It's because it actually has enough entropy to be completely improbable guessing.

Some users (of course a tiny minority) generate random passwords. I think the point holds, I just think the presentation isn't completely accurate.

> They can be guessed – Regardless of complexity requirements, users will find and use the simplest pattern allowed, and attackers routinely take advantage.

It's right in the article that they are talking about average user behavior of choosing guessable password, even if you tell them not to.

Private keys should have over 128 bits of entropy, which is enough to make it un-brute-forceable with at least the amount of energy likely to be obtained by any human organization on earth. http://en.wikipedia.org/wiki/Brute-force_attack#Theoretical_... That's what makes it un-guessable. Also, some users do choose passwords with at least as much entropy and are also unguessable. But most don't.

Security FYI:

This situation concerns RSA keys, not 128-bit symmetric keys; our problem is "factoring a really big number", not "guessing a really big number". A 128-bit RSA key can be broken in seconds.

RSA keys are commonly 2048 bits at the moment. NIST's rough rule of thumb is that a 2048-bit RSA key is equivalent in secureness to a 112-bit symmetric key; for 128-bit equivalence, you need around a 3072-bit RSA key. (2048 provides fine security for the next few decades at least.)

Edit:

You referred to "128 bits of entropy", which could be interpreted as "equivalent to a 3072-bit RSA key". I'm not accusing you of recommending insecure RSA key sizes, I just wanted to make a public service announcement. I've seen people make that mistake before.

I use automated high-entropy password-generators and a surprising amount of websites cap lengths to 30-50 characters (or even 8 chars sometimes like a Canadian bank I won't name). Allowing super-long passwords would be a good starting alternative to SSL.

The problem with this is that some sites still only support max 20 characters.

Social engineering techniques can be very effective when trying to guess a user's password (birth date, pet name, etc.). Also think about not-so-random password like "password", "qwerty", "12345" etc..