Are multi-sig transactions of N-M available or possible in BCN ?
Short answer: possible -- yes, available -- not yet.
I guess the real question is "Are multisigs and ringsigs compatible?"
Technically, yes. Independent features. Multisig: "who can sign" and ringsig: "how to sign". But some specific problems may arise.
Suppose we have three 1-of-2 outputs: (A,B), (C,D), (E,F), the same amount. (A) can spend (A,B) with ringsig (A,C,E), hiding which output was used. The problem: (B) can double-spend (A,B) with (B,D,F).
Another case: only one output 1-of-5: (A,B,C,D,E). (A) can spend it with ringsig (A,B,C,D,E), hiding the actual signer, but not the fact of spending.
The bottom line: "naive" multisig will work perfectly if we don't care about anonymity. Combining it with untraceable ringsig demands some additional rules.
