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I've recently heard of the idea of using a blockchain for secure communication. No details were given so it's a little hard for me to say that it can't be done. But as far as I know:

  1. Secure communication doesn't need blockchains. We have AES, MACs, digital signatures, etc. for that.
  2. Blockchains are only useful for documentation. Not for securing anything.

Am I missing something here?

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    $\begingroup$ sounds about right to me $\endgroup$ – Richie Frame Aug 20 '18 at 20:07
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    $\begingroup$ A possible use (speaking as a crypto non-expert) might be to "secure" an ongoing conversation between two or more people. "Secure" as in proving who said what, in what order, and being able to demonstrate that no messages have been omitted or inserted later (think a series of SMS messages, or a comment-chain on a Stack Exchange Q/A). Whether that counts as "securing" or just "documenting", I'm not sure. $\endgroup$ – TripeHound Aug 21 '18 at 8:08
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    $\begingroup$ "Am I missing something here?" Hopefully not your wallet! $\endgroup$ – Luis Casillas Aug 21 '18 at 17:33
  • $\begingroup$ @TripeHound For continuity of a certain exchange it would be easier to use a simple hash chain. Digital signatures provide non-repudiation. Blockchains are only useful in specific scenarios, where you need all properties of blockchains - otherwise there are far more efficient ways. $\endgroup$ – tylo Aug 24 '18 at 7:20
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  1. Secure communication doesn't need blockchains. We have AES and digital signatures for that.

Correct, though we also require tools other than block ciphers and digital signatures. But that is besides the point, as "blockchain" is not on that list.

  1. Blockchains are only useful for documentation. Not for securing anything.

Blockchain is a distributed, append-only ledger. That in itself does not provide confidentiality, integrity, or authenticity. However, availability is one of the pillars of information security, and it is arguable that blockchain could be used towards that end.

Am I missing something here?

Probably not. It has become trendy to try and attach blockchain to everything:

Long Island Iced Tea Corp., now called Long Blockchain Corp., received an ultimatum from Nasdaq in October, when the exchange threatened to delist it unless the market value rose above $35 million for 10 business days in a row. It achieved that on Friday, flaunting one of the hottest buzzwords of the year to get there.

Often times by people who don't/barely know what cryptography is, how it works, or what is is useful for.

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    $\begingroup$ Regarding trendiness of blockchain — obligatory xkcd. $\endgroup$ – Wildcard Aug 21 '18 at 0:07
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    $\begingroup$ Small nitpick: there's nothing inherently distributed about blockchains. $\endgroup$ – Todd Sewell Aug 21 '18 at 11:29
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    $\begingroup$ @ToddSewell I think a citation is required for that claim. "distributed" appears in every definition and mention of "blockchain" that I can find. A non-distributed (and therefore also centralized) ledger is known as a "database". $\endgroup$ – Ella Rose Aug 21 '18 at 14:51
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    $\begingroup$ @ToddSewell The security property of blockchains is defined with a strong requirement for distibution: No one must control more half or more of the computation power in the network. If that requirement is not met, blockchains are entirely useless. The most interesting question there is ignored quite often: How can this be achieved? Who takes part and why, what is the motivation? $\endgroup$ – tylo Aug 24 '18 at 7:25
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Giving a strong (either affirmative or negative) answer to questions like "is X useful to build secure systems" is hard. Basically, because what secure means depends on the specific use case and because you could find intrincate ways to achieve security.

For instance, if, for a given system being secure means ensuring information confidentiality (as in confidentiality of an encryption), then basing such system on a blockchain does not seem to be a good idea.

But also, blockchain systems do not seem a priori useful for building a system that allows performing private computations. But then, take Baum et al paper "Publicly Auditable Secure Multi-Party Computation." It describes an MPC system that uses an append-only bulletin board. This is precisely the idea that triggered the Enigma blockchain-based system, which provides secure multiparty computation using a blockchain as the append-only bulletin board mentioned in Baum's paper.

