In order to make the probability target (nBits-style) more intuitive to generate based on an expected probability, two functions were added to arith_uint256 -- SetProbabilityTarget(pt) and GetProbabilityEstimate().

The former will set the arith_uint256 to a difficulty approximately equal to pt, i.e. if pt = 1.0 (100% probability), the result will be 0b1111... and if pt = 0.5 (50%), the result will be 0b01111... and so on.

The latter will do the inverse, i.e. take the arith_uint256 value as a difficulty target and return a probability that a random hash will be lower than the target.

…lityEstimate.

A new set of classes in pow/pow.h/cpp which serve as the parents for all the proof of work algorithms (contained in the powa:: namespace).

The powa::challenge class is a container for challenges offered by peers, and powa::solution is a container for solutions to challenges.

powa::callback is used to ping-back solvers when a solution was found. There's a helper-class powa::callback_proxy which is used when an instance needs to use itself as the callback (as opposed to creating a new instance and handing ownership over to the prover).

powa::pow is the base algorithm class inherited by the algorithm implementations, and finally powa::powchain is a simple wrapper for chaining algorithms together (e.g. sha256(cuckoo-cycle)).

Origin: https://github.com/tromp/cuckoo

This commit is purposefully left as closely resembling the original source as possible, although some tweaks have been put in (in particular namespace embedding). A separate commit adapts it for the Bitcoin codebase.

A new set of classes specifically targeting anti-DoS via POW in src/ados.h/cpp in the ados:: namespace.

ados::offer is an offered service provided for solving a POW challenge, including expiration and purpose according to BIP-154.

ados::callback extends powa::callback as a general-purpose anti-DoS callback.

ados::connection_challenge is an ados::callback specialization which retains a CAddress taken from a CNode which is connected to with the given challenge/solution when/if solved.

There are also a number of helper functions:
- ados::begin_solving() takes an offer and an investment (number of ticks) and will attempt to solve the offer in the background
- ados::expected_solution_time(o) gives an estimate in seconds on how long it would take on average to solve the challenge in the offer
- ados::solvable(o) returns a bool on whether the given offer's challenge is considered solvable within the time constraints
- ados::challenge_peer(peer, purpose, pressure) constructs and sends a challenge to peer for the given purpose; pressure determines the difficulty of the challenge, and is in the range [0..1]
- ados::check_solution(o) takes an offer and checks if its given solution is a valid solution to the contained challenge

A new nPOWConnectionSlots ivar is added to connection options and fRequirePOW is added to CNode.

The logic upon incoming connections for evictions is changed to (1) include a fRequirePOW bool which defaults to false and is set to true if the number of inbound connections exceeds nMaxInbound - nPOWConnectionSlots, and (2) to attempt eviction if the number of inbound exceeds nMaxInbound. Assuming the connection is accepted, fRequirePOW is set in the node as appropriate.

CConnman::OpenNetworkConnection has an added ados::offer which, if set, is inserted into the newly connected node and subsequently sent before the version message.

NetMsgType::SOLUTION and NetMsgType::CHALLENGE are added and handled in net_processing, and NODE_DOSPROT is added as bit 5 (note: this needs to be explicitly defined probably in BIP-154).

…ne challenge).

Makes use of e.g. CSHA256 instead of openssl, as well as numerous fixes and clean ups of unused stuff.

Note: Still using a built-in siphash instead of the available one, which should probably be addressed.