Post-Quantum Signatures, Part 1: Understanding One-Time Signature

This report provides a concise technical introduction to one-time signature (OTS) schemes—Lamport OTS and Winternitz OTS (including the modern W-OTS+ variant)—explaining their key generation, signing, verification, security properties, and the checksum and hash-chain mechanisms. It also describes how OTS keys are extended into practical, multi-use hash-based signature schemes via Merkle trees and higher-level constructions (MSS, XMSS, XMSS-MT, and SPHINCS+), framing OTS as a foundational primitive for post-quantum digital signatures.