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Comment by DarthChucks | Hacker News Reader

"time to live/self delete for messages" on a "Customized blockchain implementation" seems to be contradictory. Any data deleted from a block will invalidate all blocks made after the deletion.

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DARPA appears to be aware of the paradoxes in their request.

>> " Q2. The criteria for "time to live/self delete for messages" is impossible to achieve with a Blockchain. Is that a problem for this challenge? A2. [Response Pending]"

Source: https://sbir.defensebusiness.org/topics#topic27859

Those are user posted questions. You can also get in contact with the DARPA program manager directly.

Presumably you're not putting the actual messages in the blockchain. You can use it for metadata like encrypted messages or keys, and use it as a trusted clock for TTLs.

The more confusing requirement is that they want instant communications over a blockchain.

But at a higher level, this is a sort of odd set of requirements. "You have to use a blockchain somehow" is a class project requirement, not an actual project deliverable; nobody should care if it actually uses a blockchain and how as long as it satisfies the required security goals, right? It almost sounds like this RFP was written to describe one person's/group's existing product.

What problem do they have that a blockchain solves? The big win for bitcoin is that the consensus blockchain can be chosen without trust. I would think the military wouldn't be worried about manufacturing consensus among untrusted nodes.

I guess if the messages are stored in the blockchain, a device would be able to delete the key and other information needed to access the message, effectively deleting the message from the device, protecting the user of the device from having to answer for the content of the message.

But the military can't always use secure connections. That's why the invented the tor project.

Sounds more like they want Matrix, where you just have a bunch of redundant federated private servers in their own network. Everyone ends up with copies of all room conversation history, and it uses Signal for the encryption.

When mining the block chain, it is possible to have different miners temporarily working on a chain that will later be invalidated because it's not the longest chain. Maybe it would be possible to somehow use that behavior to encode a self delete?

Not necessarily. You wouldn't have to include the data itself in the transaction, just a hashed key. You then just set the data to expire at the requested time.

The "blockchain" is almost certainly a funding buzzword there.

> seems to be contradictory

There are a lot of problems who seem contradictory until they are solved by cryptography.

Can you give some examples? I'm honestly curious.

Public key cryptography is a good example, which solved the seemingly impossible to solve key-distribution problem:

https://www.coursera.org/learn/cryptography/lecture/be3ae/th...

public-key crypto as kristofferR pointed out, but it's so common nowadays that people don't realize how weird of a concept it might have been a few decades ago.

Last year I came across a "fair" version of the Socialist Millionaires problem. How can two persons interact with one another to compare their bank statement. If you think about it, you might consider that in any interaction, a party might abort the exchange as soon as he learns the result and would then have an advantage over the other party. But not too long ago cryptography found a way to make this fair :) (https://www.cryptologie.net/article/279/how-to-fairly-compar...)

However, this sounds like wishing to violate the known laws of physics (specifically, their time invariance). Unless some of the information available at the time T is unavailable at some time T'>T, how could you delete something already shared with another party?

Yeah, HDCP (high-definition copy protection) makes an attempt to make content viewable, but not copyable, but it only works by certifying every single piece of the chain and it's still vulnerable to someone using a simple video camera to capture images of the displayed message.

Barring a human-computer interface that conveys messages directly into someone's brain, I don't see this as possible. If you can see it or if you can hear it, then it is copy-able.

I mostly meant the fact that if you can decode a message that you have with a key and algorithm that you have, you can do it later again, with the same result. Of course, HDCP is an also example of this, since you still have the message and you still have the key, even after some time (as long as you still have both).

> However, this sounds like wishing to violate the known laws of physics

Some crypto work is focused on proving that certain things are impossible. But so far I haven't seen any paper proving that this was impossible.

Bitcoin solved the double-spending problem with its revolutionary blockchain tech.

Before Bitcoin came along a working decentralized P2P currency had been considered impossible, since nobody could figure out a way to make a P2P currency unforgeable (and therefore work as a currency) before that.

It's not only true for crypto. In general the base of many inventions is finding a solution that transcends a core contradiction instead of just living with the tradeoff.

What if the data being incorporated was already encrypted... And you delete the key?

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