decrypt_android_backup.cs

        static void Decrypt(string backup_file)
        {
            RijndaelManaged aesEncryption = new RijndaelManaged();
            using (System.IO.FileStream f = System.IO.File.OpenRead(backup_file))
            {
                if ("ANDROID BACKUP" != readString(f))
                    return;

                Console.WriteLine("Version:" + readString(f));

                bool iscompressed = "1" == readString(f);

                string crypto = readString(f);
                Stream stream;
                if ("none" != crypto)
                {
                    aesEncryption.KeySize = Int32.Parse(crypto.Replace("AES-", ""));

                    aesEncryption.Mode = CipherMode.CBC;
                    aesEncryption.Padding = PaddingMode.PKCS7;

                    string UserSaltHex;
                    Console.WriteLine(UserSaltHex = readString(f));

                    string ckSaltHex;
                    Console.WriteLine(ckSaltHex = readString(f));

                    int rounds = Int32.Parse(readString(f));

                    string userIvHex;
                    Console.WriteLine(userIvHex = readString(f));

                    string masterKeyBlobHex;
                    Console.WriteLine(masterKeyBlobHex = readString(f));


                    byte[] key = new Rfc2898DeriveBytes(getPassword(), StringToByteArray(UserSaltHex), rounds).GetBytes(32);

                    byte[] IV = StringToByteArray(userIvHex);

                    byte[] mkCipher = StringToByteArray(masterKeyBlobHex);

                    byte[] mkBlob = aesEncryption.CreateDecryptor(key, IV).TransformFinalBlock(mkCipher, 0, mkCipher.Length);

                    int offset = 0;
                    int len = mkBlob[offset++];
                    IV = mkBlob.Skip(offset).Take(len).ToArray();

                    offset += len;
                    len = mkBlob[offset++];
                    byte[] mk = mkBlob.Skip(offset).Take(len).ToArray();

                    offset += len;
                    len = mkBlob[offset++];
                    byte[] mkChecksum = mkBlob.Skip(offset).Take(len).ToArray();

                    byte[] keyChecksum = new Rfc2898DeriveBytes(ByteArrayToString(mk), StringToByteArray(ckSaltHex), rounds).GetBytes(32);
                    aesEncryption.IV = IV;

                    var crypter = aesEncryption.CreateDecryptor(mk, IV);
                    CryptoStream cryptostream = new CryptoStream(f, crypter, CryptoStreamMode.Read);
                    if (iscompressed)
                    {
                        cryptostream.ReadByte();
                        cryptostream.ReadByte();
                    }
                    stream = (iscompressed ?
                       new DeflateStream(cryptostream, CompressionMode.Decompress)
                       : (Stream)cryptostream);
                }
                else
                {
                    stream = f;
                }
                byte[] buffer = new byte[1024];
                int count;
                var outfile =  File.OpenWrite("out.tar");
                while ((count = stream.Read(buffer, 0, 1024)) > 0)
                    outfile.Write(buffer, 0, count);
                    //Console.WriteLine(BitConverter.ToString(buffer, 0, count));
            }
        }

        private static string getPassword()
        {
            StringBuilder s = new StringBuilder();
            Console.Write("Password: ");
            ConsoleKeyInfo key;
            while( (key = Console.ReadKey()).Key != ConsoleKey.Enter )
            {
                if (key.Key == ConsoleKey.Backspace)
                {
                    if (s.Length > 0)
                        s.Remove(s.Length - 1, 1);
                    continue;    
                }
                s.Append(key.KeyChar);
            }
            return s.ToString();
        }

        static string ByteArrayToString(byte[] bytes)
        {
            string result = string.Empty;
            foreach (byte b in bytes)
            {
                result += (char)b;
            }
            return result;
        }

        static string readString(FileStream f)
        {
            string line = String.Empty;
            int character;
            while ((character = f.ReadByte()) > 0)
            {
                if (character == '\n')
                    break;
                line += (char)character;
            }
            return line;
        }

        public static byte[] StringToByteArray(String hex)
        {
            int NumberChars = hex.Length;
            byte[] bytes = new byte[NumberChars / 2];
            for (int i = 0; i < NumberChars; i += 2)
                bytes[i / 2] = Convert.ToByte(hex.Substring(i, 2), 16);
            return bytes;
        }