zoom_app/zoom_app/ZoomMeetingSDKJWT.swift

import CryptoKit
import Foundation

enum ZoomMeetingSDKJWTError: Error {
    case invalidUTF8
    case jsonSerializationFailed
}

/// Builds a [Meeting SDK JWT](https://developers.zoom.us/docs/meeting-sdk/auth/) from the OAuth client id/secret
/// (same credentials as the in-app Zoom OAuth app when Meeting SDK is enabled on that app).
enum ZoomMeetingSDKJWT {

    static func makeMeetingSDKToken(clientId: String, clientSecret: String) throws -> String {
        let header: [String: String] = ["alg": "HS256", "typ": "JWT"]
        let iat = Int(Date().timeIntervalSince1970) - 30
        let exp = iat + 60 * 60 * 2
        let payload: [String: Any] = [
            "appKey": clientId,
            "iat": iat,
            "exp": exp,
            "tokenExp": exp
        ]
        guard let headerData = try? JSONSerialization.data(withJSONObject: header, options: [.sortedKeys]),
              let payloadData = try? JSONSerialization.data(withJSONObject: payload, options: [.sortedKeys]),
              let headerString = String(data: headerData, encoding: .utf8),
              let payloadString = String(data: payloadData, encoding: .utf8) else {
            throw ZoomMeetingSDKJWTError.jsonSerializationFailed
        }

        let headerPart = base64URLEncode(headerString.data(using: .utf8) ?? Data())
        let payloadPart = base64URLEncode(payloadString.data(using: .utf8) ?? Data())
        let signingInput = "\(headerPart).\(payloadPart)"
        guard let signingData = signingInput.data(using: .utf8) else {
            throw ZoomMeetingSDKJWTError.invalidUTF8
        }
        let key = SymmetricKey(data: Data(clientSecret.utf8))
        let mac = HMAC<SHA256>.authenticationCode(for: signingData, using: key)
        let signaturePart = base64URLEncode(Data(mac))
        return "\(signingInput).\(signaturePart)"
    }

    private static func base64URLEncode(_ data: Data) -> String {
        data.base64EncodedString()
            .replacingOccurrences(of: "+", with: "-")
            .replacingOccurrences(of: "/", with: "_")
            .replacingOccurrences(of: "=", with: "")
    }
}