This note describes a digital signature system based on cryptographic hash functions, following the seminal work in this area. It specifies a one-time signature scheme based on the work of Lamport, Diffie, Winternitz, and Merkle (LDWM), and specifies a general signature system using a Merkle tree. These systems provide asymmetric authentication without using large integer mathematics, and achieve a high security level. They are suitable for compact implementations, are relatively simple to implement, and naturally resist side-channel attacks. Unlike most other signature systems, hash-based signatures would still be secure even if it proves feasible for an attacker to build a quantum computer.