EIP 712: Ethereum typed structured data hashing and signing

Simple Summary

Signing data is a solved problem if all we care about are bytestrings. Unfortunately in the real world we care about complex meaningful messages. Hashing structured data is non-trivial and errors result in loss of the security properties of the system.

As such, the adage “don’t roll your own crypto” applies. Instead, a peer-reviewed well-tested standard method needs to be used. This EIP aims to be that standard.

Abstract

This is a standard for hashing and signing of typed structured data as opposed to just bytestrings. It includes a

• theoretical framework for correctness of encoding functions,
• specification of structured data similar to and compatible with Solidity structs,
• safe hashing algorithm for instances of those structures,
• safe inclusion of those instances in the set of signable messages,
• an extensible mechanism for domain separation,
• new RPC call eth_signTypedData, and
• an optimized implementation of the hashing algorithm in EVM.

It does not include replay protection.

Motivation

This EIP aims to improve the usability of off-chain message signing for use on-chain. We are seeing growing adoption of off-chain message signing as it saves gas and reduces the number of transactions on the blockchain. Currently signed messages are an opaque hex string displayed to the user with little context about the items that make up the message.

Here we outline a scheme to encode data along with its structure which allows it to be displayed to the user for verification when signing. Below is an example of what a user could be shown when signing an EIP712 message.

Signatures and Hashing overview

A signature scheme consists of hashing algorithm and a signing algorithm. The signing algorithm of choice in Ethereum is secp256k1. The hashing algorithm of choice is keccak256, this is a function from bytestrings, 𝔹⁸ⁿ, to 256-bit strings, 𝔹²⁵⁶.

A good hashing algorithm should satisfy security properties such as determinism, second pre-image resistance and collision resistance. The keccak256 function satisfies the above criteria when applied to bytestrings. If we want to apply it to other sets we first need to map this set to bytestrings. It is critically important that this encoding function is deterministic and injective. If it is not deterministic then the hash might differ from the moment of signing to the moment of verifying, causing the signature to incorrectly be rejected. If it is not injective then there are two different elements in our input set that hash to the same value, causing a signature to be valid for a different unrelated message.

Transactions and bytestrings

An illustrative example of the above breakage can be found in Ethereum. Ethereum has two kinds of messages, transactions 𝕋 and bytestrings 𝔹⁸ⁿ. These are signed using eth_sendTransaction and eth_sign respectively. Originally the encoding function encode : 𝕋 ∪ 𝔹⁸ⁿ → 𝔹⁸ⁿ was defined as follows:

• encode(t : 𝕋) = RLP_encode(t)
• encode(b : 𝔹⁸ⁿ) = b

While individually they satisfy the required properties, together they do not. If we take b = RLP_encode(t) we have a collision. This is mitigated in Geth PR 2940 by modifying the second leg of the encoding function:

• encode(b : 𝔹⁸ⁿ) = "\x19Ethereum Signed Message:\n" ‖ len(b) ‖ b where len(b) is the ascii-decimal encoding of the number of bytes in b.

This solves the collision between the legs since RLP_encode(t : 𝕋) never starts with \x19. There is still the risk of the new encoding function not being deterministic or injective. It is instructive to consider those in detail.

As is, the definition above is not deterministic. For a 4-byte string b both encodings with len(b) = "4" and len(b) = "004" are valid. This can be solved by further requiring that the decimal encoding of the length has no leading zeros and len("") = "0".

The above definition is not obviously collision free. Does a bytestring starting with "\x19Ethereum Signed Message:\n42a…" mean a 42-byte string starting with a or a 4-byte string starting with 2a?. This was pointed out in Geth issue #14794 and motivated Trezor to not implement the standard as-is. Fortunately this does not lead to actual collisions as the total length of the encoded bytestring provides sufficient information to disambiguate the cases.

Both determinism and injectiveness would be trivially true if len(b) was left out entirely. The point is, it is difficult to map arbitrary sets to bytestrings without introducing security issues in the encoding function. Yet the current design of eth_sign still takes a bytestring as input and expects implementors to come up with an encoding.

Arbitrary messages

The eth_sign call assumes messages to be bytestrings. In practice we are not hashing bytestrings but the collection of all semantically different messages of all different DApps 𝕄. Unfortunately, this set is impossible to formalize. Instead we approximate it with the set of typed named structures 𝕊. This standard formalizes the set 𝕊 and provides a deterministic injective encoding function for it.

