Wyvern's Solidity contracts are structured to (a) provide the necessary least-authority permissioning and separation of concerns, (b) facilitate user-transparent core protocol upgrades, and (c) maximize code understandability, in that order of precedence.
The registry contracts proxy all user authentication. Protocol users transfer assets to a personal proxy contract and approve token transfers through a token proxy contract. This has two advantages over approving transfers from or transferring assets to the exchange contracts directly: it vastly reduces the attack surface of the complex exchange logic (as the exchange contracts never need to hold assets themselves), and it allows users to use new protocol versions without moving assets to new contracts or approving tokens for use by new contracts.
An AuthenticatedProxy contract instance is created, once, by each user of the Wyvern Protocol to hold assets on that user's behalf. The user can execute arbitrary calls through the AuthenticatedProxy contract at any time (to withdraw assets, for example), and can easily seralize multiple calls through the DELEGATECALL opcode (a functionality not provided by non-contract Ethereum accounts). Exchange contracts authorized by the ProxyRegistry can also execute arbitrary calls on behalf of the user; the logic in the exchange contracts must ensure that the user has signed a valid order to execute the call in question.
The TokenTransferProxy contract uses the ProxyRegistry's authentication table, allowing users to call ERC20 approve once for all future protocol versions.
The ProxyRegistry contract, owned by the Wyvern DAO, keeps an authentication table to facilitate protocol upgrades and a contract table to map users to AuthenticatedProxy contracts. The DAO can authorize new protocol versions, granting them access to the TokenTransferProxy contract and all AuthenticatedProxy contracts, after a mandatory 2-week delay (to allow users time to move their tokens and assets if the DAO should authorize a malicious protocol).
The exchange contracts implement the core protocol logic. The protocol can be upgraded by deploying new versions of these contracts, which then must be authorized by the Wyvern DAO before they can access user tokens and assets.
The Exchange contract simply wraps the ExchangeCore internal functions with public functions exposing arrays instead of structs, as Solidity's external struct ABI support is not yet finalized.
The ExchangeCore contract is the workhorse of the Wyvern Protocol and contains all the juicy logic for both order validation — metadata agreement, signature verification, price/fee calculation, calldata unification — and order execution — token transfer, proxy contract retrieval, call execution, and replay prevention.
The SaleKindInterface contract abstracts over order price calculation methods. Currently, only fixed-price and Dutch auction pricing methods are supported, but future protocol versions may add more (see the wishlist).