One-Time Signatures (OTS) represent the first line of defense against the future threat of quantum computing. By using hash functions—which degrade much more "gracefully" under quantum attacks—schemes like Lamport and Winternitz provide a secure way to sign data. While an OTS key is disposable, ingenious structures like Merkle Trees allow us to bundle these keys into permanent, multi-use identities. Understanding these mechanics is essential for the transition to a quantum-resistant blockchain ecosystem.