Just encoding structs is not enough. It is likely that two different DApps use identical structs. When this happens, a signed message intended for one DApp would also be valid for the other. The signatures are compatible. This can be intended behaviour, in which case everything is fine as long as the DApps took replay attacks into consideration. If it is not intended, there is a security problem.

The way to solve this is by introducing a domain separator, a 256-bit number. This is a value unique to each domain that is ‘mixed in’ the signature. It makes signatures from different domains incompatible. The domain separator is designed to include bits of DApp unique information such as the name of the DApp, the intended validator contract address, the expected DApp domain name, etc. The user and user-agent can use this information to mitigate phishing attacks, where a malicious DApp tries to trick the user into signing a message for another DApp.

Note on replay attacks

This standard is only about signing messages and verifying signatures. In many practical applications, signed messages are used to authorize an action, for example an exchange of tokens. It is very important that implementers make sure the application behaves correctly when it sees the same signed message twice. For example, the repeated message should be rejected or the authorized action should be idempotent. How this is implemented is specific to the application and out of scope for this standard.

Specification

The set of signable messages is extended from transactions and bytestrings 𝕋 ∪ 𝔹⁸ⁿ to also include structured data 𝕊. The new set of signable messages is thus 𝕋 ∪ 𝔹⁸ⁿ ∪ 𝕊. They are encoded to bytestrings suitable for hashing and signing as follows:

• encode(transaction : 𝕋) = RLP_encode(transaction)
• encode(message : 𝔹⁸ⁿ) = "\x19Ethereum Signed Message:\n" ‖ len(message) ‖ message where len(message) is the non-zero-padded ascii-decimal encoding of the number of bytes in message.
• encode(domainSeparator : 𝔹²⁵⁶, message : 𝕊) = "\x19\x01" ‖ domainSeparator ‖ hashStruct(message) where domainSeparator and hashStruct(message) are defined below.

This encoding is deterministic because the individual components are. The encoding is injective because the three cases always differ in first byte. (RLP_encode(transaction) does not start with \x19.)

The encoding is compliant with EIP-191. The ‘version byte’ is fixed to 0x01, the ‘version specific data’ is the 32-byte domain separator domainSeparator and the ‘data to sign’ is the 32-byte hashStruct(message).

Definition of typed structured data 𝕊

To define the set of all structured data, we start with defining acceptable types. Like ABIv2 these are closely related to Solidity types. It is illustrative to adopt Solidity notation to explain the definitions. The standard is specific to the Ethereum Virtual Machine, but aims to be agnostic to higher level languages. Example:

struct Mail {
    address from;
    address to;
    string contents;
}

Definition: A struct type has valid identifier as name and contains zero or more member variables. Member variables have a member type and a name.

Definition: A member type can be either an atomic type, a dynamic type or a reference type.

Definition: The atomic types are bytes1 to bytes32, uint8 to uint256, int8 to int256, bool and address. These correspond to their definition in Solidity. Note that there are no aliases uint and int. Note that contract addresses are always plain address. Fixed point numbers are not supported by the standard. Future versions of this standard may add new atomic types.

Definition: The dynamic types are bytes and string. These are like the atomic types for the purposed of type declaration, but their treatment in encoding is different.

Definition: The reference types are arrays and structs. Arrays are either fixed size or dynamic and denoted by Type[n] or Type[] respectively. Structs are references to other structs by their name. The standard supports recursive struct types.

Definition: The set of structured typed data 𝕊 contains all the instances of all the struct types.

Definition of hashStruct

The hashStruct function is defined as

• hashStruct(s : 𝕊) = keccak256(typeHash ‖ encodeData(s)) where typeHash = keccak256(encodeType(typeOf(s)))

Note: The typeHash is a constant for a given struct type and does not need to be runtime computed.

Definition of encodeType

The type of a struct is encoded as name ‖ "(" ‖ member₁ ‖ "," ‖ member₂ ‖ "," ‖ … ‖ memberₙ ")" where each member is written as type ‖ " " ‖ name. For example, the above Mail struct is encoded as Mail(address from,address to,string contents).

If the struct type references other struct types (and these in turn reference even more struct types), then the set of referenced struct types is collected, sorted by name and appended to the encoding. An example encoding is Transaction(Person from,Person to,Asset tx)Asset(address token,uint256 amount)Person(address wallet,string name).

