Swift port of the Legacy Alipay Order, which is usually only available in Objective-C. I rely on the RSA implementation of SwiftyRSA here, so that I don't have to work with their own (obj-c) implementations.

LegacyAlipayOrder.swift

import Foundation
import SwiftyRSA
/// Swift port of the always included _Order_ class from Alipay, because we suffered from the terrible code.
/// The code comments went through google translate to give us some hints on what is happening under the hood.
/// This struct is used to hold the data for an order within the alipay framework and to get the url parameter
/// containing the order object including a RSA signature.
private struct LegacyAlipayOrder {
    /********************************** Pay four elements **********************************/

    /// When the merchant signs an agreement with Alipay, Alipay assigns a unique
    /// identification number to the merchant (16-digit pure number starting with 2088).
    private var partner: String?
    /// The Alipay unique user number corresponding to the Alipay account (16-bit pure
    /// number starting with 2088), the order payment amount will be entered into the
    /// account, and a partner can correspond to multiple seller_ids.
    private var sellerID: String?
    /// The unique order number for the merchant's website item.
    private var outTradeNO: String?
    /// The total amount of funds for this order, in RMB (Yuan).
    /// The value range is [0.01, 100000000.00], which is accurate to two decimal places.
    private var totalFee: String?

    /********************************** Commodity related **********************************/

    /// Product title/transaction title/order title/order keyword, etc.
    private var subject: String?
    /// A specific description of a transaction. If it is a variety of products,
    /// please add the product description string to the body.
    private var body: String?

    /****************************** Other mandatory parameters *****************************/

    /// The interface name is fixed to mobile.securitypay.pay.
    private var service: String?
    /// The encoding format used by the merchant website is fixed to utf-8.
    private var inputCharset: String?
    /// The Alipay server proactively informs the http path of the page specified in the merchant's website.
    private var notifyURL: String?

    private var returnURL: String?

    private var currency: String?

    private var forexbiz: String?

    /********************************* Optional parameters *********************************/

    /// Payment type, 1: Purchase of goods. (the default value is not passed)
    private var paymentType: String?
    /// Specifically distinguish the type of goods in the local transaction, 1: physical
    /// transaction; (default value in the case of no transmission), 0: virtual transaction;
    /// (restriction of rules such as credit card is not allowed).
    private var goodsType: String?
    /// Whether to initiate real-name verification when paying,
    /// F: Do not initiate real-name verification; (default value in case of no transmission),
    /// T: Initiate real-name verification; (Business business requires buyer real-name authentication)
    private var rnCheck: String?
    /// Identifies the client.
    private var appID: String?
    /// Identifies the client source. The content of the parameter value is as follows:
    /// appenv=“system=client platform name^version=business system version”
    private var appenv: String?
    /// Set the timeout for unpaid transactions. Once the timeout expires,
    /// the transaction will be automatically closed. When the user enters the payment password
    /// and clicks to confirm the payment (that is, after creating the Alipay transaction),
    /// the timing starts. Value range: 1m to 15d, or use absolute time (example format: 2014-06-13 16:00:00).
    /// M-minute, h-hour, d-day, 1c-day (1c-day case, no matter when the transaction is created, it is closed at 0).
    /// The value of this parameter does not accept decimal points, such as 1.5h, which can be converted to 90m.
    private var itBPay: String?
    /// Product address
    private var showURL: String?
    /// Business extension parameters, Alipay specific business needs to add this field, json format.
    /// When the merchant accesses, it is determined by negotiation with Alipay.
    private var outContext: Dictionary<String, Any>?

    private var description: String {
        var description: String = ""
        /** Pay elements **/
        if let partner = partner {
            description = description.appendingFormat("partner=\"%@\"", partner)
        }
        if let sellerID = sellerID {
            description = description.appendingFormat("&seller_id=\"%@\"", sellerID)
        }
        if let outTradeNO = outTradeNO {
            description = description.appendingFormat("&out_trade_no=\"%@\"", outTradeNO)
        }
        if let totalFee = totalFee {
            description = description.appendingFormat("&total_fee=\"%@\"", totalFee)
        }
        /** Commodity related **/
        if let subject = subject {
            description = description.appendingFormat("&subject=\"%@\"", subject)
        }
        if let body = body {
            description = description.appendingFormat("&body=\"%@\"", body)
        }

        /** Other mandatory parameters **/
        if let service = service {
            description = description.appendingFormat("&service=\"%@\"", service)
        }
        if let inputCharset = inputCharset {
            description = description.appendingFormat("&_input_charset=\"%@\"", inputCharset)
        }
        if let notifyURL = notifyURL {
            description = description.appendingFormat("&notify_url=\"%@\"", notifyURL)
        }
        if let returnURL = returnURL {
            description = description.appendingFormat("&return_url=\"%@\"", returnURL)
        }
        if let currency = currency {
            description = description.appendingFormat("&currency=\"%@\"", currency)
        }
        if let forexbiz = forexbiz {
            description = description.appendingFormat("&forex_biz=\"%@\"", forexbiz)
        }

        /** Optional parameters **/
        if let paymentType = paymentType {
            description = description.appendingFormat("&payment_type=\"%@\"", paymentType)
        }
        if let itBPay = itBPay {
            description = description.appendingFormat("&it_b_pay=\"%@\"", itBPay)
        }
        if let showURL = showURL {
            description = description.appendingFormat("&show_url=\"%@\"", showURL)
        }
        if let appID = appID {
            description = description.appendingFormat("&app_id=\"%@\"", appID)
        }
        if let outContext = outContext {
            for (key, value) in outContext.enumerated() {
                description = description.appendingFormat("&%@=\"%@\"", key, value as? CVarArg ?? "")
            }
        }

        return description
    }

    func description(with key: PrivateKey) throws -> String? {
        guard let signed = try signedString(with: key) else { return nil }
        return description + "&sign=\"" + signed + "\"&sign_type=\"RSA\""
    }

    private func signedString(with key: PrivateKey) throws -> String? {
        /// Alipay allows all characters, but "!*'();:@&=+$,/?%#[]".
        let characterset = CharacterSet(charactersIn: "!*'();:@&=+$,/?%#[]").inverted
        /// Contains the UTF8 encoded Message object with the order's description.
        let message = try ClearMessage(string: description, using: .utf8)
        /// The message's signature using SHA1.
        let signature = try message.signed(with: key, digestType: Signature.DigestType.sha1)
        /// Percent encoded signature as base64 string.
        let wrappedSignedString = signature.base64String.addingPercentEncoding(withAllowedCharacters: characterset)
        // Let's be sure everything worked out as expected.
        guard let signedString = wrappedSignedString, !signedString.isEmpty else { return nil }

        return signedString
    }
}