You could read David Leigh’s book, in which he published the full decryption key: www.amazon.com/…/161039061X

That’s literally how he leaked it.

The wikipedia article on it has the whole “he said - she said”:
en.wikipedia.org/…/WikiLeaks:_Inside_Julian_Assan…

Including the lie that @ChairmanMeow@programming.dev is parroting about Assange not caring about people dying.