What special characters will users be able to use for passwords?
All. No restrictions.
To include special characters not found on a standard keyboard?
As always there is a relevant XKCD:
Yes, any binary sequence is acceptable. The higher level character encoding is irrelevant to the algorithms in self-authentication.
I would expect limited to within Unicode and UTF-8, though UTF-16 input likely will just work.
Maybe the default client application password field will be limited to that, but on the core level the network really doesn’t care. You could even use a huge binary file as the password input.
The only thing I question about the maidsafe login is why limit the pin to only 4 characters? and why only digits? Everything is going to be hashed anyways, so the length and characters shouldn’t matter. Personally I don’t really care much about the digits limitation (that’s why they call it a pin), but why only 4 digits? I would prefer to use 5 or 6 like my screenlock / (weak) encryption password for my phone. Is there a reason behind this @dirvine?
I wouldn’t worry about it, the credentials never leave the local machine, that part of self-authentication is all done by client software. Other network nodes aren’t involved and the algorithms can take arbitrary input. And since this is all open source, anyone can easily create a client with full freedom concerning credentials.
If somewhere it was stated that the PIN will be 4 digits long, it was probably done to communicate the general idea. If people want longer PINs, it’s a trivial change to add that. It’s not like @dirvine is going to stamp his feet and demand everyone can only use a 4 digit PIN.