//
//  RSAHelper.swift
//  PSCB-OOS-iOS
//
//  Created by Antonov Ilia on 26.10.2020.
//

import Foundation

enum RSAKeyError: Error {
    
    /// Thrown when provided public key is invalid
    case invalidKey
    
    /// Thrown when encryption of message failed
    case encryption(OSStatus)
    
    /// Thrown when unable to initialize `SecKey` with given key string
    case unmanaged(Unmanaged<CFError>)
}

public final class RSAHelper {
    
    internal static let key = "MFwwDQYJKoZIhvcNAQEBBQADSwAwSAJBAPQsWyHaynwoc2tuMbengf1SFase9tPnwtPh4o1tR+94xsWztADdhhUaUBk/68ipaoZE8uSnM9UgdEPmOotFXyUCAwEAAQ=="
    
    internal static var secKey: SecKey? = nil

    static func initPSCBOnlinePublicKey() throws {
        // Treat as assert
        guard let keyData = Data(base64Encoded: key) else {
            print("Invalid key string: \(key)")
            throw RSAKeyError.invalidKey
        }
        
        var attributes: CFDictionary {
            return [
                kSecAttrKeyType:  kSecAttrKeyTypeRSA,
                kSecAttrKeyClass: kSecAttrKeyClassPublic,
                kSecAttrKeySizeInBits: 2048,
                kSecReturnPersistentRef: true
            ] as CFDictionary
        }
        
        var error: Unmanaged<CFError>? = nil
        guard let secKey = SecKeyCreateWithData(keyData as CFData, attributes, &error) else {
            print("Error creating a security key with \(String(describing: keyData))")
            print(error.debugDescription)
            throw RSAKeyError.unmanaged(error!)
        }
        
        Self.secKey = secKey
    }
    
    /// Encrypts given message with OOS public key.
    ///
    /// - Parameters:
    ///     - message: A message to encrypt.
    ///
    /// - Returns: A base64 encoded string
    ///
    /// - Throws: `RSAKeyError` type error
    public static func encryptEncodeBase64(message: String) throws -> String {
        // Initialize OOS public key
        if Self.secKey == nil {
            try Self.initPSCBOnlinePublicKey()
        }
        
        // Initialize buffers
        let sKey = Self.secKey!
        let buff = [UInt8](message.utf8)
        
        // Mutable encryption refs
        var size = SecKeyGetBlockSize(sKey)
        var kBuf = [UInt8](repeating: 0, count: size)
        
        let status = SecKeyEncrypt(sKey, SecPadding.PKCS1, buff, buff.count, &kBuf, &size)
        
        // Sanity check
        guard status == errSecSuccess else {
            throw RSAKeyError.encryption(status)
        }
        
        return Data(bytes: kBuf, count: size).base64EncodedString()
    }
    
}