Added evaluation of the Filecoin paper

evaluations/filecoin.md

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+An evaluation of the Filecoin whitepaper
+========================================
+
+2020-12-08 Mark Spanbroek
+
+https://filecoin.io/filecoin.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 Filecoin.
+
+#### Pros:
+
++ Clients do not need to actively monitor hosts. Once a deal has been agreed
+  upon, the network checks proofs of storage.
++ The network actively tries to repair storage faults by introducing new
+  orders in the storage market. (§4.3.4).
++ Integrity is achieved because files are addressed using their content
+  hash (§4.4).
++ Marketplaces are explicitly designed and specified (§5).
++ Micropayments via payment channels (§5.3.1).
++ Integration with other blockchain systems such as Ethereum (§7.2) are being
+  worked on.
+
+#### Cons:
+
+- Filecoin requires its own very specific blockchain, which influences a lot
+  of its design. There is tight coupling between the blockchain, storage
+  accounting, proofs and markets.
+- Proof of spacetime is much more complex than simple challenges, and only
+  required to make the blockchain work (§3.3, §6.2)
+- A miners influence is proportional to the amount of storage they're
+  providing (§1.2), which is an incentive to become big. This could lead to
+  the same centralization issues that plague Bitcoin.
+- Incentives are geared towards making the particulars of the Filecoin
+  design work, instead of directly aligned with users' interest. For instance,
+  there are incentives for storage and retrieval, but it seems that a miner
+  would be able to make money by only storing data, and never offering it for
+  retrieval. Also, the incentive for a miner to store multiple independent
+  copies does not mean protection against loss if they're all located on the
+  same failing disk.
+- The blockchain contains a complete allocation table of all things that are
+  stored in the network (§4.2), which raises questions about scalability.
+- Zero cash entry (such as in Swarm) doesn't seem possible.
+- Consecutive micropayments are presented as a solution for the trust problems
+  while retrieving (§5.3.1), which doesn't entirely mitigate withholding
+  attacks.
+- The addition of smart contracts (§7.1) feels like an unnecessary
+  complication.