Yes, they're authentic. Some fragments were published in English translation as early as 1948 in the UK, and other portions were published in the West during the next few decades. These were based on originals that were discovered in Berlin in 1945, but they weren't complete, because some of the originals had been burned. The almost-fully-complete versions we have now weren't published until much later, because they're based on the microfilmed copies of the diaries, which were captured by the Soviets and held in the Soviet archives, where they were rediscovered in 1992 by Elke Fröhlich. The provenance and authenticity is discussed in the introduction to the German version of the diaries, which were edited by a number of German historians, but primarily by Fröhlich.