Camper-Monitor/VanControl/Bluetooth/AESCounterMode.swift

import CommonCrypto
import Foundation

/// AES-128 im Counter-Modus. CryptoKit bietet CTR nicht an, deshalb CommonCrypto.
enum AESCounterMode {
    /// - Parameters:
    ///   - data: Der verschlüsselte Nutzteil des Advertisements.
    ///   - key: 16 Byte Geräteschlüssel aus VictronConnect.
    ///   - nonce: Der 16-Byte-Zählerblock (Victron: Nonce little-endian in den
    ///            ersten beiden Bytes, Rest 0).
    static func crypt(_ data: [UInt8], key: [UInt8], nonce: [UInt8]) -> [UInt8]? {
        guard key.count == kCCKeySizeAES128, nonce.count == kCCBlockSizeAES128 else { return nil }

        var cryptor: CCCryptorRef?
        let createStatus = key.withUnsafeBytes { keyBuffer in
            nonce.withUnsafeBytes { ivBuffer in
                CCCryptorCreateWithMode(
                    CCOperation(kCCEncrypt),          // CTR ist symmetrisch
                    CCMode(kCCModeCTR),
                    CCAlgorithm(kCCAlgorithmAES),
                    CCPadding(ccNoPadding),
                    ivBuffer.baseAddress,
                    keyBuffer.baseAddress, key.count,
                    nil, 0, 0,
                    CCModeOptions(kCCModeOptionCTR_BE),
                    &cryptor
                )
            }
        }
        guard createStatus == kCCSuccess, let cryptor else { return nil }
        defer { CCCryptorRelease(cryptor) }

        var output = [UInt8](repeating: 0, count: data.count)
        var moved = 0
        let updateStatus = data.withUnsafeBytes { input in
            output.withUnsafeMutableBytes { out in
                CCCryptorUpdate(cryptor,
                                input.baseAddress, data.count,
                                out.baseAddress, data.count,
                                &moved)
            }
        }
        guard updateStatus == kCCSuccess else { return nil }
        return Array(output.prefix(moved))
    }
}