An apparatus for executing a function associated with a handwritten user input on a lock screen, the apparatus comprising: a touch screen configured to display a lock screen; and a controller configured to: control the touch screen to display the lock screen, the lock screen comprising a first layer (600, 900), receive an unlock command comprising a handwritten signature via the touch screen, verify if the unlock command is the same as a predetermined unlock command by comparing the handwritten signature to a handwritten signature registered in advance, control the touch screen to display a second layer (610, 920) of the lock screen that is laid over the first layer (600, 900) and to receive a further handwritten input on the second layer (610, 920) of the lock screen if the unlock command is the same as the predetermined unlock command, if the further handwritten input is received via the touch screen within a predetermined period of time after determining that the unlock command is the same as the predetermined unlock command, search for a command corresponding to the further handwritten input, unlock the lock screen, and execute the command corresponding to the further handwritten input, and if the further handwritten input is not received via the touch screen within the predetermined period of time after determining that the unlock command is the same as the predetermined unlock command, unlock the lock screen. a touch screen configured to display a lock screen; and a controller configured to: control the touch screen to display the lock screen, the lock screen comprising a first layer (600, 900), receive an unlock command comprising a handwritten signature via the touch screen, verify if the unlock command is the same as a predetermined unlock command by comparing the handwritten signature to a handwritten signature registered in advance, control the touch screen to display a second layer (610, 920) of the lock screen that is laid over the first layer (600, 900) and to receive a further handwritten input on the second layer (610, 920) of the lock screen if the unlock command is the same as the predetermined unlock command, if the further handwritten input is received via the touch screen within a predetermined period of time after determining that the unlock command is the same as the predetermined unlock command, search for a command corresponding to the further handwritten input, unlock the lock screen, and execute the command corresponding to the further handwritten input, and if the further handwritten input is not received via the touch screen within the predetermined period of time after determining that the unlock command is the same as the predetermined unlock command, unlock the lock screen. control the touch screen to display the lock screen, the lock screen comprising a first layer (600, 900), receive an unlock command comprising a handwritten signature via the touch screen, verify if the unlock command is the same as a predetermined unlock command by comparing the handwritten signature to a handwritten signature registered in advance, control the touch screen to display a second layer (610, 920) of the lock screen that is laid over the first layer (600, 900) and to receive a further handwritten input on the second layer (610, 920) of the lock screen if the unlock command is the same as the predetermined unlock command, if the further handwritten input is received via the touch screen within a predetermined period of time after determining that the unlock command is the same as the predetermined unlock command, search for a command corresponding to the further handwritten input, unlock the lock screen, and execute the command corresponding to the further handwritten input, and if the further handwritten input is not received via the touch screen within the predetermined period of time after determining that the unlock command is the same as the predetermined unlock command, unlock the lock screen.