed system has two main problems: establishing that each title deed in the chain is valid in isolation, and establishing that no other valid title deeds exist. While the analogy isn’t exact - establishing the validity of title deeds isn’t as crisp a process as simple checking a cryptographic signature - these two basic problems are closely related to the actions of checking a transaction’s signatures in isolation, and ensuring it hasn’t been double-spent.

To solve these problems the Torrens title system was developed, first in Australia and later Canada, to establish a singular central registry of deeds, or property transfers. Simplifying a bit we can say inclusion - publication - in the official registery is a necessary pre-condition to a given property transfer being valid. Multiple competing transfers are made obvious, and the true valid transfer can be determined by whichever transfer happened first.

Similarly in places where the Torrens title system has not been adopted, almost always a small number of title insurance providers have taken on the same role. The title insurance provider maintains a database of all known title deeds, and in practice if a given title deed isn’t published in the database it’s not considered valid.

## Common myths

### Proof-of-publication is the same as timestamping

No. Timestamping is a significantly weaker primitive than proof-of-publication. This myth seems to persist because unfortunately many members of the Bitcoin development and theory community - and even members of the Blockstream project - have frequently used the term “timestamping” for applications that need proof-of-publication.

### Publication means publishing meaningful data to the whole world

No. The data to be published can often be an otherwise meaningless nonce, indistinguishable from any other random value. (e.g. an ECC pubkey)

For example colored coins can be implemented by committing the hash of the map of colored inputs to outputs inside a transaction. These maps can be passed from payee to payor to prove that a given output is colored with a set of recursive proofs, as is done in the author’s Smartcolors library. The commitment itself can be a simple hash, or even a pay-to-contract style derived pubkey.

A second example is Zerocash, which depends on global consensus of a set of revealed serial numbers. As this set can include “false-positives” - false revealed serial numbers that do not actually correspond to a real Zerocash transaction - the blockchain itself can be that set. The Zerocash transactions themselves - and associated proofs - can then be passed around via a p2p network separate from the blockchain itself. Each Zerocash Pour proof then simply needs to specify what set of priorly evaluated proofs makes up its particular commitment merkle tree and the proofs are then evaluated against that proof-specific tree. (in practice likely some kind of DAG-like structure) (note that there is a sybil attack risk here: a sybil attack reduces your k-anonymity set by the number of transactions you were prevented from seeing; a weaker proof-of-publication mechanism may be appropriate to prevent that sybil attack).

The published data may also not be meaningful because it is encrypted. Only a small community may need to come to consensus about it; everyone else can ignore it. For instance proof-of-publication for decentralized asset exchange is an application where you need publication to be timely, however the audience may still be small. That audience can share an encryption key.

### Proof-of-publication is always easy to censor

No, with some precautions. This myth is closely related to the above idea that the data must be globally meaningful to be useful. The colored coin and Zerocash examples above are cases where censoring the publication is obviously impossible as it can be made prior to giving anyone at all sufficient info to determine if the publicaiton has been made; the data itself is just nonces.

In the case of encrypted data the encryption key can also often be revealed well after the publication has been made. For instance in a Certificate Transparency scheme the certificate authority (CA) may use proof-of-publication to prove that a certificate was in a set of certificates. If that set of certificates is hashed into a merkelized binary prefix tree indexed by domain name the correct certificate for a given domain name - or lack thereof - is easily proven. Changes to that set can be published on the blockchain by publishing successive prefix tree commitments.

If these commitments are encrypted, each commitment $C_i$ can also commit to the encryption key to be used for $C_{i+1}$. That key need not be revealed until the commitment is published; validitity is assured as every client knows that only one $C_{i+1}$ is possible, so any malfeasance is guaranteed to be revealed when $C_{i+2}$ is published.

Secondly the published data can be timelock encrypted with timelocks that take more than the average block interval to decrypt. This puts would-be censoring miners into a position of either delaying all transactions, or accepting that they will end up mining publication proofs. The only way to circumvent this is highly restrictive whitelisting.

### Proof-of-publication is easier to censor than (merge)-mined sidechains

False under all circumstances. Even if the publications use no anti-censorship techniques to succesfully censor a proof-of-publication system at least 51% of the total hashing power must decide to censor it, and they must do so by attacking the other 49% of hashing power - specifically rejecting their blocks. This is true no matter how “niche” the proof-of-publication system is - whether it is used by two people or two million people it has the same security.

On the other hand a (merge)-mined sidechain with x% of the total hashing power supporting it can be attacked at by anyone with >x% hashing power. In the case of a merge-mined sidechain this cost will often be near zero

• only by providing miners with a significant and ongoing reward can the marginal cost be made high. In the case of sidechains with niche audiences this is particularly true - sidechain advocates have often advocated that sidechains be initially protected by centralized checkpoints until they become popular enough to begin to be secure.

Secondly sidechains can’t make use of anti-censorship techniques the way proof-of-publication systems can: they inherently must be public for miners to be able to mine them in a decentralized fashion. Of course, users of them may use anti-censorship techniques, but that leads to a simple security-vs-cost tradeoff between using the Bitcoin blockchain and a sidechain. (note the simularity to the author’s treechains proposal!)

