What kind of hashing algorithm used to generate 12 character length alphanumeric?

What kind of hashing algorithm used to generate 12 character length alphanumeric?

for example stackoverflow use 2 keys to store cookies

t=IhweorwSw6K7
s=............  (#intention开发者_开发技巧ally replace with . because its a session cookies)

How does that hashing algorithm looks like?

"Each is a base64-encoded 128-bit (presumably random) number, with the trailing == removed."

From: https://meta.stackexchange.com/questions/12047/what-are-the-s-and-t-parameters-in-the-user-cookie-used-by-stackoverflow

Maybe.