Adaptor Signature

An adaptor signature is a cryptographic tool used to securely exchange secrets during blockchain transactions, especially for protocols like atomic swaps and coin swaps where two parties trade assets simultaneously without trusting each other. It acts like a “conditional” signature: when combined with the right secret, it unlocks a complete, valid signature.

Here’s how it works in practice: There are three components involved—a secret value, the adaptor signature (a partial signature), and the complete (or “normal”) signature. If you have any two, you can figure out the third. One notable property is that both parties can create adaptor signatures tied to the same secret, without needing to share the secret directly.

Suppose Alice and Bob want to trade 1 BTC each. Alice first generates an adaptor signature on her transaction to send 1 BTC to Bob. This signature is incomplete—it can’t spend the coins yet—but it commits to a hidden secret.

Bob then prepares his own transaction sending 1 BTC to Alice. By using Alice’s adaptor signature, Bob is able to create his own adaptor signature tied to the same secret, even without actually knowing that secret. He sends both his transaction and his adaptor signature to Alice.

With Bob’s adaptor signature and her own secret, Alice can now finalize Bob’s signature, allowing her to claim Bob’s 1 BTC. When Bob sees his signed transaction on the blockchain, he can retrieve the secret value. Using this, he can complete Alice’s signature and claim his 1 BTC. This exchange means both parties get their coins at the same time, and neither can cheat by backing out midway. Adaptor signatures are an important building block for decentralized trust in cross-chain transactions.