NITWITS NITPICKING Ard Bahlman

Assumptions  “A customer will be required to insert an ATM card and enter a personal identification number (PIN) which will be sent to the bank for validation as part of each transaction.”  “Card slot reads card data and can signal if card is unreadable”. Yet no sign of it in messages spec.  “Detects if card is stuck or unidentified objects are present in the reader.” The system cannot know if a card is half- inserted, because event spec states this: card inserted (card data)—fired upon complete insertion only

Assumptions  “Confiscated Card Stack signals critical high level”  No, the software checks for the remaining space. (Messages spec)  “Various I/O devices (keyboard, touch screen, etc.) displays UI”  Keyboard?  “Intrusion sensor / alarm detects and signals physical stress to the machine like shakes.”  Maybe to vague. Air pressure, small earthquake etc should be excluded. Intrusion through one of the sides (other than those resulting from valid specified actions like card insertion) should be noticed. This is feasible.

Assumptions  “ALL hardware may signal physical errors if devices malfunction e.g.  Notes or coins stuck in transport canals  Sensor failure”  what about hardware observing hardware?  outgoing withdrawal vault: “Sensors for detecting when the levels at each stack reaches critical lows”  No, there is no such a message. We have instead: check remaining supply response: stock levels (list of (banknote, amount)) – ‘banknote’ indicates the currency and value of the note, ‘amount’ indicates how many of those there are left.  It is not made explicit that withdrawing only involves notes and not coins.

Requirements  Requirements doc doesn’t state what to do with a printed receipt.  “Upon an authentication request the central service responds with either” (…) “Confiscate card, show message  send card to Confiscated Card Stack”  Does message comes from central service?  “ATM machine will authenticate itself to the central service with some unique id.”  No message for it.  The word encryption is not mentioned, only that there are public and private keys. Make encryption requirement explicit.  Put a restriction on the number of trials

Requirements  “All log entries are stored locally and sent to Central Service.”  Memory space in atm is limited, you need e.g. log rotation  The specification assumes that vaults cannot be shared, but doesn’t state so.  Same holds for communication with central service, they should be synchronous.  After maintenance has been performed the ‘status’ should be updated.  Engineer enters new values?  Does jammed paper count as stock?

Requirements  “Before starting timeout a jammed card will be pulled back by the card slot and sent to the Confiscated Card Stack.”  Well, that’s a problem

Messages  maintenance needed (device, problem) – used when paper runs out etc.  Possible problems should be known to the programmer.  Recurrent variable is card data, but we actually mean card id.