I'm doing some research about the GSM protocol and how SIM cards work. As far as I've found, the only type of SIMs where the secret value, Ki, can be extracted are COMP128v1, since it is not really long byte long and can be easily brute-forced knowing the signed response.

I found [more details about the closed 128v2/3 protocol](http://www.hackingprojects.net/2013/04/secrets-of-sim.html), and the author states that the Ki is actually a 16 byte long number. So, is the Ki essentially a 16 digits long number?