Cryptographic method that allows combining a genuine signature with an additional signature (called an "adaptor signature") to reveal a secret piece of data. This method works so that knowing two elements among the valid signature, the adaptor signature, and the secret allows deducing the missing third element. One interesting property of this method is that if we know our peer's adaptor signature and the specific point on the elliptic curve related to the secret used to compute that adaptor signature, we can then derive our own adaptor signature that corresponds to the same secret, all without ever having direct access to the secret itself. In an exchange between two parties who do not trust each other, this technique allows for the simultaneous disclosure of two sensitive pieces of information between the participants. This process removes the need for trust in instant transactions such as a Coinswap or an Atomic Swap. Let's look at an example to better understand. Alice and Bob each want to send 1 BTC to the other, but they do not trust each other. They will therefore use adaptor signatures to eliminate the need to trust one another during the exchange (thus making it an "atomic" exchange). They proceed as follows: Alice initiates this atomic exchange. She creates a transaction that sends 1 BTC to Bob. She creates a signature to validates this transaction using her private key (), a nonce , and a secret ( and ):

Alice calculates the adaptor signature using the secret and her real signature :

This verification gives Bob some assurance from Alice, allowing him to safely proceed with the atomic swap process. He then creates his own transaction sending 1 BTC to Alice and calculates his own adaptor signature , which is linked to the same secret (a value known only by Alice at this point, Bob only knows its corresponding point , which Alice provided):

Alice then broadcasts the signed transaction to the Bitcoin blockchain in order to claim the BTC Bob promised. Once this transaction is published, Bob can observe it on the blockchain. He is thus able to extract the signature . From this information, Bob can isolate the famous secret he needed:

This secret was the only missing information Bob needed to compute a valid signature , from Alice's adaptor signature . With it, he can now validate transaction which sends 1 BTC from Alice to Bob. He then calculates and broadcasts the transaction :