The audio, which was released by the investigative journalism website Project Veritas, was tweeted out by NSA whistleblower Edward Snowden.
The call between Assange and Johnson indicates that the WikiLeaks boss wanted to minimise any harm that could be caused by unredacted material being disseminated.
He told Johnson that the US State Department could organise for an emissary to come to the site where he was under house arrest in London at the time and collect information about the five locations on the Web where the encrypted file in question was being kept along with an encryption key.
This is the raw, unedited audio of the over one hour long Assange phone call warning the @StateDept (again) of publication risks, and requesting them to aid in harm minimization. Primary source material. https://t.co/iUTHKBhOZ0— Edward Snowden (@Snowden) December 16, 2020
He warned Johnson that the encrypted material — which he said was in a mass of files — could be released by the rogue employee within hours or a week.
Assange said he had tried to take legal action in Germany — where the rogue employee was — but had no standing to do so as WikiLeaks did not own the cables.
He urged Johnson to act through the US Government, suggesting that an approach to the rogue employee could have beneficial effects.
Johnson argued that releasing material either way — through the rogue employee or WikiLeaks —- was all the same, but Assange did not agree.
He argued that while WikiLeaks put out material because of its ideological orientation and only after sanitising it on the advice of its media partners, the rogue employee had only one motive, to damage the reputation of WikiLeaks.
Assange, one of Australia's best known hackers, was involved in attacks on several American government organisations in the 1980s, at a time when he went by the moniker Mendax.
He is currently incarcerated in Belmarsh prison in the UK, awaiting a verdict from a hearing held earlier this year to decide whether the UK will extradite him to the US to face espionage charges there over publishing classified information that was leaked to WikiLeaks by an American soldier, then known as Bradley Manning, but now, after gender reassignment surgery, known as Chelsea Manning.
That verdict will be announced in January 2021. The new US president Joe Biden is due to be sworn in on the 20th of that month.