Solidity入门级别｜用智能合约实现房屋贷款系统

Home Loans Based on Smart Contract for Banks

代码目的：

创造一个自运行的房屋贷款系统，减轻银行许多传统金融合约操作流程的负担

执行摘要：

贷款申请人在线登录或注册银行账户；填写房产信息、个人信息并上传数字文件
智能合约验证身份，并根据确定的阈值评估资格
满足门槛后，申请人可以在线签署协议和条款
智能合约将计算贷款支付、转移贷款资金并向申请人收取贷款还款
最后，当贷款完成或发生贷款违约时，智能合约会自行销毁

1) Customer Registration/Login With Basic Information

2) Loan Application

2.1 Property Information (Location, Price, Etc.)

2.2 Loan Application (Amount, Maximum Down Payment, Etc.)

2.3 Application Qualification (Income, Working Years, Etc.)

2.3 Identity Verification of Applicant

2.4 Pre-defined Condition Fulfilment & Result (Set Limit to Parameters)

2.5 Sign Contract With Digital Signature & Verify Signature(验证过程此处未实现)

3) Loan Operation

3.1 Loan Payment Calculation and Distribution

3.2 Loan Repayment

3.3 Price Conversion

4) Destruction of Smart Contract

4.1 Loan Repayment Completion4.2 Overdue Repayment & Loan Default

1) Customer Registration/Login With Basic Information

pragma solidity >=0.7.0 <0.9.0; //指定编译器版本

创建合约一，让用户注册并输入个人信息

contract Customer_Detail {
enum gender {male, female}
enum Residence_Status {Permanant_residence_permit,
European_residence_permit,
Limited_resicence_permit,
No_German_residence_permit,
Others
}
struct customer_info{
string first_name;
string last_name;
int age;
address customer_address;
string residence_id;
string email;
int contact_number;
gender genders;
Residence_Status Residence_Status;
bool married;
}

（理论上应该先检查用户地址是否已经存在（mapping语句似乎没有成功，楼主已放弃），没有则继续注册过程，输入个人信息）

    mapping (address=>uint) registered_info;function getRegisteredInfo(address customer_address) public
view returns (string memory,string memory,int,
address,string memory,string
memory,int,gender,Residence_Status,bool) {return(registeredinfo[customer_address].first_name,registeredinfo[customer_address].last_name,registeredinfo[customer_address].age,registeredinfo[customer_address].customer_address,registeredinfo[customer_address].residence_id,registeredinfo[customer_address].email,registeredinfo[customer_address].contact_number,registeredinfo[customer_address].genders,registeredinfo[customer_address].Residence_Status,registeredinfo[customer_address].married,
);
}
// input new customer's basic informationfunction getNewCustomer(string first_name, string last_name,
int age, address customer_address, string residence_id,string email, int contact_number, gender
genders,ResidenceStatus Residence_Status,bool married) {
customerinfo[customer_address] = customerinfo(
{
first_name: first_name,
last_name: last_name,
age: age,
residence_id:residence_id,
email:email,
contact_number:contact_number,
genders:genders,
Residence_Status:Residence_Status,
married:married});
}

2) Loan Application

2.1 Property Information (Location, Price, Etc.)

2.2 Loan Application (Amount, Maximum Down Payment, Etc.)

Contract Loan_Application {
enum employment_status{
Self_employed,
Freelance,
Civil_servant,
Independent_wealthy_person,
Retired,
Others
}
enum property_source{
developers,
resale
}
string applicantion_id;
address applicant;
// To prevent data overflow
library SafeMath {
function add(uint8 a, uint8 b) internal pure returns (uint8){
uint8 c=a+b;
}
assert (c>=a);
return c;
}
using SafeMath for uint8struct property_info {uint8 zipcodes;string city;else {
string property_source;uint size;}
uint price;struct loan_info {uint loan_amount;uint max_down_payment;uint8 loan_years;}if (loan_amount>1.4*price) {}
return "exceed maximum limits";return loan_amount;}if (loan_years > 60) {return "exceed maximum limits";}else {}
return loan_years;}

2.3 Application Qualification (Income, Working Years, Etc.)

struct applicant_qualification {uint debt;uint working_years;uint8 applicant_amount; //employment_status employment_status;uint total_monthly_net_income;uint assets;bool married;}contract applicantqualifiction{applicant_qualification qualification;constructor (uint debt, uint working_years, uint8 applicant_amount, uint total_monthly_net_income, uint assets, bool married) {qualification.debt = debt;qualification.working_years = working_years;qualification.applicant_amount = applicant_amount;qualification.total_monthly_net_income = total_monthly_net_income;qualification.assets = assets;qualification.married = married;}
}

2.3 Identity Verification of Applicant

这个步骤会上传文件并审核真实性(这里是需要联合其他官方数据库来认证的，此处代码未体现)

contract Verification {string file_info;string file_hash;string name;string idandfile_hash;function set_uploaded_file_info() public view returns(string memory)  {return file_info;}function get_uploaded_file_info (uint certificateid) public pure returns (uint) {return certificateid;
}function verifyHash () public view returns(string memory) {if(uint(keccak256(abi.encodePacked(file_hash))) == uint(keccak256(abi.encodePacked(file_hash)))) {return "successful verification";}else {return "fail verification";}}
}

