  Securing smart contracts remains a fundamental challenge. At its core, it is
about building software that is secure in composition with untrusted code, a
challenge that extends far beyond blockchains. We introduce SCIF, a language
for building smart contracts that are compositionally secure. SCIF is based on
the fundamentally compositional principle of secure information flow, but
extends this core mechanism to include protection against reentrancy attacks,
confused deputy attacks, and improper error handling, even in the presence of
malicious contracts that do not follow SCIF's rules. SCIF supports a rich
ecosystem of interacting principals with partial trust through its mechanisms
for dynamic trust management. SCIF has been implemented as a compiler to
Solidity. We describe the SCIF language, including its static checking rules
and runtime. Finally, we implement several applications with intricate security
reasoning, showing how SCIF supports building complex smart contracts securely
and gives programmer accurate diagnostics about potential security bugs.
