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In a Monero transaction, according to explanation videos:

The non-signers are past transaction outputs pulled from the blockchain, which act as decoys. These outputs together make the inputs of a transaction.

To make a transaction (like Bitcoin), you need to sign all inputs, to prove that you can spend them. Not a big deal. All input addresses have a public and private key, and you must prove that you have the private key of all inputs to spend them (you are the owner of all the coins being sent). So, you sign the transaction with all the input private keys.

But, in Monero, my understanding is that inputs are made from other past outputs. These outputs are unsigned (have not been spent), so, you do not own the private key of those outputs. And if they are spent, the signature would not be valid because the transaction is using other users outputs.

So, my question is:

Where are the signatures of a ring coming from? You don't own the private keys of the outputs pulled from the blockchain, and therefore, you can't sign the new transaction.

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A ring-signature is a group of output keys signed using one of the outputs private keys. An observer can verify that one of the outputs was the real signer, but importantly not which one. A key image is also bundled with the transaction. This is used to ensure an output is not spent more than once.

A little more detail on key images can be found in this answer.

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  • So, who is the input and who is the output? Are the outputs signed individually?
    – user9634
    Jan 20, 2019 at 23:53
  • outputs get used as inputs to a tx. And no, outputs are not individually signed, they are combined into a ring-signature.
    – jtgrassie ♦
    Jan 21, 2019 at 0:37

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