Any file sync system worth its salt is able to skip transfers of data the
receiving side has already. This is known as source (or client-side) deduplication. Many
storage systems (both cloud-based and local ones) perform data deduplication,
which allows to store a single copy of each piece of data even
when it’s duplicated across files or users. (They are usually implemented
together, but are actually independent in a strict sense.)
Source and storage deduplication speed up operations and decrease transfer and
storage costs. But they also open the door to a number of side channel attacks
that threaten privacy, allow service abuse, and expose both users and
service operators to legal risk.
Popular public cloud services are particularly exposed, because anybody can
register and use (plus, as shown below, attack) them. The Sync Appliance, on
the other hand, runs on your own computers, and access is thus restricted to
allowed users (both regular and guests, with more limited capabilities). It
incorporates nonetheless protections against the attacks detailed below.