Sway Standards

⌘K

Src 16 Typed Structured Data

Icon LinkSRC-16: Typed Structured Data

The following standard sets out to standardize encoding and hashing of typed structured data. This enables secure off-chain message signing with human-readable data structures.

Icon LinkMotivation

As the Fuel ecosystem expands, there's an increasing need for applications to handle complex, human-readable data structures rather than raw bytes. When users sign messages or transactions, they should be able to clearly understand what they're signing, whether it's a simple asset transfer, or a complex DeFi interaction. Without a standard method for hashing structured data, developers risk implementing their own solutions, which could lead to confusion or compromise security. This standard provides a secure and consistent way to handle encoding and hashing of structured data, ensuring both safety and usability within ecosystem.

This standard aims to:

Provide a secure, standardized method for hashing structured data
Enable clear presentation of structured data for user verification during signing
Support complex data types that mirror Sway structs
Enable domain separation to prevent cross-protocol replay attacks
Define a consistent encoding scheme for structured data types
Remain stateless, not requiring any storage attributes to enable use across all Fuel program types.

Icon LinkPrior Art

This standard uses ideas from Ethereum's EIP-712 standard Icon Link, adapting its concepts for the Fuel ecosystem. EIP-712 has proven successful in enabling secure structured data signing for applications like the various browser based wallets and signers that are utilized throughout various DeFi protocols.

Icon LinkSpecification

Icon LinkDefinition of Typed Structured Data 𝕊

The set of structured data 𝕊 consists of all instances of struct types that can be composed from the following types:

Atomic Types:

u8 to u256
bool
b256 (hash)

Dynamic Types:

Bytes   // Variable-length byte sequences
String  // Variable-length strings

Reference Types:

Arrays (both fixed size and dynamic) Structs (reference to other struct types)

Example struct definition:

struct Mail {
    from: Address,
    to: Address,
    contents: String,
}

Icon LinkDomain Separator Encoding

The domain separator provides context for the signing operation, preventing cross-protocol replay attacks. It is computed as hash_struct(domain) where domain is defined as:

pub struct SRC16Domain {
    name: String,                   // The protocol name (e.g., "MyProtocol")
    version: String,                // The protocol version (e.g., "1")
    chain_id: u64,                  // The Fuel chain ID
    verifying_contract: ContractId, // The contract id that will verify the signature
}

The chain_id field is a u64 that must be encoded by left-padding with zeros and packed in big-endian order to fill a 32-byte value.

The domain separator encoding follows this scheme:

Add SRC16_DOMAIN_TYPE_HASH
Add Keccak256 hash of name string
Add Keccak256 hash of version string
Add chain ID as 32-byte big-endian
Add verifying contract id as 32 bytes

Icon LinkType Encoding

Each struct type is encoded as name ‖ "(" ‖ member₁ ‖ "," ‖ member₂ ‖ "," ‖ … ‖ memberₙ ")" where each member is written as type ‖ " " ‖ name.

Example:

Mail(address from,address to,string contents)

Icon LinkData Encoding

Icon LinkDefinition of hash_struct

The hash_struct function is defined as:

hash_struct(s : 𝕊) = keccak256(type_hash ‖ encode_data(s)) where:

type_hash = keccak256(encode_type(type of s))
‖ represents byte concatenation
encode_type and encode_data are defined below

Icon LinkDefinition of encode_data

The encoding of a struct instance is enc(value₁) ‖ enc(value₂) ‖ … ‖ enc(valueₙ), the concatenation of the encoded member values in the order they appear in the type. Each encoded member value is exactly 32 bytes long.

The values are encoded as follows:

Atomic Values:

Boolean false and true are encoded as u64 values 0 and 1, padded to 32 bytes
Address, ContractId, Identity, and b256 are encoded directly as 32 bytes
Unsigned Integer values (u8 to u256) are encoded as big-endian bytes, padded to 32 bytes

Dynamic Types:

Bytes and String are encoded as their Keccak256 hash

Reference Types:

Arrays (both fixed and dynamic) are encoded as the Keccak256 hash of their concatenated encodings
Struct values are encoded recursively as hash_struct(value)

The implementation of TypedDataHash for 𝕊 SHALL utilize the DataEncoder for encoding each element of the struct based on its type.

