using System.Security.Cryptography;
using System.Text;
using System.Runtime.InteropServices;

namespace CredentialManager.Services;

/// <summary>
/// Interfaccia per il servizio di crittografia
/// </summary>
public interface IEncryptionService
{
    string Encrypt(string plainText);
    string Decrypt(string encryptedText);
    bool CanDecrypt(string encryptedText);
    string MigrateEncryptedText(string oldEncryptedText, string newPlainText);
}

/// <summary>
/// Servizio per la crittografia delle password cross-platform con supporto per migrazione
/// </summary>
public class EncryptionService : IEncryptionService
{
    private readonly byte[] _key;
    private readonly byte[] _iv;
    private const string ENTROPY_STRING = "CredentialManager2025";
    private const string AES_PREFIX = "AES:";
    private const string PROTECTED_PREFIX = "PROTECTED:";

    public EncryptionService()
    {
        // Chiave e IV derivati da una stringa fissa
        var keySource = "CredentialManager2025KeyForEncryption!";
        var ivSource = "CredMgr2025IV!";
        
        using var sha256 = SHA256.Create();
        _key = sha256.ComputeHash(Encoding.UTF8.GetBytes(keySource));
        _iv = new byte[16];
        Array.Copy(Encoding.UTF8.GetBytes(ivSource), _iv, Math.Min(16, ivSource.Length));
    }

    public string Encrypt(string plainText)
    {
        if (string.IsNullOrEmpty(plainText))
            return string.Empty;

        try
        {
            // Sempre usa AES per nuove crittografie per garantire portabilità
            return AES_PREFIX + EncryptWithAes(plainText);
        }
        catch (Exception ex)
        {
            throw new InvalidOperationException("Errore durante la crittografia", ex);
        }
    }

    public string Decrypt(string encryptedText)
    {
        if (string.IsNullOrEmpty(encryptedText))
            return string.Empty;

        try
        {
            // Determina il metodo di crittografia utilizzato
            if (encryptedText.StartsWith(AES_PREFIX))
            {
                // Rimuovi il prefisso e decrittografa con AES
                var aesEncryptedText = encryptedText.Substring(AES_PREFIX.Length);
                return DecryptWithAes(aesEncryptedText);
            }
            else if (encryptedText.StartsWith(PROTECTED_PREFIX))
            {
                // Rimuovi il prefisso e decrittografa con ProtectedData
                var protectedEncryptedText = encryptedText.Substring(PROTECTED_PREFIX.Length);
                return DecryptWithProtectedData(protectedEncryptedText);
            }
            else
            {
                // Formato legacy - prova prima ProtectedData (se su Windows), poi AES
                return DecryptLegacyFormat(encryptedText);
            }
        }
        catch (Exception ex)
        {
            throw new InvalidOperationException("Errore durante la decrittografia", ex);
        }
    }

    public bool CanDecrypt(string encryptedText)
    {
        if (string.IsNullOrEmpty(encryptedText))
            return true;

        try
        {
            Decrypt(encryptedText);
            return true;
        }
        catch
        {
            return false;
        }
    }

    public string MigrateEncryptedText(string oldEncryptedText, string newPlainText)
    {
        // Cripta il nuovo testo in chiaro con il metodo attuale (AES)
        return Encrypt(newPlainText);
    }

    private string DecryptLegacyFormat(string encryptedText)
    {
        // Su Windows, prova prima ProtectedData
        if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
        {
            try
            {
                return DecryptWithProtectedData(encryptedText);
            }
            catch (CryptographicException ex) when (ex.Message.Contains("Chiave non utilizzabile") || 
                                                   ex.Message.Contains("Key not valid for use in specified state") ||
                                                   ex.Message.Contains("The data is invalid"))
            {
                // ProtectedData non riesce (probabilmente diversa macchina/utente)
                // Prova con AES come fallback
                try
                {
                    return DecryptWithAes(encryptedText);
                }
                catch
                {
                    throw new InvalidOperationException(
                        "Impossibile decrittografare le credenziali. " +
                        "Le credenziali potrebbero essere state create su una macchina/utente diverso. " +
                        "È necessario re-inserire le credenziali.", ex);
                }
            }
        }
        else
        {
            // Su altre piattaforme, usa AES
            return DecryptWithAes(encryptedText);
        }
    }    private string EncryptWithProtectedData(string plainText)
    {
        byte[] plainTextBytes = Encoding.UTF8.GetBytes(plainText);
        byte[] entropy = Encoding.UTF8.GetBytes(ENTROPY_STRING);
        
        if (OperatingSystem.IsWindows())
        {
            byte[] encryptedBytes = ProtectedData.Protect(plainTextBytes, entropy, DataProtectionScope.CurrentUser);
            return Convert.ToBase64String(encryptedBytes);
        }
        
        // Fallback ad AES se non su Windows
        return EncryptWithAes(plainText);
    }

    private string DecryptWithProtectedData(string encryptedText)
    {
        if (OperatingSystem.IsWindows())
        {
            byte[] encryptedBytes = Convert.FromBase64String(encryptedText);
            byte[] entropy = Encoding.UTF8.GetBytes(ENTROPY_STRING);
            byte[] decryptedBytes = ProtectedData.Unprotect(encryptedBytes, entropy, DataProtectionScope.CurrentUser);
            return Encoding.UTF8.GetString(decryptedBytes);
        }
        
        // Fallback ad AES se non su Windows
        return DecryptWithAes(encryptedText);
    }

    private string EncryptWithAes(string plainText)
    {
        using var aes = Aes.Create();
        aes.Key = _key;
        aes.IV = _iv;

        using var encryptor = aes.CreateEncryptor();
        using var msEncrypt = new MemoryStream();
        using var csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write);
        using var swEncrypt = new StreamWriter(csEncrypt);
        
        swEncrypt.Write(plainText);
        swEncrypt.Close();
        
        return Convert.ToBase64String(msEncrypt.ToArray());
    }

    private string DecryptWithAes(string encryptedText)
    {
        byte[] cipherBytes = Convert.FromBase64String(encryptedText);
        
        using var aes = Aes.Create();
        aes.Key = _key;
        aes.IV = _iv;

        using var decryptor = aes.CreateDecryptor();
        using var msDecrypt = new MemoryStream(cipherBytes);
        using var csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read);
        using var srDecrypt = new StreamReader(csDecrypt);
        
        return srDecrypt.ReadToEnd();
    }
}