I agree that someone could do this, but why is that a problem? Isn't the goal of this exercise to ensure more full nodes on the network? In order to be able to answer the challenges, an entity would need to be running a full node somewhere. Thus, they have contributed at least one additional full node to the network. I could certainly see a case for a company to host hundreds of lightweight (e.g., EC2) servers all backed by a single copy of the block chain. Why force every single machine to have its own copy? All you really need to require is that each agency/participant have its own copy.
On Friday, 27 March 2015, at 2:32 pm, Robert McKay wrote: > Basically the problem with that is that someone could setup a single > full node that has the blockchain and can answer those challenges and > then a bunch of other non-full nodes that just proxy any such challenges > to the single full node. > > Rob > > On 2015-03-26 23:04, Matt Whitlock wrote: > > Maybe I'm overlooking something, but I've been watching this thread > > with increasing skepticism at the complexity of the offered solution. > > I don't understand why it needs to be so complex. I'd like to offer > > an > > alternative for your consideration... > > > > Challenge: > > "Send me: SHA256(SHA256(concatenation of N pseudo-randomly selected > > bytes from the block chain))." > > > > Choose N such that it would be infeasible for the responding node to > > fetch all of the needed blocks in a short amount of time. In other > > words, assume that a node can seek to a given byte in a block stored > > on local disk much faster than it can download the entire block from > > a > > remote peer. This is almost certainly a safe assumption. > > > > For example, choose N = 1024. Then the proving node needs to perform > > 1024 random reads from local disk. On spinning media, this is likely > > to take somewhere on the order of 15 seconds. Assuming blocks are > > averaging 500 KiB each, then 1024 blocks would comprise 500 MiB of > > data. Can 500 MiB be downloaded in 15 seconds? This data transfer > > rate > > is 280 Mbps. Almost certainly not possible. And if it is, just > > increase N. The challenge also becomes more difficult as average > > block > > size increases. > > > > This challenge-response protocol relies on the lack of a "partial > > getdata" command in the Bitcoin protocol: a node cannot ask for only > > part of a block; it must ask for an entire block. Furthermore, nodes > > could ban other nodes for making too many random requests for blocks. > > > > > > On Thursday, 26 March 2015, at 7:09 pm, Sergio Lerner wrote: > >> > >> > If I understand correctly, transforming raw blocks to keyed blocks > >> > takes 512x longer than transforming keyed blocks back to raw. The > >> key > >> > is public, like the IP, or some other value which perhaps changes > >> less > >> > frequently. > >> > > >> Yes. I was thinking that the IP could be part of a first layer of > >> encryption done to the blockchain data prior to the asymetric > >> operation. > >> That way the asymmetric operation can be the same for all users (no > >> different primers for different IPs, and then the verifiers does not > >> have to verify that a particular p is actually a pseudo-prime > >> suitable > >> for P.H. ) and the public exponent can be just 3. > >> > >> > > >> >> Two protocols can be performed to prove local possession: > >> >> 1. (prover and verifier pay a small cost) The verifier sends a > >> seed to > >> >> derive some n random indexes, and the prover must respond with > >> the hash > >> >> of the decrypted blocks within a certain time bound. Suppose that > >> >> decryption of n blocks take 100 msec (+-100 msec of network > >> jitter). > >> >> Then an attacker must have a computer 50 faster to be able to > >> >> consistently cheat. The last 50 blocks should not be part of the > >> list to > >> >> allow nodes to catch-up and encrypt the blocks in background. > >> >> > >> > > >> > Can you clarify, the prover is hashing random blocks of > >> *decrypted*, > >> > as-in raw, blockchain data? What does this prove other than, > >> perhaps, > >> > fast random IO of the blockchain? (which is useful in its own > >> right, > >> > e.g. as a way to ensure only full-node IO-bound mining if baked > >> into > >> > the PoW) > >> > > >> > How is the verifier validating the response without possession of > >> the > >> > full blockchain? > >> > >> You're right, It is incorrect. Not the decrypted blocks must be > >> sent, > >> but the encrypted blocks. There correct protocol is this: > >> > >> 1. (prover and verifier pay a small cost) The verifier sends a seed > >> to > >> derive some n random indexes, and the prover must respond with the > >> the > >> encrypted blocks within a certain time bound. The verifier decrypts > >> those blocks to check if they are part of the block-chain. > >> > >> But then there is this improvement which allows the verifier do > >> detect > >> non full-nodes with much less computation: > >> > >> 3. (prover pays a small cost, verifier smaller cost) The verifier > >> asks > >> the prover to send a Merkle tree root of hashes of encrypted blocks > >> with > >> N indexes selected by a psudo-random function seeded by a challenge > >> value, where each encrypted-block is previously prefixed with the > >> seed > >> before being hashed (e.g. N=100). The verifier receives the Markle > >> Root > >> and performs a statistical test on the received information. From > >> the N > >> hashes blocks, it chooses M < N (e.g. M = 20), and asks the proved > >> for > >> the blocks at these indexes. The prover sends the blocks, the > >> verifier > >> validates the blocks by decrypting them and also verifies that the > >> Merkle tree was well constructed for those block nodes. This proves > >> with > >> high probability that the Merkle tree was built on-the-fly and > >> specifically for this challenge-response protocol. > >> > >> > I also wonder about the effect of spinning disk versus SSD. Seek > >> time > >> > for 1,000 random reads is either nearly zero or dominating > >> depending > >> > on the two modes. I wonder if a sequential read from a random > >> index is > >> > a possible trade-off,; it doesn't prove possession of the whole > >> chain > >> > nearly as well, but at least iowait converges significantly. Then > >> > again, that presupposes a specific ordering on disk which might > >> not > >> > exist. In X years it will all be solid-state, so eventually it's > >> moot. > >> > > >> Good idea. > >> > >> Also we don't need that every node implements the protocol, but only > >> nodes that want to prove full-node-ness, such as the ones which want > >> to > >> receive bitnodes subsidy. > > > > > > > > ------------------------------------------------------------------------------ > > Dive into the World of Parallel Programming The Go Parallel Website, > > sponsored > > by Intel and developed in partnership with Slashdot Media, is your > > hub for all > > things parallel software development, from weekly thought leadership > > blogs to > > news, videos, case studies, tutorials and more. Take a look and join > > the > > conversation now. http://goparallel.sourceforge.net/ > > _______________________________________________ > > Bitcoin-development mailing list > > Bitcoinfirstname.lastname@example.org > > https://lists.sourceforge.net/lists/listinfo/bitcoin-development > > > ------------------------------------------------------------------------------ > Dive into the World of Parallel Programming The Go Parallel Website, sponsored > by Intel and developed in partnership with Slashdot Media, is your hub for all > things parallel software development, from weekly thought leadership blogs to > news, videos, case studies, tutorials and more. Take a look and join the > conversation now. http://goparallel.sourceforge.net/ > _______________________________________________ > Bitcoin-development mailing list > Bitcoinemail@example.com > https://lists.sourceforge.net/lists/listinfo/bitcoin-development ------------------------------------------------------------------------------ Dive into the World of Parallel Programming The Go Parallel Website, sponsored by Intel and developed in partnership with Slashdot Media, is your hub for all things parallel software development, from weekly thought leadership blogs to news, videos, case studies, tutorials and more. Take a look and join the conversation now. http://goparallel.sourceforge.net/ _______________________________________________ Bitcoin-development mailing list Bitcoinfirstname.lastname@example.org https://lists.sourceforge.net/lists/listinfo/bitcoin-development