Definition of encodeData

The encoding of a struct instance is enc(value₁) ‖ enc(value₂) ‖ … ‖ enc(valueₙ), i.e. the concatenation of the encoded member values in the order that they apear in the type. Each encoded member value is exactly 32-byte long.

The atomic values are encoded as follows: Boolean false and true are encoded as uint256 values 0 and 1 respectively. Addresses are encoded as uint160. Integer values are sign-extended to 256-bit and encoded in big endian order. bytes1 to bytes31 are arrays with a beginning (index 0) and an end (index length - 1), they are zero-padded at the end to bytes32 and encoded in beginning to end order. This corresponds to their encoding in ABI v1 and v2.

The dynamic values bytes and string are encoded as a keccak256 hash of their contents.

The array values are encoded as the keccak256 hash of the concatenated encodeData of their contents (i.e. the encoding of SomeType[5] is identical to that of a struct containing five members of type SomeType).

The struct values are encoded recursively as hashStruct(value). This is undefined for cyclical data.

Definition of domainSeparator

domainSeparator = hashStruct(eip712Domain)

where the type of eip712Domain is a struct named EIP712Domain with one or more of the below fields. Protocol designers only need to include the fields that make sense for their signing domain. Unused fields are left out of the struct type.

• string name the user readable name of signing domain, i.e. the name of the DApp or the protocol.
• string version the current major version of the signing domain. Signatures from different versions are not compatible.
• uint256 chainId the EIP-155 chain id. The user-agent should refuse signing if it does not match the currently active chain.
• address verifyingContract the address of the contract that will verify the signature. The user-agent may do contract specific phishing prevention.
• bytes32 salt an disambiguating salt for the protocol. This can be used as a domain separator of last resort.

Future extensions to this standard can add new fields with new user-agent behaviour constraints. User-agents are free to use the provided information to inform/warn users or refuse signing.

Specification of the eth_signTypedData JSON RPC

The method eth_signTypedData is added to the Ethereum JSON-RPC. The method parallels eth_sign.

eth_signTypedData

The sign method calculates an Ethereum specific signature with: sign(keccak256("\x19Ethereum Signed Message:\n" + len(message) + message))).

By adding a prefix to the message makes the calculated signature recognisable as an Ethereum specific signature. This prevents misuse where a malicious DApp can sign arbitrary data (e.g. transaction) and use the signature to impersonate the victim.

Note the address to sign with must be unlocked.

Parameters

1. Address - 20 Bytes - Address of the account that will sign the messages.
2. TypedData - Typed structured data to be signed.

Typed data is a JSON object containing type information, domain seprator parameters and the message object. Below is the json-schema definition for TypedData param.

{
  type: 'object',
  properties: {
    types: {
      type: 'object',
      properties: {
        EIP712Domain: {type: 'array'},
      },
      additionalProperties: {
        type: 'array',
        items: {
          type: 'object',
          properties: {
            name: {type: 'string'},
            type: {type: 'string'}
          },
          required: ['name', 'type']
        }
      },
      required: ['EIP712Domain']
    },
    primaryType: {type: 'string'},
    domain: {type: 'object'},
    message: {type: 'object'}
  },
  required: ['types', 'primaryType', 'domain', 'message']
}

Returns

DATA: Signature. As in eth_sign it is a hex encoded 129 byte array starting with 0x. It encodes the r, s and v parameters from appendix F of the yellow paper in big-endian format. Bytes 0…64 contain the r parameter, bytes 64…128 the s parameter and the last byte the v parameter. Note that the v parameter includes the chain id as specified in EIP-155.

Example

Request:

curl -X POST --data '{"jsonrpc":"2.0","method":"eth_signTypedData","params":["0xCD2a3d9F938E13CD947Ec05AbC7FE734Df8DD826", {"types":{"EIP712Domain":[{"name":"name","type":"string"},{"name":"version","type":"string"},{"name":"chainId","type":"uint256"},{"name":"verifyingContract","type":"address"}],"Person":[{"name":"name","type":"string"},{"name":"wallet","type":"address"}],"Mail":[{"name":"from","type":"Person"},{"name":"to","type":"Person"},{"name":"contents","type":"string"}]},"primaryType":"Mail","domain":{"name":"Ether Mail","version":"1","chainId":1,"verifyingContract":"0xCcCCccccCCCCcCCCCCCcCcCccCcCCCcCcccccccC"},"message":{"from":{"name":"Cow","wallet":"0xCD2a3d9F938E13CD947Ec05AbC7FE734Df8DD826"},"to":{"name":"Bob","wallet":"0xbBbBBBBbbBBBbbbBbbBbbbbBBbBbbbbBbBbbBBbB"},"contents":"Hello, Bob!"}}],"id":1}'

