Currently signmessage/verifymessage only supports messages signed by a
single key. We should extend that to messages signed by n-of-m keys, or
from the users point of view, P2SH multisig addresses.

rpc.cpp:signmessage() returns the output of SignCompact(). That in turn
starts with a header byte marking the signs of the various keys to allow
for key recovery. The header byte can be one of 0x1B, 0x1C, 0x1D or 0x1E


For multisig signmessage signatures this is extended:

    <01> <varint n> <varint m> <sig or key> {<sig or key>, ...}

Each signature or key can be one of the following forms:

    sig: <1B/1C/1D/1E> <32 byte r> <32 byte s>
    compress key: <02/03> <32 byte x>
    uncompressed key: <04> <32 byte x> <32 byte y>

Note that we have to provide all pubkeys, even if they aren't used for a
given signature, to allow the P2SH address to be reconstructed.

Decoding/encoding is a bit code-intensive due to the all the cases, but
on the other hand this format keeps the size down to an absolute
minimum. Alternatively I could use length bytes.

The format is backwards compatible in the sense that older versions will
fail safely on new signatures, even ones that have been truncated.
Similarly new signatures are easily distinguished from old, and going
forward if we for some reason need yet another signature format the
leading byte can be incremented.

Signing incomplete signatures on messages can be handled by converting
pubkeys to signatures. Similarly the RPC signmessage command can be
extended with an optional "existing signature" option.

-- 
'peter'[:-1]@petertodd.org