It's not only the UK that considers cryptography to be a munition. It is also classified as munitions in the US:

> Encryption items specifically designed, developed, configured, adapted or modified for military applications (including command, control and intelligence applications) are controlled by the Department of State on the United States Munitions List.

It was part of the whole crypto wars, and the lawsuit brought by Bernstein vs the United States.

See more:

- https://en.wikipedia.org/wiki/Bernstein_v._United_States - https://en.wikipedia.org/wiki/Crypto_Wars - https://en.wikipedia.org/wiki/Export_of_cryptography_from_th...