Result:

{
  "id":1,
  "jsonrpc": "2.0",
  "result": "0x4355c47d63924e8a72e509b65029052eb6c299d53a04e167c5775fd466751c9d07299936d304c153f6443dfa05f40ff007d72911b6f72307f996231605b915621c"
}

An example how to use Solidity ecrecover to verify the signature calculated with eth_signTypedData can be found in the EIP712 Example.js. The contract is deployed on the testnet Ropsten and Rinkeby.

personal_signTypedData

There also should be a corresponding personal_signTypedData method which accepts the password for an account as the last argument.

Specification of the Web3 API

Two methods are added to Web 3 version 1 that parallel the web3.eth.sign and web3.eth.personal.sign methods.

web3.eth.signTypedData

web3.eth.signTypedData(typedData, address [, callback])

Signs typed data using a specific account. This account needs to be unlocked.

Parameters

1. Object - Domain separator and typed data to sign. Structured according to the JSON-Schema specified above in the eth_signTypedData JSON RPC call.
2. String|Number - Address to sign data with. Or an address or index of a local wallet in :ref:web3.eth.accounts.wallet <eth_accounts_wallet>.
3. Function - (optional) Optional callback, returns an error object as first parameter and the result as second.

Note The 2. address parameter can also be an address or index from the web3.eth.accounts.wallet <eth_accounts_wallet>. It will then sign locally using the private key of this account. —-

Returns

Promise returns String - The signature as returned by eth_signTypedData.

Example

See the eth_signTypedData JSON-API example above for the value of typedData.

web3.eth.signTypedData(typedData, "0xCD2a3d9F938E13CD947Ec05AbC7FE734Df8DD826")
.then(console.log);
> "0x4355c47d63924e8a72e509b65029052eb6c299d53a04e167c5775fd466751c9d07299936d304c153f6443dfa05f40ff007d72911b6f72307f996231605b915621c"

web3.eth.personal.signTypedData

web3.eth.personal.signTypedData(typedData, address, password [, callback])

Identical to web3.eth.signTypedData except for an additional password parameter analogous to web3.eth.personal.sign.

Rationale

The encode function is extended with a new case for the new types. The first byte of the encoding distinguishes the cases. For the same reason it is not safe to start immediately with the domain separator or a typeHash. While hard, it may be possible to construct a typeHash that also happens to be a prefix of a valid RLP encoded transaction.

The domain separator prevents collision of otherwise identical structures. It is possible that two DApps come up with an identical structure like Transfer(address from,address to,uint256 amount) that should not be compatible. By introducing a domain separator the DApp developers are guaranteed that there can be no signature collision.

The domain separator also allows for multiple distinct signatures use-cases on the same struct instance within a given DApp. In the previous example, perhaps signatures from both from and to are required. By providing two distinct domain separators these signatures can be distinguished from each other.

Alternative 1: Use the target contract address as domain separator. This solves the first problem, contracts coming up with identical types, but does not address the second use-case. The standard does suggest implementors to use the target contract address where this is appropriate.

The function hashStruct starts with a typeHash to separate types. By giving different types a different prefix the encodeData function only has to be injective within a given type. It is okay for encodeData(a) to equal encodeData(b) as long as typeOf(a) is not typeOf(b).

Rationale for typeHash

The typeHash is designed to turn into a compile time constant in Solidity. For example:

bytes32 constant MAIL_TYPEHASH = keccak256(
  "Mail(address from,address to,string contents)");

For the type hash several alternatives were considered and rejected for the reasons:

Alternative 2: Use ABIv2 function signatures. bytes4 is not enough to be collision resistant. Unlike function signatures, there is negligible runtime cost incurred by using longer hashes.

Alternative 3: ABIv2 function signatures modified to be 256-bit. While this captures type info, it does not capture any of the semantics other than the function. This is already causing a practical collision between ERC20’s and ERC721’s transfer(address,uint256), where in the former the uint256 refers to an amount and the latter to a unique id. In general ABIv2 favors compatibility where a hashing standard should prefer incompatibility.

Alternative 4: 256-bit ABIv2 signatures extended with parameter names and struct names. The Mail example from a above would be encoded as Mail(Person(string name,address wallet) from,Person(string name,address wallet) to,string contents). This is longer than the proposed solution. And indeed, the length of the string can grow exponentially in the length of the input (consider struct A{B a;B b;}; struct B {C a;C b;}; …). It also does not allow a recursive struct type (consider struct List {uint256 value; List next;}).

