back channel

a secret, unofficial, or informal channel of communication as used in politics or diplomacy: sensitive information passed on through a back channel.

OTHER WORDS FROM back channel

back-channel, adjective

But Cantor has since quietly held a series of back-channel conversations with the vice president.

Why the Debt Deal Faltered |Howard Kurtz |July 8, 2011 |DAILY BEAST
Esfandiari vehemently denies involvement in any type of espionage, back-channel organizations, or the CIA.

My Iranian Prison |Sandra McElwaine |September 25, 2009 |DAILY BEAST
Reagan occasionally used her trips to the Soviet Union for back-channel diplomacy.

The Lady Who Warmed Up the Cold War |James Mann |March 10, 2009 |DAILY BEAST
The back-channel filled up with URLs and phone numbers and email addresses.

Makers |Cory Doctorow

The far shore was a full mile away, while between the island and the near shore lay a back-channel perhaps a hundred yards across.

The God of His Fathers |Jack London