  Thanks to the widespread deployment of TLS, users can access private data
over channels with end-to-end confidentiality and integrity. What they cannot
do, however, is prove to third parties the {\em provenance} of such data, i.e.,
that it genuinely came from a particular website. Existing approaches either
introduce undesirable trust assumptions or require server-side modifications.
  As a result, the value of users' private data is locked up in its point of
origin. Users cannot export their data with preserved integrity to other
applications without help and permission from the current data holder.
  We propose DECO (short for \underline{dec}entralized \underline{o}racle) to
address the above problems. DECO allows users to prove that a piece of data
accessed via TLS came from a particular website and optionally prove statements
about such data in zero-knowledge, keeping the data itself secret. DECO is the
first such system that works without trusted hardware or server-side
modifications.
  DECO can liberate data from centralized web-service silos, making it
accessible to a rich spectrum of applications. To demonstrate the power of
DECO, we implement three applications that are hard to achieve without it: a
private financial instrument using smart contracts, converting legacy
credentials to anonymous credentials, and verifiable claims against price
discrimination.
