They were advancing the plan secretly.
The decision was made covertly.
内々に means 'privately' or 'internally', often within a group, whereas 暗々裏 implies a more covert or tacit action.
Standard kanji spelling using 暗々裏.
Variant with 裡 instead of 裏; both are used.
Non-repetition kanji variant; less common but valid.
Non-repetition kanji variant with 裡.
Compound of 暗々 (reduplication of 暗 'dark') and 裏 'back, behind', literally 'in the dark behind'. The exact historical derivation is uncertain, but the meaning is transparent from its components.