Icon LinkFinal Message Encoding

The encoding of structured data follows this pattern:

encode(domain_separator : 𝔹²⁵⁶, message : 𝕊) = "\x19\x01" ‖ domain_separator ‖ hash_struct(message)

where:

\x19\x01 is a constant prefix
‖ represents byte concatenation
domain_separator is the 32-byte hash of the domain parameters
hash_struct(message) is the 32-byte hash of the structured data

Icon LinkExample implementation

const MAIL_TYPE_HASH: b256 = 0x536e54c54e6699204b424f41f6dea846ee38ac369afec3e7c141d2c92c65e67f;
 
impl TypedDataHash for Mail {
 
    fn type_hash() -> b256 {
        MAIL_TYPE_HASH
    }
 
    fn struct_hash(self) -> b256 {
        let mut encoded = Bytes::new();
        encoded.append(
            MAIL_TYPE_HASH.to_be_bytes()
        );
        encoded.append(
            DataEncoder::encode_address(self.from).to_be_bytes()
        );
        encoded.append(
            DataEncoder::encode_address(self.to).to_be_bytes()
        );
        encoded.append(
            DataEncoder::encode_string(self.contents).to_be_bytes()
        );
 
        keccak256(encoded)
    }
}

Icon LinkRationale

Domain separators provides protocol-specific context to prevent signature replay across different protocols and chains.
Type hashes ensure type safety and prevent collisions between different data structures
The encoding scheme is designed to be deterministic and injective
The standard maintains compatibility with existing Sway types and practices

Icon LinkBackwards Compatibility

This standard is compatible with existing Sway data structures and can be implemented alongside other Fuel standards. It does not conflict with existing signature verification methods.

Icon LinkType System Compatibility Notes

When implementing SRC16 in relation to EIP712, the following type mappings and considerations apply:

Icon LinkString Encoding

Both standards use the same String type and encoding
SRC16 specifically uses String type only (not Sway's str or str[])
String values are encoded identically in both standards using keccak256 hash

Icon LinkFixed Bytes

EIP712's bytes32 maps directly to Sway's b256
Encoded using encode_b256 in the DataEncoder
Both standards handle 32-byte values identically
Smaller fixed byte arrays (bytes1 to bytes31) are not supported in SRC16

Icon LinkAddress Types

EIP712 uses 20-byte Ethereum addresses
When encoding an EIP712 address, SRC16:
- Takes only rightmost 20 bytes from a 32-byte Fuel Address
- Pads with zeros on the left for EIP712 compatibility
- Example: Fuel Address of 32 bytes becomes rightmost 20 bytes in EIP712 encoding

Icon LinkContractId Handling

ContractId is unique to Fuel/SRC16 (no equivalent in EIP712)
When encoding for EIP712 compatibility:
- Uses rightmost 20 bytes of ContractId
- Particularly important in domain separators where EIP712 expects a 20-byte address

Icon LinkDomain Separator Compatibility

// SRC16 Domain (Fuel native)
pub struct SRC16Domain {
    name: String,                   // Same as EIP712
    version: String,                // Same as EIP712
    chain_id: u64,                  // Fuel chain ID
    verifying_contract: ContractId, // Full 32-byte ContractId
}
 
// EIP712 Domain (Ethereum compatible)
pub struct EIP712Domain {
    name: String,
    version: String,
    chain_id: u256,
    verifying_contract: b256,      // Only rightmost 20 bytes used
}

Note on verifying_contract field; When implementing EIP712 compatibility within SRC16, the verifying_contract address in the EIP712Domain must be constructed by taking only the rightmost 20 bytes from either a Fuel ContractId. This ensures proper compatibility with Ethereum's 20-byte addressing scheme in the domain separator.

// Example ContractId conversion:
// Fuel ContractId (32 bytes):
//   0x000000000000000000000000a2233d3bf2aa3f0cbbe824eb04afc1acc84c364c
//                            └─────────────── 20 bytes ───────────────┘
//
// EIP712 Address (20 bytes):
//   0xa2233d3bf2aa3f0cbbe824eb04afc1acc84c364c
//    └─────────────── 20 bytes ───────────────┘

Note on EIP712 Domain Separator salt; Within EIP712 the field salt is an optional field to be used at the discretion of the protocol designer. Within SRC16 the EIP712Domain does not use the salt field. The other fields in EIP712Domain are mandatory within SRC16.

Icon LinkSecurity Considerations

Icon LinkReplay Attacks

Implementations must ensure signatures cannot be replayed across:

Different chains (prevented by chain_id) Different protocols (prevented by domain separator) Different contracts (prevented by verifying_contract)

Icon LinkType Safety

Implementations must validate all type information and enforce strict encoding rules to prevent type confusion attacks.

