We 've been discussing something in another thread and since I am not, by any means, a bitcoin contributor, I 'd like to ask the more technically savvy users about it.

What would it need to add Ring Signatures to Bitcoin?

Here is my approach, but of course, I guess it will be invalid.

Adding Ring Signatures to Bitcoin would require changes both in the Protocol and Consensus layers.

1. New opcodes will be needed to support ring signatures. I guess this needs a soft fork.
2. Any input will need to reference multiple UTXOs, because in ring signatures only one of the UTXOs is truly spent. I guess this needs a change in the consensus layer.
3. Changes in wallets to support this new update.

I don't have any other thoughts. What makes me sceptical though is:

a) what about the efficiency in terms of fees and speed?
b) could it be vulnerable to double-spending?
c) what about the transactions that have already been added to the blockchain?