secret communication

See: confidence
References in classic literature ?
There was a secret communication between the cardinal's apartments and those of the queen; and through this corridor* Mazarin passed whenever he wished to visit Anne of Austria.
Sam had scarcely recovered his usual composure of countenance, which had been greatly disturbed by the secret communication of his respected relative, when Mr.
That I shall not discuss, and perhaps cannot account for; but certainly they are a proof of the converse of spirits, and a secret communication between those embodied and those unembodied, and such a proof as can never be withstood; of which I shall have occasion to give some remarkable instances in the remainder of my solitary residence in this dismal place.
But then it was covered by the occult influence of her who was referred to in confidential talks, secret communications, and discreet whispers of Royalist salons as: "Madame de Lastaola."
"Yes, I know what you are going to tell me, monsieur; I know that there is an order of the king's to prevent all secret communications with the defenders of Belle-Isle, and that is exactly why I do not offer to communicate except in presence of my staff."
[36] Shangping Zhong, Xueqi Cheng, and Tierui Chen, "Data Hiding in a Kind of PDF Texts for Secret Communication," International Journal of Network Security, vol.4, no.1, pp.17-26, 2007.
As for Lee's controversial trip to the United States in 1995, which led to the missile crisis, Su said China had been in the know beforehand through the secret communication channel.
He prayed the court to seek an affidavit from the former president denying the secret communication by the Pakistani ambassador that bypassed the Foreign Office as well as the Prime Minister's Secretariat.
Also, Kushner's relationship with former National Security Adviser Michael Flynn - who was fired for misleading Trump officials about his calls with Russian Ambassador Sergey Kislyak - and his reported discussion with the ambassador about setting up a secret communication channel are being examined by the investigators.
I found the texting, a secret communication with people I couldn't see, quite terrifying.
The courses include: Dealing with explosives, topography, navigation, light-weapons, rope-climbing, parachuting, swimming and diving training, military discipline, intelligence, Interpol, secret communication, changing one's appearance, various means of assassination, Shiite Islam and Iranian revolutionary ideology.