OTS

One-time signatures, abbreviated OTS, are digital signature schemes designed to be secure for a single signing operation with a given key pair. The concept was introduced by Leslie Lamport in 1979 and has since become a foundational component of hash-based cryptography. In a Lamport-style OTS, the signer generates a set of secret values in pairs and publishes the hashes of those values as public data. To sign a message, the signer computes a hash of the message, writes it in binary, and reveals, for each bit, the corresponding preimage from the appropriate secret pair. Verification rehashes the revealed preimages and checks them against the published public hashes. Because only one set of secret preimages is disclosed, the same key material should not be used to sign more than one message.

Variants of OTS include the Winternitz scheme, which reduces signature size by trading off signing time and public-key size through parameter choices. Security relies on the chosen hash function’s resistance to preimage attacks and on the one-time usage of the signing key; reusing keys can enable forgery.

In practice, OTS signatures are relatively large and require careful key management, which limits their direct use for frequent signing. However, they play a crucial role as building blocks in hash-based, post-quantum signature schemes. Hash-based schemes such as XMSS (eXtended Merkle Signature Scheme) and XMSSMT extend OTS into multi-use, tree-structured systems that remain secure against quantum attacks while enabling practical signing of multiple messages over time.

Other meanings of the acronym OTS exist in different domains, but this article focuses on one-time signatures in cryptography.