Sia - an introduction for decentralized storage based on the blockchain.

1. Simple Decentralized Storage
Speaker: William
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2. Content
• 1. Introduction
• 2. General structure
• 3. Transactions
• Inputs and Outputs
• File Contracts
• Proof of Storage
• Arbitrary Transaction Data
• 4. Protection mechanism
• 5. Host selection solutions.
• 6. Siafund.
• 7. Pros & Cons of Sia.
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3. 1. Introduction
• Sia is a decentralized cloud storage platform develop by Nebulous.
• Peers on Sia rent storage from each other. Sia itself stores only the
storage contracts formed between parties
• Sia recommend storing data redundantly across multiple hosts
• By forming a contract, a storage provider(also known as a host)
agrees to store a client’s data, and to periodically submit proof of
their continued storage until the contract expires.
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4. 2. General structure
• Sia is an altcoin which is resemble Bitcoin structure.
• Sia opts instead to use an M–of–N multi-signature scheme for all
transactions
• Sia also extends transactions to enable the creation and enforcement
of storage contracts, and the extension is:
• Contracts
• Proofs
• Contract updates
• Contracts declare the intention of a host to store a file with a certain
size and hash.(They define the regularity with which a host must
submit storage proofs.)
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5. Multi-Signature Transaction Scheme
Contract, proof
contract-update
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6. 3. Transactions
• Format of Transaction:
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7. 3.1 Inputs and Outputs
• Every input must come from a prior output, so an input is simply an
output ID.
• Inputs contain spend conditions .
• Outputs contain merkle root hash .
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8. Spend Conditions
• Spend conditions include a time lock and a set of public keys, and the
number of signatures required.
• An output cannot be spent until the time lock has expired and enough of
the specified keys have added their signature.
• The spend conditions are hashed into a merkle tree, using below as leaves
• time lock
• number of signatures required
• public keys
• In order to spend the coins, the spend conditions corresponding to the
address hash must be provided.
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9. Time
lock
# of Sig
requirement
Public
key
Public
key
Root
Hash
Public
Key
As an address where the money are sent.
.....
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10. 3.2 File Contracts
• A file contract is an agreement(like SLA) between a storage provider
and their client. Contain:
• Allowance: a pre-paid amount of Siacoin meant to pay storage and
bandwidth costs
• Core of the file contract is “files merkle root”
• The file is split into segments of constant size and hashed into a merkle tree.
• The root hash, along with the total size of the file, can be used to verify
storage proofs.
• File contracts are also created with a list of “edit conditions,”
analogous to the spend conditions of a transaction.
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11. File contracts also specify :
• Duration (by default 13 weeks)
• Challenge frequency
• specifies how often a storage proof must be submitted.
• creates discrete challenge windows during which a host must submit storage
proofs .
• submitting “valid proof” triggers an automatic payment to the “valid proof” .
• Payout parameters
• reward for a valid proof .
• reward for an invalid or missing proof .
• maximum number of proofs that can be missed .
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12. 3.3 Proof of Storage
• During the contract, the hosts must submit Proofs of Storage to the
blockchain to demonstrate:
• They are online.
• They still preserve the client’s files.
• Storage proof transactions are periodically submitted to fulfill file
contracts.
• Each storage proof targets a specific file contract.
• Storage proof only contain
• contract ID
• the proof data.
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13. Algorithm
• Hosts prove their storage by providing a segment of the original file and a
list of hashes from the file’s Merkle tree.
• Each storage proof uses a randomly selected segment. The random seed
for challenge window Wi is given by:
• If the host is consistently able to demonstrate possession of a random
segment, then they are very likely storing the whole file.
• 會產生隨機檔案中部分為64kb的file作為proof，
• 若他沒有持有完整的檔案，只有持有一半，那他會有50%的機率可以作弊猜中，但他得冒猜錯的
風險，不僅損失他從客戶收到的錢，而且也得損失他擔保的金錢（proof of burn)
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14. 14
File
Segment
Root
Hash
As an address where the money are sent.
.....
File
Segment
File
Segment
File
Segment
Randomly ask provider to
Provide a file segment and its
relative hash as proof to
“file contract”.

