文本用于示例，如果线下计算USDC中的Permit签名

主要分为两步：首先，计算出permit摘要；然后，对摘要进行签名。
代码如下：

const { keccak256, defaultAbiCoder, toUtf8Bytes, solidityPack } = require("ethers/lib/utils");
const { ethers } = require("ethers");

// 注意参数
var DOMAIN_SEPARATOR = "0xd8bcfd71283db806c953550a7728f6d9c566844f11de95a94f898d43860a4321";
var PERMIT_TYPEHASH = "0x6e71edae12b1b97f4d1f60370fef10105fa2faae0126114a169c64845d6126c9";

function getPermitDigest(
    owner,
    spender,
    value,
    nonce,
    deadline
) {
    return keccak256(
        solidityPack(
            ['bytes1', 'bytes1', 'bytes32', 'bytes32'],
            [
                '0x19',
                '0x01',
                DOMAIN_SEPARATOR,
                keccak256(
                    defaultAbiCoder.encode(
                        ['bytes32', 'address', 'address', 'uint256', 'uint256', 'uint256'],
                        [PERMIT_TYPEHASH, owner, spender, value, nonce, deadline]
                    )
                ),
            ]
        )
    )
}


async function getPermitSign() {
    var usdc_owner_pk = "0x8d2e94f6b15ef64577e5adc58c6723ca50efcebdf9ae3373147c1c";
    var usdc_owner_addr = "0x349c666566fcD9eB2f1BCc68823b919641254d4e";

    // permit 参数
    var owner = usdc_owner_addr;
    var spender = "0xBC1F42383f9567B43C219200D83C71CF144C1146";
    var value = 90;
    var nonce = 0;
    var deadline = 1709360911; //2024-03-02 14:28:31

    // 1. 计算摘要
    var digest = getPermitDigest(owner, spender, value, nonce, deadline);
    console.log("digest:", digest);

    // 2. 签名
    var signer = new ethers.utils.SigningKey(usdc_owner_pk)
    var signature = signer.signDigest(digest);
    console.log("signature: ", signature);

}

if (require.main === module) {
    getPermitSign();
}

输出结果如下：

digest: 0x67a9ee0d3bbe100aaba3e82a7e496f030cf754f5640ed0003c0d57613425b6bd
signature:  {
  r: '0x8edd1b531b8f639f37eee808b866bc534035524e17baccfb9b85a8f683412baf',
  s: '0x743066a136fa8fb64cdfa249f821269a634b0f2e01ef38536b8fee39742c0157',
  _vs: '0x743066a136fa8fb64cdfa249f821269a634b0f2e01ef38536b8fee39742c0157',
  recoveryParam: 0,
  v: 27,
  yParityAndS: '0x743066a136fa8fb64cdfa249f821269a634b0f2e01ef38536b8fee39742c0157',
  compact: '0x8edd1b531b8f639f37eee808b866bc534035524e17baccfb9b85a8f683412baf743066a136fa8fb64cdfa249f821269a634b0f2e01ef38536b8fee39742c0157'
}