[nknight]

He's actually wrong. Starcraft introduced usernames/passwords and unique names to Battle.net in early 1998. The support was then patched into Diablo 1.05. Diablo I's Battle.net functionality did not originally include usernames/passwords at all.

It was quite a strange little architecture, initially. Your displayed name was whatever you'd named your character, with the distinguishing feature being an "account number" that could be re-generated by deleting a file in your Diablo directory (the corollary being if you didn't back the file up, your account number would change upon a reformat or migration to a new computer).

[timewasted]

Ultra minor nit-pick, but your account number was stored in a registry entry (HKEY_LOCAL_MACHINE\SOFTWARE\Battle.net\Configuration), not a file.

The account number consisted of four parts:

  Registration Version: This was always 1 for all the account numbers that I still have lying around.
  Registration Authority: I don't actually remember what this was used for.
  Client ID: The actual account number.
  Client Token: Random number used to verify the validity of the Client ID.

[ohashi]

Or you used a tool to change your account to 1537 and nobody could ever find you because hundreds, if not thousands, of people all used that same shared account.