As far as I know current Bitcoin protocol doesn't let you to include any arbitrary data with the transactions (as it would become non-standard and clients would not relay it). So if you have multiple addresses you can't sign them with a single private key and include that signature in the transaction so other party can verify it against your public key. This could become very handy though - a reputable wallet service could issue transactions that require zero confirmations from the other party, because with the added signature they know that the transaction is from this reputable service and they trust that this service won't try to double spend. I'm thinking of something like Mt.Gox's "green address", but baked into protocol (Mt.Gox does this by sending your funds to some known by the others Bitcoin address and then relaying them to the final destination).
Do you think it's possible/feasible to add a feature like this to the current protocol without forking the chain? This could be as simple as adding support for following scripts:
<data block> OP_DROP OP_DUP OP_HASH160 <pubKeyHash> OP_EQUALVERIFY OP_CHECK
<data block> OP_DROP OP_HASH160 <pubKeyHash> OP_EQUAL
The <data block> should not be longer than 34 bytes (or more, depending if we want to have some room for future use cases). This is all that needs to be changed in the Bitcoin client. Now for actually using the feature a further definition of <data block> is required:
22 AC 20 <32 byte signature>
22 is data block length and "AC 20" is just a sub-opcode that can be either defined by the protocol (in this case I'm reusing OP_CHECKSIG's opcode but that's not required since this is all part of data block) or just agreed upon between people that want to use this feature.
It's possible that the above could be achieved in some simpler way using other opcodes or mechanisms present in Bitcoin protocol that I'm not aware of. Either way, I'd like to hear your opinions whether a feature like this should be considered and added.