Penumbra is a fully private proof-of-stake network providing privacy to the Cosmos ecosystem.
Penumbra integrates privacy with proof-of-stake through a novel private delegation mechanism that provides staking derivatives, tax-efficient staking, and on-chain governance with private voting. Penumbra connects to the Cosmos ecosystem via IBC, acting as an ecosystem-wide shielded pool and allowing private transactions in any IBC-compatible asset. Users can also swap these assets using ZSwap, a private decentralized exchange supporting sealed-bid batch auctions and Uniswap-v3-style concentrated liquidity. Sealed-bid batch auctions prevent frontrunning, provide better execution, and reveal only the net flow across a pair of assets in each block, and liquidity positions are created anonymously, allowing traders to approximate their desired trading function without revealing their individual beliefs about prices.
Penumbra records all value in a single multi-asset shielded pool based on the Zcash Sapling design, but allows private transactions in any kind of IBC asset. Inbound IBC transfers shield value as it moves into the zone, while outbound IBC transfers unshield value.
Unlike Zcash, Penumbra has no notion of transparent transactions or a
transparent value pool; instead, inbound IBC transfers are analogous to
Zcash transactions, outbound IBC transfers are analogous to
transactions, and the entire Cosmos ecosystem functions analogously to
Zcash’s transparent pool.
Unlike the Cosmos Hub or many other chains built on the Cosmos SDK, Penumbra has no notion of accounts. Only validators have any kind of long-term identity, and this identity is only used (in the context of transactions) for spending the validator’s commission.
In a proof-of-stake system like the Cosmos Hub, stakeholders delegate staking tokens by bonding them to validators. Validators participate in Tendermint consensus with voting power determined by their delegation size, and delegators receive staking rewards in exchange for taking on the risk of being penalized for validator misbehavior (slashing).
Integrating privacy and proof of stake poses significant challenges. If delegations are public, holders of the staking token must choose between privacy on the one hand and staking rewards and participation in consensus on the other hand. Because the majority of the stake will be bonded to validators, privacy becomes an uncommon, opt-in case. But if delegations are private, issuing staking rewards becomes very difficult, because the chain no longer knows the amount and duration of each address’ delegations.
Penumbra sidesteps this problem using a new mechanism that eliminates staking rewards entirely, treating unbonded and bonded stake as separate assets, with an epoch-varying exchange rate that prices in what would be a staking reward in other systems. This mechanism ensures that all delegations to a particular validator are fungible, and can be represented by a single token representing a share of that validator’s delegation pool, in effect a first-class staking derivative. Although delegation fungibility is key to enabling privacy, as a side effect, delegators do not realize any income while their stake is bonded, only a capital gain (or loss) on unbonding.
The total amount of stake bonded to each validator is part of the public chain state and determines consensus weight, but the bonded stake itself is just another token to be recorded in a multi-asset shielded pool. This provides accountability for validators and privacy and flexibility for delegators, who can trade and transact with their bonded stake just like they can with any other token.
It also provides an alternate perspective on the debate between fixed-supply and inflation-based rewards. Choosing the unbonded token as the numéraire, delegators are rewarded by inflation for taking on the risk of validator misbehavior, and the token supply grows over time. Choosing the bonded token as the numéraire, non-delegators are punished by depreciation for not taking on any risk of misbehavior, and the token supply is fixed.
Like the Cosmos Hub, Penumbra supports on-chain governance with delegated voting. Unlike the Cosmos Hub, Penumbra’s governance mechanism supports secret ballots. Penumbra users can anonymously propose votes by escrowing a deposit of bonded stake. Stakeholders vote by proving ownership of their bonded stake prior to the beginning of the voting period and encrypting their votes to a threshold key controlled by the validator set. Validators sum encrypted votes and decrypt only the per-epoch totals.
Penumbra provides private, sealed-bid batch swaps using ZSwap. ZSwap allows users to privately swap between any pair of assets. Individual swaps do not reveal trade amounts. Instead, all swaps in each block are executed in a single batch. Only the total amount in each batch is revealed, and only after the batch has been finalized. This prevents front-running and provides better execution, but also provides long-term privacy for individual swaps. Users can also provide liquidity by anonymously creating Uniswap-v3-style concentrated liquidity positions. These positions reveal the amount of liquidity and the bounds in which it is concentrated, but are not otherwise linked to any identity, so that (with some care) users can privately approximate arbitrary trading functions without revealing their specific views about prices.