2.4 Pre-defined Condition Fulfilment & Result (Set Limit to Parameters)

contract Examine_Qualification {    uint price;uint loan_amount;uint loan_years;uint applicant_amount;uint debt;uint assets;uint total_monthly_net_income;uint working_years;function set_limit() public view returns (string memory qualification) {uint loan_limit = price; // An applicant can request house price as maximum loan amount. This standard could be various from banks to banksuint256 total_monthly_net_income_min = (120 + 125) * loan_amount * applicant_amount/12/100; // at most 80% income are used to pay loan,this standard could be various from banks to banksuint256 debt_max = 4 * loan_amount/100; // at most 40% loan amount of existing debt is acceptable, this standard could be various from banks to banksuint asset_min = 8  * debt/100; // the applicant should own at least 80% assets(including fixed assets, current assets, etc.) of existing debt. This aims to lower the possibility of default.This standard could be various from banks to banks.uint working_years_min = 2; // the applicant is expected to have worked for at least two years. This aims to guarantee the applicant has a stable job. This standard could be various from banks to banks.if (loan_amount>loan_limit || debt>debt_max ||total_monthly_net_income < total_monthly_net_income_min ||assets < asset_min || working_years < working_years_min) {return qualification="notEligible";}else {return qualification="Eligible";}   }// Only when all the conditions are satisfied will the applicant be eligible. Otherwise it would terminate the process.}

2.5 Sign Contract With Digital Signature & Verify Signature(验证过程此处未实现)

contract homeLoanAgrement {//This Annexure and the terms and conditions hereunder shall come into force on the signing of the same by the Borrower and Bankfunction terms() public view returns (string memory borrowerNameChloe,string memory totalLoanAmountonelac,string memory date12December,uint loanDurationMonths30)
{}
}

3) Loan Operation

在此，在计算前首先设置了以下指标：

a) 首付 = 10,00,000；

b) 贷款期限 = 10 年或 120 个月，

c) 利率 6%

当运行这个计算时，我们可以看到每月的 EMI 以及贷款人必须支付的年息

3.1 Loan Payment Calculation and Distribution

contract MortgageCalculator {uint256 PrincipalAmount = 1000000;//600 basis points = 6.0 pctuint256 monthlyInterestRate = 6;uint loanTenure = 10;uint adding;function yearlyInterest() public view returns (uint abc) {abc = PrincipalAmount * monthlyInterestRate * loanTenure;return abc/100;
}function emi() public view returns (uint def) {//from the above results (return abc/100) will add the PrincipalAmount and will divide by the duration 120(10Years*12) to het the Monthly EMI.def = 600000     + PrincipalAmount;return def/120;
}}

3.2 Loan Repayment

contract LoanDisbursement{// Bank will transferring the Ethers(after conversion) to the Smart Contractfunction sendviaCall(address payable _to) public payable {bool sent = _to.send(msg.value);require (sent, "Failed to sent Ether");}
}// Customer transferring monthly EMI to the Smart Contracts
contract LoanRepayment {uint256 num1;address owner;function payEmi () external payable{}function getBalance() external view returns (uint256) {return address(this).balance;}
}// Customer transferring monthly EMI to the Smart Contracts
contract LoanReceipts {address sender;string receiptNumber;function confirmation(string memory paymentReceiptNumber) public pure returns (string memory) {return paymentReceiptNumber;}
}

3.3 Price Conversion

对于货币转换，代码使用了 chainlink 文档中的以下代码https://docs.chain.link/docs/get-the-latest-price

4) Destruction of Smart Contract

智能合约房屋贷款过程中的另一个关键要素是智能合约自毁。这里考虑两种情况：

4.1：贷款人成功偿还了全部贷款金额。之后，智能合约将立即销毁，客户将收到无异议证书。

4.2：如果贷款人未能归还贷款，系统将对两次逾期还款发出两次警告，如果贷款人仍不还款，将被宣布为违约人，智能合约将被终止，财产将被查封和拍卖。

4.1 Loan Repayment Completion
4.2 Overdue Repayment & Loan Default

contract smartContractDefault_Destruct{import random;address applicant;string private_key;uint total_default = 0;struct default_info {bool overdue_repayment;uint total_default;bool loan_default;}struct capital_private_key{}
string wallet_private_key;
string assets_private_key;function get_default_info(uint id) public pure returns(bool) {return default_info[id].overdue_repayment;
for (id=1; id<=960;id++){
if (default_info[id].overdue_repayment=true) {
total_default++;
continue;
}
if (total_default>=5) {
break;
return loan_default=true;else {return total_default=false;}}

在违约情况下，智能合约通过更改贷款人财产的私人密钥来夺取控制权：

function modify_private_key public pure returns (string) {
if (loan_default=true) {
bits = random.getrandbits(256)
bits_hex = hex(bits)
wallet_private_key = bits_hex[2:];
}
bits = random.getrandbits(256)
bits_hex = hex(bits)
assets_private_key = bits_hex[2:];
}contract smartContractSelf_Destruct{address owner_addr;function destroySmartContract(address payable _to) public{require(msg.sender == owner_addr);selfdestruct(_to);}
}