In a letter to Italy's La Stampa newspaper, he said "there is absolutely no doubt regarding the validity of my resignation," the ANSA news agency reported.
"The only condition for the validity of my resignation is the complete freedom of my decision," he added.
Benedict resigned a year ago Friday.
An article published last week in the daily Libero said Benedict was forced from office by scandals, including financial irregularities and the leaking of confidential Vatican documents to the media.
He also said he continues to wear the white cassock "purely for practical reasons" because at the time of his resignation, no other clothes were available, Vatican Radio reported Wednesday.
Benedict said he wears the cassock in a visibly different manner than Pope Francis, his successor.