109 lines
7.7 KiB
Markdown
109 lines
7.7 KiB
Markdown
Wormhole Support for Algorand
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=============================
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This directory contains the components needed to support full Wormhole functionality under the Algorand blockchain platform.
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## Component overview
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---------------------
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This system is comprised of the following main components:
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* **Core contract (`wormhole_core.py`)**: Algorand stateful contract with entrypoints for publishing messages (VAAs), verification of VAA signatures, and triggering of governance chores. This will be referred as _CoreContract_ in this document.
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* **Token bridge contract (`token_bridge.py`)**: Algorand stateful contract supporting cross-chain bridging, exposing entrypoints for exchanging attestations, native tokens and ASAs, and triggering of governance. This will be referred as _TokenBridge_ in this document.
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* **VAA verification stateless program (`vaa_verify.py`)**: Stateless program for verifying the signatures of a Wormhole VAA payload against the set of active guardian public keys. This will be referred as _VaaVerify_ in this document.
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* **Dynamic storage stateless program (`TmplSig.py`)**: A stateless program that is bound to the main core and token bridge contracts to provide dynamic storage spaces addressable as a raw blob of bytes. See `local_blob.py`. This will be referred as _TmplSig_ in this document.
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Helper utilities and code include support PyTEAL code, deployment tools and tests.
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## System Architecture
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----------------------
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### _TmplSig_ details
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--------------------
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This stateless program code is parametrized with several values that give different output address. The stateless code will check for several transaction group parameters accordingly.
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|Text |Replaced by |
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|------|------------|
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|`TMPL_ADDR_IDX`| Where storage starts interpreting the space as a raw array of bytes |
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|`TMPL_EMITTER_ID` | Concatenation of chain Id + emitter Id in VAAs to be processed, or a hardcoded string identifying the type of information stored e.g `guardian` utf8 string stored in hex. |
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|`TMPL_SEED_AMT` | The amount that is paid in Gtxn[0] transaction group fee |
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|`TMPL_APP_ID` | Application Id of _CoreContract_, _TokenBridge_, etc that is specified in Gtxn[1] for **opt-in** transaction |
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|`TMPL_APP_ADDRESS`| Escrow address of the stateful contract specified in `APP_ID`. Used for **rekey transaction** in Gtxn[2]|
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* Local-state associated with the _Tmplsig_ accounts are used as dynamic storage. The technique is to access this local storage as a plain array of bytes instead of the typical key/value structure. With the current Algorand parameters, we have 127 * 15 ~ 2K of storage to be used random-access-like.
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* The contract accounts addresses are generated by compilation of a stateless code parametrized by several parameters. In the system, the following contract accounts are generated:
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* Account (`seq_addr`) for storing verified sequence number bits based on chainId,emitter,`int(vaa.sequence / MAX_BITS)` where MAX_BITS = 15240. This allows the system to reject duplicated VAAs for the last 2K sequence numbers.
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* Account (`guardian_addr` and `new_guardian_addr`) for storing total guardian count , the guardian public keys and guardian set expiration time.
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* Once generated, the accounts are opted-in and rekeyed to the core application.
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Briefly, this is the transaction group when _TmplSig_ is "attached" to a stateful app:
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|Tx#| Semantics |
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|-|-|
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|0 | Payment Txn of `TMPL_SEED_AMT` ALGOs to fund operation |
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|1 | Optin of LogicSig to target stateful contract `TMPL_APP_ID` for the app to use Logicsig account local storage |
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|2 | Rekey of LogicSig to escrow address for the smart contract to become the sole "governor" of the Logicsig account address |
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## Core Contract: Functional Description
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----------------------------------------
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### Initialization stage
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The initialization call needs a governance VAA to be passed in, typically to setup initial guardian list. The init call will:
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* store the _VaaVerify_ hash in the `vphash` global state key
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* check for the creator address, set `booted` global state to `1`
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* check for duplicate VAA
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* handle the governance VAA passed as argument.
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See below on how governance VAAs are processed, and how duplicate detection technique is used.
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### publishMessage
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The `publishMessage` call will retrieve the current sequence number from related _TmplSig_ local store, increment in by 1, store the new sequence number and emit a Log message which can be picked by Wormhole network for subsequently creating a guardian-signed VAA message.
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### hdlGovernance
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Governance messages can carry requests for:
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* Update the active guardian set
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* Upgrade contracts: For Algorand, an upgrade-contract governance VAA must contain the hash of the program that is approved as an upgrade (stored in global `validUpdateApproveHash`). The upgrade process itself is triggered with the **update** action, where the clear and approval program hashes are checked against what the governance VAA carried. If they differ, an assertion is thrown and the update call is aborted. A successful call writes an onchain Log with the new hashes and allows the update process to go on.
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* Setting the per-message fee
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* Retrieving previously paid message fees
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A governance request packed in a VAA must be verified by a `verifyVaa` call in the transaction group.
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### vaaVerify
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The VAA verify call will work by design *only* in a transaction group structured as:
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| TX | args | accounts | sender |
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| ---------- | ------------------------------------------------------- | ----------------------- | -------------------- |
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| verifySigs | [sigs<sub>0..n</sub>, keyset<sub>0..n</sub>, digest] | seq_addr, guardian_addr | vaa_verify_stateless |
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| verifySigs | ... | seq_addr, guardian_addr | vaa_verify_stateless
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| verifyVAA | vaa | seq_addr, guardian_addr | foundation |
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Keep in mind that depending on the number of signatures to verify there can be one or several _verifySigs_ calls working in tandem with the _VaaVerify_ stateless program. This depends on how many signatures we can verify on a single TX. At time of this writing, considering the opcode budget limitation of AVM 1.1, a total of nine (9) signatures can be verified at once, so for the current 19 guardians three _verifySigs_ calls would be needed for verifying signatures 0..8, 9..17, 18.
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A successful call must:
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* Retrieve the guardian keys from the proper local dynamic storage
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* Validate if the VAA passed in Argument #1 has enough guardians to be verified
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* Check that it's not expired.
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* Verify that each _verifySigs_ TX is validated by the correct stateless _VerifyVaa_
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* Verify that each _verifySigs_ TX is verifying the expected signature subset.
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* Verify that each _verifySigs_ TX is verifying against the same guardian keys.
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* Verify that each _verifySigs_ TX is verifying the same VAA.
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The vaaVerify call does allow *nop* (dummy) TX in the group to maximize opcode budgets and/or storage capacity. After the `verifyVAA` call, a client can issue more transactions with the fact that the VAA was verified.
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## Appendix: Duplicate verification
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------------------------------------
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To detect duplicate VAA sequence numbers the following technique is used:
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* For each key in local state, there is an associated value entry. The total space of value-entries is 127*15, we have 2K of addressable space using the `LocalBlob` class.
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* A _TmplSig_ stateless account is generated using the 2K space as a bit field, yielding 15240 bits. So for ~16K consecutive VAA numbers, the contract code sets a bit for identifying already verified VAAs. Based on setting the stateless `TMPL_ADDR_IDX` to formula `vaa_sequence_number / 15240`, we have designated storage for marking VAAs in consecutive 16k-bit blocks.
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