And, if you want to take things further, it has been shown by Ishai et al that certain types of zero-knowledge proofs can be built from MPC too (disclaimer: I have not studied Ishai's paper, so I don't know whether or not a blockchain-based MPC system like Enigma's would be a valid implementation for its specific construction, but this serves me to make a point.) Now, zero-knowledge proofs are one of the most used cryptographic primitives to build secure systems. Probably, using a blockchain to build an MPC to then build a ZK proof to then build some other secure protocol is way far from efficient. But there may be variants or special cases for which it is still useful (and at least, from a theoretical point of view, seems very interesting.)

Also, just quoting the abstract of a very recent paper, "SoK: A Consensus Taxonomy in the Blockchain Era," by Garay and Kiayias:

Consensus (a.k.a. Byzantine agreement) is arguably one of the most fundamental problems in distributed systems, playing also an important role in the area of cryptographic protocols as the enabler of a (secure) broadcast functionality.

Now, blockchain systems can be seen as a special type of consensus systems, where security is defined (in the referenced paper by Garay and Kiayias) in terms of termination, agreement and validity properties. Therefore, a blockchain system can be used to implement a secure broadcast channel. So this is another counter example of blockchains being useful to provide some specific type of security.

To summarize, there may be non-intuitive ways to achieve security and, moreover, what does it mean for a system to be secure depends on what are the properties that such system needs to ensure. Confidentiality may be one, but there may be a lot more, as some of the other answers have pointed out too.

PS: This said, sure, there is way too much hype and too much people trying to use blockchain everywhere without making sense of anything.

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The appeal of a block chains is in its high availability of a single publicly shared communication channel (a decentralised bus if you will) combined with integrity in regards to message order (through distributed consensus). You can slap encryption onto pretty much any communication channel to make it private or even authentic (given a back or side channel to exchange key material at least once).

Since availability and (partial) integrity are desirable traits of secure communication and thus typically considered part of communication security block chains have some appeal here given a use case that requires these particular features.

A substantiated rant beyond the topic of cryptography: It is my understanding that many people and businesses jump onto the block chain band wagon not because of these unique features but because they want to burden their users with (some of) the cost of infrastructure or, in the case of crypto-currencies, look for an over-hyped and poorly regulated funding method. What availability and integrity block chains afford can often be achieved more easily by other models and means of communications.

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Am I missing something here?

Yes, you're actually missing the entire concept of the technology.

The "security" aspect of block chain technology is absolutely not keeping something secret. In most respects it is the very opposite. You've posted on a crypto forum, so people here will (understandably) view that word through a cryptographer's prism and think about keeping secrets.

There's an excellent explanation of block chains here by a very proper British bloke. Quoting from his video, the fundamental concept of thee block chain is:-

blockchain

What is secure is the totality of transactions, especially their order. What's in the transaction is irrelevant. At the moment it's mostly Bitcoin transfers, but almost everyone in the world is working on applying the secure distributed ledger technology to other things. Also see Blockchain Technology Market Size Worth $7.59 Billion By 2024.

As far as you and this question are concerned, the transaction could be a communique from one person to another. The content could be in plain text, or could be encrypted with whatever. It doesn't matter. What is secure, is that others can easily verify that some content passed between two two parties, and it's irrevocable.

The cryptography bit just comes in at the end in producing, appending and distributing another block to every one's copy of the chain. Personally, the real threat to this technology (when the hype dies down) is whether anyone will bother if you don't get a shiny new Bitcoin for adding blocks to the chain.

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If you are simply discussing for communication between TWO communication terminals then Block Chain is over kill. The encryption at source or channel is a proven technique.

If you are discussing for a secure broadcast messaging which can be seen by many people then this concept can be used . check the technology is based on secure distributed transactions. See in bold the Paul Uszak comment. For telecommunication Services it can be used effectively with little modification in definition of blocks of data.

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