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VAAs (Verified Action Approvals)
VAAs are the core messaging primative in Wormhole. You can think of them as packets of xData that are emitted any time an xDapp contract interacts with the Core Contract.
The basic VAA has two components--a Header and a Body.
Header
byte version (VAA Version)
u32 guardian_set_index (Indicates which guardian set is signing)
u8 len_signatures (Number of signatures stored)
[][66]byte signatures (Collection of ecdsa signatures)
The Header is used by the Core Contract to determine the authenticity of the VAA, but can generally be ignored by other consumers.
Body
u32 timestamp (Timestamp of the block where the source transaction occurred)
u32 nonce (A grouping number)
u16 emitter_chain (Wormhole ChainId of emitter contract)
[32]byte emitter_address (Emitter contract address, in Wormhole format)
u64 sequence (Strictly increasing sequence, tied to emitter address & chain)
u8 consistency_level (How many blocks were waited before emitting this VAA)
[]byte payload (VAA message content)
The Body is the relevant information for consumers and is handed back from parseAndVerifyVM. Because the emitterAddress is included as part of the Body, the developer is able to tell if this VAA originated from a trusted contract.
VAAs are uniquely indexed by their emitterChain, emittedAddress and sequence. They can be obtained by querying a node in the Guardian Network with this information.
Because baseline VAAs have no destination, they are effectively multicast. They will be verified as authentic by any Core Contract on any chain in the network -- the responsibility to deliver VAAs to the appropriate place is entirely that of relayers.
Batch VAAs
Certain blockchains support version 2 VAAs, also referred to as Batch VAAs. When multiple messages with the same nonce are emitted in the same transaction, a batch VAA will be created in addition to the individual VAAs. The Batch VAA contains the body of each individual VAA, but only has a single header. This reduces the gas cost of verifying the VAA, and simplifies the process of relaying and consuming multiple VAAs.
Batch VAAs are not currently live on mainnet, but will have initial support on all EVM chains when they launch.
In the next section, we'll give an overview of how the Wormhole Guardian network creates VAAs along with a look at the key design considerations that underpin the network.