  We study the impact of finite-size effects on the security of thermal one-way
quantum cryptography. Our approach considers coherent/squeezed states at the
preparation stage, on the top of which the sender adds trusted thermal noise.
We compute the key rate incorporating finite-size effects, and we obtain the
security threshold at different frequencies. As expected finite-size effects
deteriorate the performance of thermal quantum cryptography. Our analysis is
useful to quantify the impact of this degradation on relevant parameters like
tolerable attenuation, transmission frequencies at which one can achieve
security.
