confidential
The police could not be informed of anything that clients discussed confidentially.
When the player came to him confidentially, he told her he would do anything to help her.
The information was obtained confidentially.

SMART Vocabulary: related words and phrases

Secrecy and privacy
anonymity
anonymization
anonymize
anonymous
anonymously
data protection
dead men tell no talesidiom
dissimulate
dissimulation
down-low
on the quietidiom
privacy
privacy settings
private
privately
under wrapsidiom
undisclosed
unrevealed
unsourced
unstated