When Otto Frank first published his daughter's red-checked diary and notebooks, he wrote a prologue assuring readers that the book mostly contained her words". Although many Holocaust deniers, such as Robert Faurisson, have claimed that Anne Frank's diary was fabricated, critical and forensic studies of the text and the original manuscript have supported its authenticity.
The Netherlands Institute for War Documentation commissioned a forensic study of the manuscripts after the death of Otto Frank in 1980. The material composition of the original notebooks and ink, and the handwriting found within them and the loose version were extensively examined. In 1986, the results were published: The handwriting attributed to Anne Frank was positively matched with contemporary samples of Anne Frank's handwriting, and the paper, ink, and glue found in the diaries and loose papers were consistent with materials available in Amsterdam during the period in which the diary was written.
The survey of her manuscripts compared an unabridged transcription of Anne Frank's original notebooks with the entries she expanded and clarified on loose paper in a rewritten form and the final edit as it was prepared for the English translation. The investigation revealed that all of the entries in the published version were accurate transcriptions of manuscript entries in Anne Frank's handwriting, and that they represented approximately a third of the material collected for the initial publication. The magnitude of edits to the text is comparable to other historical diaries such as those of Katherine Mansfield, Anaïs Nin and Leo Tolstoy in that the authors revised their diaries after the initial draft, and the material was posthumously edited into a publishable manuscript by their respective executors, only to be superseded in later decades by unexpurgated editions prepared by scholars.
Not that this will settle it for the antisemitic assholes.
It does show that accusations that Anne Frank did indeed write it all and there was nothing forged by her father as some have claimed.