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fix(evm-charge): remove redundant externalId from Permit2 witness #313

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fix(evm-charge): remove redundant externalId from Permit2 witness #313

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15 changes: 8 additions & 7 deletions specs/methods/evm/draft-evm-charge-00.md
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Original file line number Diff line number Diff line change
Expand Up @@ -151,7 +151,7 @@ This specification defines four credential types:
- The server naturally sponsors gas (fee payer)
- Split payments are atomic via batch transfers
- No nonce management burden on the client
- `externalId` is cryptographically bound via witness data
- `externalId` is cryptographically bound via the challenge hash

- **`type="authorization"`**: The client signs an
off-chain EIP-3009 {{EIP-3009}} `transferWithAuthorization`
Expand Down Expand Up @@ -450,18 +450,19 @@ in array order.

The Permit2 witness mechanism provides cryptographic
binding between the payment authorization and the
challenge. When `externalId` is present in the challenge
request, the client MUST include it in the EIP-712
witness struct. When `externalId` is absent from the challenge request,
`PaymentWitness.externalId` is the empty string (`""`). The server MUST
challenge. The witness commits to `challengeHash`, which
is derived from `challenge.id` and `challenge.realm`.
Because `challenge.id` is an HMAC over the serialized
challenge request (see {{I-D.httpauth-payment}}), every
request field, including `externalId` when present, is
already bound through `challengeHash`. The server MUST
verify the witness matches before submitting the transaction.

The witness type is defined as:

~~~solidity
struct PaymentWitness {
bytes32 challengeHash;
string externalId;
}
~~~

Expand All @@ -480,7 +481,7 @@ against a different challenge, even if the payment
parameters are identical.

The witness type string for EIP-712 is:
`"PaymentWitness witness)PaymentWitness(bytes32 challengeHash, string externalId)TokenPermissions(address token,uint256 amount)"`
`"PaymentWitness witness)PaymentWitness(bytes32 challengeHash)TokenPermissions(address token,uint256 amount)"`

This specification defines that witness schema directly for
challenge binding. Implementations MUST use the exact type
Expand Down