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Is the Lamport-Diffie signature secure in the standard model? I ask this question because I am reading the Postquantum-Cryptography book and the reductions the authors use one-way functions and not hash functions. According to my understanding, ROM is used to replace hash function for random oracles, but in Lamport-Diffie only pre-image resistance functions are sufficient.

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Is the Lamport-Diffie signature secure in the standard model?

Technically, no, the Lamport(-Diffie) one-time signature scheme is not EUF-CMA secure, however, also technically "it depends".

Nominally, a signature scheme is typically called "secure" - especially in the public-key world - if for a polynomial number of adverserial interactions with it in the relevant model the advantage of said adversary is still negligible - with the negligibility coming from some security assumption typically.

However, you can easily construct an adversary which performs 2 interactions with the signature scheme in the EUF-CMA model and achieves an advantage of 1 from that. Such an adversary would e.g. query $0^n$ and $1^n$ as messages, revealing all private keys.

Also however, we can still construct a security statement, namely $$\mathbf{Adv}^{\textsf{EUF-CMA}}_{\text{LD-OTS}_F}(\mathcal A;q,n)\leq\begin{cases}\mathbf{Adv}^{\textsf{OWF}}_{F}(\mathcal A';n)&q\leq 1\\1&\text{else}\\\end{cases}$$ where $n$ is the security parameter, $q$ is the number of queries, $\mathcal A'$ is an existentially qualified adversary against the underlying OWF $F$ and $\mathcal A$ is the polynomially-bounded universally qualified adversary. The order of quantifiers is for all $\mathcal A$ exists $\mathcal A'$.

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