### Proof-of-publication can be made expensive

True, in some cases! By tightly constraining the Bitcoin scripting system the available bytes for stenographic embedment can be reduced. For instance P2SH^2 requires an brute force exponentially increasing amount of hashes-per-byte-per-pushdata. However this is still ineffective against publishing hashes, and to fully implement it - scriptSigs included - would require highly invasive changes to the entire scripting system that would greatly limit its value.

### Proof-of-publication can be outsourced to untrusted third-parties

Timestamping yes, but proof-of-publication no.

We’re talking about systems that attempt to publish multiple pieces of data from multiple parties with a single hash in the Bitcoin blockchain, such as Factom. Essentially this works by having a “child” blockchain, and the hash of that child blockchain is published in the master Bitcoin blockchain. To prove publicaiton you prove that your message is in that child chain, and the child chain is itself published in the Bitcoin blockchain. Proving membership is possible for yourself by determining if you have the contents corresponding to the most recent child-chain hash.

The problem is proving non-publication. The set of all potential child-chain hashes must be possible to obtain by scanning the Bitcoin blockchain. As a hash is meaningless by itself, these hashes must be signed. That introduces a trusted third-party who can also sign an invalid hash that does not correspond to a block and publish it in the blockchain. This in turn makes it impossible for anyone using the child blockchain to prove non-publication - they can’t prove they did not publish a message because the content of all child blockchains is now unknown.

In short, Factom and systems like it rely on trusted third parties who can put you in a position where you can’t prove you did not commit fraud.

### Proof-of-publication “bloats” the blockchain

Systems that do not make use of the UTXO are no different technically than any other transaction: they pay fees to publish messages to the Bitcoin blockchain with no amortized increase in the UTXO set. Some systems do grow the UTXO set - a potential scaling problem as currently that all full nodes must have the entire UTXO set - although there are a number of existing mechanisms and proposals to mitigate this issue such as the (crippled) OP_RETURN scriptPubKey format, the dust rule, the authors TXO commitments, UTXO expiry etc.

From an economic point of view proof-of-publication systems compete with other uses of the blockchain as they pay fees; supply of blockchain space is fixed so the increased demand must result in a higher per-transaction price in fees. On the other hand this is true of all uses of the blockchain, which collectively share the limited transaction capacity. For instance Satoshidice and similar sites have been widely condemned for doing conventional transactions on Bitcoin when they could have potentially used off-chain transactions.

It’s unknown what the effect on the Bitcoin price will actually be. Some proof-of-publication uses have nothing to do with money at all - e.g. certificate transparency. Others are only indirectly related, such as securing financial audit logs such as merkle-sum-trees of total Bitcoins held by exchanges. Others in effect add new features to Bitcoin, such as how colored coins allows the trade of assets on the blockchain, or how Zerocash makes Bitcoin transactions anonymous. The sum total of all these effects on the Bitcoin price is difficult to predict.

The authors belief is that even if proof-of-publication is a net-negative to Bitcoin as it is significantly more secure than the alternatives and can’t be effectively censored people will use it regardless of effects to discourage them through social pressure. Thus Bitcoin must make technical improvements to scalability that negate these potentially harmful effects.

### Proof-of-publication systems are inefficient

If you’re talking about inefficient from the perspective of a full-node that does full validation, they are no different than (merge)-mined sidechain and altcoin alternatives. If you’re talking about efficiency from the perspective of a SPV client, then yes, proof-of-publication systems will often require more resources than mining-based alternatives.

However it must be remembered that the cost of mining is the introduction of a trusted third party - miners. Of course, mined proof-of-publication has miners already, but trusting those miners to determine the meaning of that data places significantly more trust in them than mearly trusting them to create consensus on the order in which data is published.

Many usecases involve trusted third-parties anyway - the role of proof-of-publication is to hold those third-parties to account and keep them honest. For these use-cases - certificate transparency, audit logs, financial assets - mined alternatives simply add new trusted third parties and points of failure rather than remove them.

Of course, global consensus is inefficient - Bitcoin itself is inefficient. But this is a fundemental problem to Bitcoin’s architecture that applies to all uses of it, a problem that should be solved in general.

### Proof-of-publication needs “scamcoins” like Mastercoin and Counterparty

First of all, whether or not a limited-supply token is a “scam” is not a technical question. However some types of embedded consensus systems, a specific use-case for proof-of-publication, do require limited-supply tokens within the system for technical reasons, for instance to support bid orders functionality in decentralized marketplaces.

Secondly using a limited-supply token in a proof-of-publicaton system is what lets you have secure client side validation rather than the alternative of 2-way-pegging that requires users to trust miners not to steal the pegged funds. Tokens also do not need to be, economically speaking, assets that can appreciate in value relative to Bitcoin; one-way-pegs where Bitcoins can always be turned into the token in conjunction with decentralized exchange to buy and sell tokens for Bitcoins ensure the token value will always closely approximate the Bitcoin value as long as the protocol itself is considered valuable.

Finally only a subset of proof-of-publication use-cases involve tokens at all - many like colored coins transact directly to and from Bitcoin, while other use-cases don’t even involve finance at all.