Register a signer (session) key for an account. The account authorises the signer and the signer proves consent, so this requires two EIP-712 signatures.

Prerequisites:

1. GET /v1/auth/eip712-domain — EIP-712 domain (name, version, chainId, verifyingContract = Authorization contract). Both signatures use this same domain.
2. GET /v1/nonce-state/{account} — nonce_anchor and current_bitmap_index. Send them back as nonce_anchor / nonce_bitmap_index and sign the exact same values.

1. Account signs RegisterSigner:

RegisterSigner(address account,address signer,string message,uint32 expiration,uint48 nonceAnchor,uint8 nonceBitmap)

Struct hash: keccak256(abi.encode(TYPEHASH, account, signer, keccak256(bytes(message)), expiration, nonceAnchor, nonceBitmap))

2. Signer signs VerifySigner:

VerifySigner(address account,uint48 nonceAnchor,uint8 nonceBitmap)

Struct hash: keccak256(abi.encode(TYPEHASH, account, nonceAnchor, nonceBitmap))

Signature encoding: account_signature and signer_signature are 0x-prefixed hex strings. This differs from permit.signature on the trading endpoints, which is a protobuf bytes field and is therefore base64 in JSON.

Idempotent: if the signer is already active, the call is a no-op and returns success: true with an empty transaction_hash.

Failure modes: a wrong struct or field order fails signature recovery. A nonceAnchor other than the current anchor or the current anchor + 1 reverts with InvalidNonceAnchor, which looks like a nonce bug but is usually a mismatch between the signed and submitted values.