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Overview

The Solana Gateway protocol implementation is split into two programs: GatewayWallet and GatewayMinter. To ensure alignment with EVM contracts’ logic and state, and to facilitate upgrades and maintenance, the code and state of Solana programs reflect the EVM counterparts as closely as possible. Key differences for Solana:
  • Where canonical addresses are expected in the API, Solana addresses should be represented as base58.
  • Where 32-byte addresses are expected in the API, Solana addresses should be represented as hex.
  • All block height expiration fields in the API (such as maxBlockHeight and expirationBlock) refer to the last slot before the signed message is invalid.
  • BurnIntentSets originating from Solana are not supported. Each transfer from Solana must be specified as a separate signed BurnIntent.
  • Solana uses a reduced attestation format to accommodate transaction size limits.

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Message formats

Gateway on Solana uses the same message formats as on EVM for the TransferSpec and BurnIntent messages. These messages are described in the Gateway Technical Guide. Gateway on Solana uses a reduced attestation format to accommodate transaction size limits.

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ReducedMintAttestation

A mint attestation contains the details needed for the GatewayMinter to mint. Mint attestations on Solana use a “reduced” encoding to account for Solana’s transaction size limits. On Solana, all attestations are ReducedMintAttestationSets. This reduces ambiguity when decoding MintAttestations and saves transaction size on common fields between elements that have been hoisted to the set level.

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Attestation set encoding

FieldOffsetBytesDescription
magic040x10cbb1ec
version44Protocol version
destinationDomain84Destination domain identifier
destinationContract1232Minter contract address
destinationCaller4432Allowed caller address
maxBlockHeight768Expiration slot
numAttestations844Number of attestation elements
attestations88varArray of attestation elements

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Attestation element encoding

FieldOffsetBytesDescription
destinationToken032Token address on destination
destinationRecipient3232Recipient address
value648Amount to mint
transferSpecHash7232keccak256 hash of the TransferSpec
hookDataLength1044Length of hook data
hookData108varHook data bytes
Unlike standard MintAttestation messages, the ReducedMintAttestation does not include the full TransferSpec. The TransferSpec hash is encoded directly into the message and can be used as a unique crosschain identifier. Retrieve the original TransferSpec through the Get transfer spec endpoint.

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Signing

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BurnIntent signing

Once a BurnIntent has been constructed, prepend the signing domain header (0xff000000000000000000000000000000) before signing using the ed25519 keypair associated with the source depositor. Alternatively, sign with a delegated address.
FieldOffsetBytesDescription
signingDomain0160xff000000000000000000000000000000
burnIntent16varEncoded BurnIntent
The signing domain header ensures:
  1. The signature cannot authorize a transaction due to illegal first byte (0xff).
  2. The program can be upgraded to support Off-Chain Message Signing Format (OCMSF) without introducing ambiguity between signing payload types.

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MintAttestation signing

ReducedMintAttestation messages are signed by the Gateway API service using EVM-style ECDSA signatures. The keccak256 hash of the message is signed by the Circle attestation signer according to EIP-191, using the personal_sign (0x45) format.

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Transaction size limits

Solana transactions can be at most 1232 bytes. To maximize composability of gatewayMint instructions, the protocol minimizes the data size needed to execute mints. The ReducedMintAttestation message reduces the size of the original MintAttestation designed for EVM by removing fields that do not need to be validated on the destination chain. The parameter mode for gatewayMint further reduces redundancy by inferring data from the execution context or account list. To ensure that USDC supply can be maintained 1:1, the Gateway Transfer API rejects user BurnIntents that are too large to be safely executed on Solana. Verify your transaction sizes before broadcasting transactions using the gatewayMint instruction.

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Executing large mint attestations

You can execute large MintAttestations that exceed the transaction size limit, as long as strict atomicity is not required. Deploy a separate program that writes MintAttestation data into a buffer account across multiple transactions. Once the message has been written to the buffer account, this program can execute a cross-program invocation (CPI) into the Gateway Minter program, passing in the MintAttestation bytes from the buffer account as an argument. CPI calls can be at most 10 * 1024 bytes.