Icon LinkExample Implementation

Example of the SRC16 implementation where a contract utilizes the encoding scheme to produce a typed structured data hash of the Mail type.

contract;
 
use src16::{
    DataEncoder,
    DomainHash,
    SRC16,
    SRC16Base,
    SRC16Domain,
    SRC16Encode,
    SRC16Payload,
    TypedDataHash,
};
use std::{bytes::Bytes, contract_id::*, hash::*, string::String};
 
configurable {
    /// The name of the signing domain.
    DOMAIN: str[8] = __to_str_array("MyDomain"),
    /// The current major version for the signing domain.
    VERSION: str[1] = __to_str_array("1"),
    /// The active chain ID where the signing is intended to be used. Cast to u256 in domain_hash
    CHAIN_ID: u64 = 9889u64,
}
 
/// A demo struct representing a mail message
pub struct Mail {
    /// The sender's address
    pub from: Address,
    /// The recipient's address
    pub to: Address,
    /// The message contents
    pub contents: String,
}
 
/// The Keccak256 hash of the type Mail as UTF8 encoded bytes.
///
/// "Mail(address from,address to,string contents)"
///
/// 536e54c54e6699204b424f41f6dea846ee38ac369afec3e7c141d2c92c65e67f
///
const MAIL_TYPE_HASH: b256 = 0x536e54c54e6699204b424f41f6dea846ee38ac369afec3e7c141d2c92c65e67f;
 
impl TypedDataHash for Mail {
    fn type_hash() -> b256 {
        MAIL_TYPE_HASH
    }
 
    fn struct_hash(self) -> b256 {
        let mut encoded = Bytes::new();
        // Add the Mail type hash.
        encoded.append(MAIL_TYPE_HASH.to_be_bytes());
        // Use the DataEncoder to encode each field for known types
        encoded.append(DataEncoder::encode_address(self.from).to_be_bytes());
        encoded.append(DataEncoder::encode_address(self.to).to_be_bytes());
        encoded.append(DataEncoder::encode_string(self.contents).to_be_bytes());
 
        keccak256(encoded)
    }
}
 
/// Implement the encode function for Mail using SRC16Payload
///
/// # Additional Information
///
/// 1. Get the encodeData hash of the Mail typed data using
///    <Mail>..struct_hash();
/// 2. Obtain the payload to by populating the SRC16Payload struct
///    with the domain separator and data_hash from the previous step.
/// 3. Obtain the final_hash [Some(b256)] or None using the function
///    SRC16Payload::encode_hash()
///
impl SRC16Encode<Mail> for Mail {
    fn encode(s: Mail) -> b256 {
        // encodeData hash
        let data_hash = s.struct_hash();
        // setup payload
        let payload = SRC16Payload {
            domain: _get_domain_separator(),
            data_hash: data_hash,
        };
 
        // Get the final encoded hash
        match payload.encode_hash() {
            Some(hash) => hash,
            None => revert(0),
        }
    }
}
 
impl SRC16Base for Contract {
    fn domain_separator_hash() -> b256 {
        _get_domain_separator().domain_hash()
    }
 
    fn data_type_hash() -> b256 {
        MAIL_TYPE_HASH
    }
}
 
impl SRC16 for Contract {
    fn domain_separator() -> SRC16Domain {
        _get_domain_separator()
    }
}
 
abi MailMe {
    fn send_mail_get_hash(from_addr: Address, to_addr: Address, contents: String) -> b256;
}
 
impl MailMe for Contract {
    /// Sends a some mail and returns its encoded hash
    ///
    /// # Arguments
    ///
    /// * `from_addr`: [Address] - The sender's address
    /// * `to_addr`: [Address] - The recipient's address
    /// * `contents`: [String] - The message contents
    ///
    /// # Returns
    ///
    /// * [b256] - The encoded hash of the mail data
    ///
    fn send_mail_get_hash(from_addr: Address, to_addr: Address, contents: String) -> b256 {
        // Create the mail struct from data passed in call
        let some_mail = Mail {
            from: from_addr,
            to: to_addr,
            contents: contents,
        };
 
        Mail::encode(some_mail)
    }
}
 
/// A program specific implementation to get the Fuel SRC16Domain
///
/// In a Contract the ContractID can be obtain with ContractId::this()
///
/// In a Predicate or Script it is at the implementors discretion to
/// use the code root if they wish to contrain the validation to a
/// specifc program.
///
fn _get_domain_separator() -> SRC16Domain {
    SRC16Domain::new(
        String::from_ascii_str(from_str_array(DOMAIN)),
        String::from_ascii_str(from_str_array(VERSION)),
        CHAIN_ID,
        ContractId::this(),
    )
}

contract;
 
use src16::{
    DataEncoder,
    DomainHash,
    EIP712,
    EIP712Domain,
    SRC16Base,
    SRC16Encode,
    SRC16Payload,
    TypedDataHash,
};
use std::{bytes::Bytes, contract_id::*, hash::*, string::String};
 
configurable {
    /// The name of the signing domain.
    DOMAIN: str[8] = __to_str_array("MyDomain"),
    /// The current major version for the signing domain.
    VERSION: str[1] = __to_str_array("1"),
    /// The active chain ID where the signing is intended to be used. Cast to u256 in domain_hash
    CHAIN_ID: u64 = 9889u64,
}
 
