  We report the design, implementation, and deployment of Lepton, a
fault-tolerant system that losslessly compresses JPEG images to 77% of their
original size on average. Lepton replaces the lowest layer of baseline JPEG
compression-a Huffman code-with a parallelized arithmetic code, so that the
exact bytes of the original JPEG file can be recovered quickly. Lepton matches
the compression efficiency of the best prior work, while decoding more than
nine times faster and in a streaming manner. Lepton has been released as
open-source software and has been deployed for a year on the Dropbox
file-storage backend. As of February 2017, it had compressed more than 203 PiB
of user JPEG files, saving more than 46 PiB.
