The ransom message on Baltimore’s computer system said the RobbinHood virus had encrypted files. The note demanded payment of 3 Bitcoins (worth about $17,600) per system, or 13 Bitcoins (worth about $76,280) to free all the city’s systems. It said Tuesday that the ransom must be paid within four days, or the price would go up, and that after 10 days, the city would not be able to get its data back.