/// A demo struct representing a mail message
pub struct Mail {
    /// The sender's address
    pub from: b256,
    /// The recipient's address
    pub to: b256,
    /// The message contents
    pub contents: String,
}
 
/// The Keccak256 hash of the type Mail as UTF8 encoded bytes.
///
/// "Mail(bytes32 from,bytes32 to,string contents)"
///
/// cfc972d321844e0304c5a752957425d5df13c3b09c563624a806b517155d7056
///
const MAIL_TYPE_HASH: b256 = 0xcfc972d321844e0304c5a752957425d5df13c3b09c563624a806b517155d7056;
 
impl TypedDataHash for Mail {
    fn type_hash() -> b256 {
        MAIL_TYPE_HASH
    }
 
    fn struct_hash(self) -> b256 {
        let mut encoded = Bytes::new();
 
        // Add the Mail type hash.
        encoded.append(MAIL_TYPE_HASH.to_be_bytes());
        // Use the DataEncoder to encode each field for known types
        encoded.append(DataEncoder::encode_b256(self.from).to_be_bytes());
        encoded.append(DataEncoder::encode_b256(self.to).to_be_bytes());
        encoded.append(DataEncoder::encode_string(self.contents).to_be_bytes());
 
        keccak256(encoded)
    }
}
 
/// Implement the encode function for Mail using SRC16Payload
///
/// # Additional Information
///
/// 1. Get the encodeData hash of the Mail typed data using
///    <Mail>..struct_hash();
/// 2. Obtain the payload to by populating the SRC16Payload struct
///    with the domain separator and data_hash from the previous step.
/// 3. Obtain the final_hash [Some(b256)] or None using the function
///    SRC16Payload::encode_hash()
///
impl SRC16Encode<Mail> for Mail {
    fn encode(s: Mail) -> b256 {
        // encodeData hash
        let data_hash = s.struct_hash();
        // setup payload
        let payload = SRC16Payload {
            domain: _get_domain_separator(),
            data_hash: data_hash,
        };
 
        // Get the final encoded hash
        match payload.encode_hash() {
            Some(hash) => hash,
            None => revert(0),
        }
    }
}
 
impl SRC16Base for Contract {
    fn domain_separator_hash() -> b256 {
        _get_domain_separator().domain_hash()
    }
 
    fn data_type_hash() -> b256 {
        MAIL_TYPE_HASH
    }
}
 
impl EIP712 for Contract {
    fn domain_separator() -> EIP712Domain {
        _get_domain_separator()
    }
}
 
abi MailMe {
    fn send_mail_get_hash(from_addr: b256, to_addr: b256, contents: String) -> b256;
}
 
impl MailMe for Contract {
    /// Sends a some mail and returns its encoded hash
    ///
    /// # Arguments
    ///
    /// * `from_addr`: [b256] - The sender's address
    /// * `to_addr`: [b256] - The recipient's address
    /// * `contents`: [String] - The message contents
    ///
    /// # Returns
    ///
    /// * [b256] - The encoded hash of the mail data
    ///
    fn send_mail_get_hash(from_addr: b256, to_addr: b256, contents: String) -> b256 {
        // Create the mail struct from data passed in call
        let some_mail = Mail {
            from: from_addr,
            to: to_addr,
            contents: contents,
        };
 
        Mail::encode(some_mail)
    }
}
 
/// A program specific implementation to get the Ethereum EIP712Domain
///
/// In a Contract the ContractID can be obtain with ContractId::this()
///
/// In a Predicate or Script it is at the implementors discretion to
/// use the code root if they wish to contrain the validation to a
/// specifc program.
///
fn _get_domain_separator() -> EIP712Domain {
    EIP712Domain::new(
        String::from_ascii_str(from_str_array(DOMAIN)),
        String::from_ascii_str(from_str_array(VERSION)),
        (asm(r1: (0, 0, 0, CHAIN_ID)) {
                r1: u256
            }),
        ContractId::this(),
    )
}