Alternative 5: Include natspec documentation. This would include even more semantic information in the schemaHash and further reduces chances of collision. It makes extending and amending documentation a breaking changes, which contradicts common assumptions. It also makes the schemaHash mechanism very verbose.

Rationale for encodeData

The encodeData is designed to allow easy implementation of hashStruct in Solidity:

function hashStruct(Mail memory mail) pure returns (bytes32 hash) {
    return keccak256(abi.encode(
        MAIL_TYPEHASH,
        mail.from,
        mail.to,
        keccak256(mail.contents)
    ));
}

it also allows for an efficient in-place implementation in EVM

function hashStruct(Mail memory mail) pure returns (bytes32 hash) {

    // Compute sub-hashes
    bytes32 typeHash = MAIL_TYPEHASH;
    bytes32 contentsHash = keccak256(mail.contents);

    assembly {
        // Back up select memory
        let temp1 := mload(sub(mail, 32))
        let temp2 := mload(add(mail, 128))

        // Write typeHash and sub-hashes
        mstore(sub(mail, 32), typeHash)
        mstore(add(mail, 64), contentsHash)

        // Compute hash
        hash := keccak256(sub(mail, 32), 128)

        // Restore memory
        mstore(sub(mail, 32), temp1)
        mstore(add(mail, 64), temp2)
    }
}

The in-place implementation makes strong but reasonable assumptions on the memory layout of structs in memory. Specifically it assumes structs are not allocated below address 32, that members are stored in order, that all values are padded to 32-byte boundaries, and that dynamic and reference types are stored as a 32-byte pointers.

Alternative 6: Tight packing. This is the default behaviour in Soldity when calling keccak256 with multiple arguments. It minimizes the number of bytes to be hashed but requires complicated packing instructions in EVM to do so. It does not allow in-place computation.

Alternative 7: ABIv2 encoding. Especially with the upcoming abi.encode it should be easy to use abi.encode as the encodeData function. The ABIv2 standard by itself fails the determinism security criteria. There are several valid ABIv2 encodings of the same data. ABIv2 does not allow in-place computation.

Alternative 8: Leave typeHash out of hashStruct and instead combine it with the domain separator. This is more efficient, but then the semantics of the Solidity keccak256 hash function are not injective.

Alternative 9: Support cyclical data structures. The current standard is optimized for tree-like data structures and undefined for cyclical data structures. To support cyclical data a stack containing the path to the current node needs to be maintained and a stack offset substituted when a cycle is detected. This is prohibitively more complex to specify and implement. It also breaks composability where the hashes of the member values are used to construct the hash of the struct (the hash of the member values would depend on the path). It is possible to extend the standard in a compatible way to define hashes of cyclical data.

Similarly, a straightforward implementation is sub-optimal for directed acyclic graphs. A simple recursion through the members can visit the same node twice. Memoization can optimize this.

Rationale for domainSeparator

Since different domains have different needs, an extensible scheme is used where the DApp specifies a EIP712Domain struct type and an instance eip712Domain which it passes to the user-agent. The user-agent can then apply different verification measures depending on the fields that are there.

A field string eip719dsl can be added and be rejected if the value does not match the hash of the EIP-719 DSL interface string.

Backwards Compatibility

The RPC calls, web3 methods and SomeStruct.typeHash parameter are currently undefined. Defining them should not affect the behaviour of existing DApps.

The Solidity expression keccak256(someInstance) for an instance someInstance of a struct type SomeStruct is valid syntax. It currently evaluates to the keccak256 hash of the memory address of the instance. This behaviour should be considered dangerous. In some scenarios it will appear to work correctly but in others it will fail determinism and/or injectiveness. DApps that depend on the current behaviour should be considered dangerously broken.

Test Cases

An example contract can be found in Example.sol and an example implementation of signing in JavaScript in Example.js

Implementation

To be done before this EIP can be considered accepted:

• Finalize specification of structure hashing
• Domain separators
• Add test vectors
• Review specification

To be done before this EIP can be considered “Final”:

• Implement eth_signTypedData in major RPC providers.
• Implement web3.eth.signTypedData in Web3 providers.
• Implement keccak256 struct hashing in Solidity.

Copyright

Copyright and related rights waived via CC0.