Added evaluation of the Sia whitepaper

evaluations/sia.md

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+An evaluation of the Sia whitepaper
+===================================
+
+2020-12-07 Mark Spanbroek
+
+https://sia.tech/sia.pdf
+
+Goal of this evaluation is to find things to adopt or avoid while designing
+Dagger. It is not meant to be a criticism of Sia.
+
+#### Pros:
+
++ Clients do not need to actively monitor hosts (§1). Once a contract has been
+  agreed upon, the host earns/loses coins based on proofs of storage that the
+  network can check.
++ Denial of service attacks can be mitigated by burning funds associated with
+  missed proofs (§4).
++ Proof of storage is simple; provide a random piece of the file, and the
+  corresponding Merkle proof (§5.1).
++ Promotes erasure codes to safeguard against data loss (§7.2).
++ Suggests to use payment channels for micro-payments (§7.3).
++ The basic reputation system is protected against Sybil attacks (§7.4).
+
+#### Cons:
+
+- Sia has its own blockchain (§1), which makes some attacks more likely
+  (§5.2, §5.3). This can be mitigated by adopting a widely used, general purpose
+  blockchain such as Ethereum.
+- Requires a multi-signature scheme (§2).
+- The proof-of-storage algorithm requires that hosts store the entire file (§4),
+  instead of a few chunks.
+- Contracts can be edited (§4). This feels like an unnecessary complication of
+  the protocol.
+- Randomness for the storage proofs comes from the latest block hash (§5.1).
+  This can be manipulated, especially when using a specialized blockchain for
+  storage.
+- There is an arbitrary data field that might be used for advertisements in a
+  storage marketplace (§6). This feels like a very restrictive environment for a
+  marketplace, and an unnecessary complication for the underlying blockchain.
+- It is suggested that clients use erasure coding before encryption (§7.2). If
+  this were reversed (first encryption, then erase coding) then this would open
+  up scenario's for caching and re-hosting by those who do not possess the
+  decryption key.
+- Consecutive micropayments are presented as a solution for the trust problems
+  while downloading (§7.3). This assumes that the whole file, or a large part of
+  it, is stored on a single host. It also doesn't entirely mitigate withholding
+  attacks.
+- The basic reputation system favors hosts that have already earned or bought
+  coins (§7.4). It is also unclear how the reputation system discourages abuse.
+- Governance seems fairly centralized, with most funds and proceeds going to a
+  single company (§8).