15. Attack of Proof of Storage
• Block Withholding Attacks
• Attacker withholds blocks until they find one that will produce a favorable
random number.
• Clients can specify a high challenge frequency and large penalties for missing
proofs.
• Closed Window Attacks
• Miners could maliciously exclude storage proofs from blocks, depriving
themselves of transaction fees but forcing a penalty on hosts.
• Miners could extort hosts by requiring large fees to include storage proofs
• Use a large window size. Hosts can reasonably assume that some percentage
of miners will include their proofs in return for a transaction fee.
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16. 3.4 Arbitrary Transaction Data
• Each transaction has an arbitrary data field which can be used for any
type of information.
• Nodes will be required to store the arbitrary data if it is signed by any
signature in the transaction.
• Nodes will initially accept up to 64 KB of arbitrary data per block.
• This arbitrary data provides hosts and clients with a decentralized way
to organize themselves.
• Arbitrary data could also be used to implement other types of soft
forks.
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17. File Repair
• In versions earlier than 1.3.0 Capricorn, files had to be stored locally
in order to restore the redundancy.
• Now they are automatically downloaded from the available hosts and
uploaded to new ones.
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18. 4. Protection mechanism
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Host Protections Client Protections
• A contract requires consent from
both the storage provider and
their client.
• They can setting price on
contract to show how reliable
their services is.
• Host should protect themselves
from such Dos attacks .
(or they will receive penalties).
• Use erasure codes (Reed-Solomon
erasure coding),to safeguard against
hosts going offline.
• Downloads can be run in parallel
to maximize available bandwidth.

19. 5. Host selection solutions.
• Renter tracks hosts uptime and pick frequently chanllenge hosts to do
off-chain storage proof. (via micropayment channel).
• Proof of Burn: To combat Sybil attack, clients can require that hosts
that announce themselves in the arbitrary data section also include a
large volume of time locked coins.
• If 10 coins are time locked 14 days into the future, then the host can be said to have created
a lock valued at 140 coin-days. By favoring hosts that have created high-value locks
• Large coin-days means high quality guarantees.
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20. 6. Siafunds
• By imposing a fee on all contracts to get revenue.
• When a contract is created ,3.9% of the contract fund(A 3.9% of the
renter’s allowance and a 3.9% of the host’s) is removed and
distributed to the holders of siafunds.
• By using Siafunds instead of premining.
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21. 7.Pros and cons of Sia
Pros Cons
• Storage market place.
• Fully decentralized.
• Require collateral for hosting
• Only accepting siacoin as payment.
• Require entirely blockchain download.
• Currently not support streaming files.
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22. Roadmap
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Release
Feature
• Ability to repair files without having locally.
• Initial block sync time reduced.
• Simple File sharing.
• Ability to recover files using only wallet seed.
• Form file contract within 5 min instead of 60 min.
Short term
(Q1 2018)
• Video streaming support.
• Proper support tiny files. (from 40MB to ~ 100kbs).
• Automatic host pricing.
Medium term

23. Reference:
• Sia - Decentralized Cloud Storage - Presented by David Vorick at Mit
Bitcoin Expo 2016
https://www.youtube.com/watch?v=cR8YL05nq4A&t=14s
• Siacoin vs Storj https://www.youtube.com/watch?v=Kyy-mWn9Kv0
• Sia Roadmap https://trello.com/b/Io1dDyuI/sia-public-roadmap
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24. Q&A
• Proof of storage由誰保存
• 為了避免storage provider丟棄本體，只保存(file + random) = hash
值 ， 下一次proof 就直接提繳 hash值。
• 只記root hash 不過會有上面的狀況。
• 契約終止的條件。
• 確保用戶 提供者 價格保障
• 切片過程加密 是否會影響 上傳下載下率？？
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