def load(self, buf = None):
        """This method opens an existing database.

        self.password/self.keyfile and self.filepath must be set.
        
        """

        if self.password is None and self.keyfile is None:
            raise KPError('Need a password or keyfile')
        elif self.filepath is None and buf is None:
            raise KPError('Can only load an existing database!')
        
        if buf is None:
            buf = self.read_buf()
        
        # The header is 124 bytes long, the rest is content
        header = buf[:124]
        crypted_content = buf[124:]
        del buf
        
        # The header holds two signatures
        if not (struct.unpack('<I', header[:4])[0] == 0x9AA2D903 and
                struct.unpack('<I', header[4:8])[0] == 0xB54BFB65):
            del crypted_content
            del header
            raise KPError('Wrong signatures!')

        # Unpack the header
        self._enc_flag = struct.unpack('<I', header[8:12])[0]
        self._version = struct.unpack('<I', header[12:16])[0]
        self._final_randomseed = struct.unpack('<16s', header[16:32])[0]
        self._enc_iv = struct.unpack('<16s', header[32:48])[0]
        self._num_groups = struct.unpack('<I', header[48:52])[0]
        self._num_entries = struct.unpack('<I', header[52:56])[0]
        self._contents_hash = struct.unpack('<32s', header[56:88])[0]
        self._transf_randomseed = struct.unpack('<32s', header[88:120])[0]
        self._key_transf_rounds = struct.unpack('<I', header[120:124])[0]
        del header

        # Check if the database is supported
        if self._version & 0xFFFFFF00 != 0x00030002 & 0xFFFFFF00:
            del crypted_content
            raise KPError('Unsupported file version!')
        #Actually, only AES is supported.
        elif not self._enc_flag & 2:
            del crypted_content
            raise KPError('Unsupported file encryption!')
        
        if self.password is None:
            masterkey = self._get_filekey()
        elif self.password is not None and self.keyfile is not None:
            passwordkey = self._get_passwordkey()
            filekey = self._get_filekey()
            sha = SHA256.new()
            sha.update(passwordkey+filekey)
            masterkey = sha.digest()
        else:
            masterkey = self._get_passwordkey()

        # Create the key that is needed to...
        final_key = self._transform_key(masterkey)
        # ...decrypt the content
        decrypted_content = self._cbc_decrypt(final_key, crypted_content)

        # Check if decryption failed
        if ((len(decrypted_content) > 2147483446) or
            (len(decrypted_content) == 0 and self._num_groups > 0)):
            del decrypted_content
            del crypted_content
            raise KPError("Decryption failed!\nThe key is wrong or the file is"
                          " damaged.")

        sha_obj = SHA256.new()
        sha_obj.update(decrypted_content)
        if not self._contents_hash == sha_obj.digest():
            del masterkey
            del final_key
            raise KPError("Hash test failed.\nThe key is wrong or the file is "
                          "damaged.")
        del masterkey
        del final_key

        # Read out the groups
        pos = 0
        levels = []
        cur_group = 0
        group = v1Group()

        while cur_group < self._num_groups:
            # Every group is made up of single fields
            field_type = struct.unpack('<H', decrypted_content[:2])[0]
            decrypted_content = decrypted_content[2:]
            pos += 2

            # Check if offset is alright
            if pos >= len(crypted_content)+124:
                del decrypted_content
                del crypted_content
                raise KPError('Unexpected error: Offset is out of range.[G1]')
            
            field_size = struct.unpack('<I', decrypted_content[:4])[0]
            decrypted_content = decrypted_content[4:]
            pos += 4
            
            if pos >= len(crypted_content)+124:
                del decrypted_content
                del crypted_content
                raise KPError('Unexpected error: Offset is out of range.[G2]')

            # Finally read out the content
            b_ret = self._read_group_field(group, levels, field_type,
                                           field_size, decrypted_content)

            # If the end of a group is reached append it to the groups array
            if field_type == 0xFFFF and b_ret == True:
                group.db = self
                self.groups.append(group)
                group = v1Group()
                cur_group += 1
            
            decrypted_content = decrypted_content[field_size:]

            if pos >= len(crypted_content)+124:
                del decrypted_content
                del crypted_content
                raise KPError('Unexpected error: Offset is out of range.[G1]')

        # Now the same with the entries
        cur_entry = 0
        entry = v1Entry()
        
        while cur_entry < self._num_entries:
            field_type = struct.unpack('<H', decrypted_content[:2])[0]
            decrypted_content = decrypted_content[2:]
            pos += 2
             
            if pos >= len(crypted_content)+124:
                del decrypted_content
                del crypted_content
                raise KPError('Unexpected error: Offset is out of range.[G1]')
            
            field_size = struct.unpack('<I', decrypted_content[:4])[0]
            decrypted_content = decrypted_content[4:]
            pos += 4
            
            if pos >= len(crypted_content)+124:
                del decrypted_content
                del crypted_content
                raise KPError('Unexpected error: Offset is out of range.[G2]')
            
            b_ret = self._read_entry_field(entry, field_type, field_size,
                                      decrypted_content)
            
            if field_type == 0xFFFF and b_ret == True:
                self.entries.append(entry)
                if entry.group_id is None:
                    del decrypted_content
                    del crypted_content
                    raise KPError("Found entry without group!")

                entry = v1Entry()
                cur_entry += 1
            
            decrypted_content = decrypted_content[field_size:]
            pos += field_size
            
            if pos >= len(crypted_content)+124:
                del decrypted_content
                del crypted_content
                raise KPError('Unexpected error: Offset is out of range.[G1]')

        if self._create_group_tree(levels) is False:
            del decrypted_content
            del crypted_content
            return False

        del decrypted_content
        del crypted_content

        if self.filepath is not None:
            with open(self.filepath+'.lock', 'w') as handler:
                handler.write('')
        return True
This method opens an existing database.

        self.password/self.keyfile and self.filepath must be set.