Yet Enigma was a huge challenge. It looked like a typewriter, but with no space for paper. It had lights for each letter and, inside it, the three rotors (and later more than three) could be arranged in a range of different ways, each one linked to a different set of electrical connections. The key code, with the rotor setting, would be in a three letter key for each day, which all the machine operators would look up in the code book. It was believed to be impregnable. The three rotors could mean more than 17,000 different solutions for a given message, but – since the three rotors could be rearranged in any of six different ways – the number of combinations reached over 105,000.