[carlosrt]

When I worked at a large bank many years ago, internal calls were verified to be bank employees. It was low tech, but when a bank employee called and asked about a customer we had them verify they were a bank employee by telling them to look up, and tell us what was on a certain page and line of an internal bank book. If their answer matched what we were looking at as well then the conversation continued. The books were changed/printed often.

[ionforce]

That's just like early days video game anti-piracy measure.

What is the third word on the second paragraph of page 42 of the Dungeon Master's manual? Etc.

[nknighthb]

Military codebooks are used in this way for authenticating over unsecured links. Letters or numbers laid out in a grid-like format, and you make the far side read off a certain cell.

http://en.wikipedia.org/wiki/DRYAD

http://en.wikipedia.org/wiki/BATCO#Other_functions

[jcfiala]

The Pragmatic Programmer website uses this system to allow you to prove that you bought a hard-copy of a book, so they can offer you a discounted ebook.

[chilldream]

In my case we had a security code on an internal system that was updated in real-time. So the protocol was:

"Hi I'm an employee calling from [X]" "OK, can I get the security code?" (caller gives security code)

Any employee in the company could also request a no-questions-asked reset at any time. I actually had cause hit the big red button once when the call went:

"Hi, this is [employee] calling from [branch]" "All right, can I get the security code?" "Oh, (mutters "security code"), it's $foo"

See, that counted as a compromise because someone in the lobby may have overheard her.

A couple other fun stories:

- Once I called a branch and got transferred to someone else. The conversation at the other end:

Him: "Did you give the code already?" Me: "...are you seriously going to believe me if I say 'Yes'?"

- Apparently there was a phishing attempt where people would call our center opening with:

"Hi this is [person] from the fraud department, before we begin can I get the security code?"

I don't know if it ever worked, but we got several memos warning us not to fall for it.