Nonmalleable Progress Leakage

Information-flow control systems often enforce progress-insensitive noninterference, as it is simple to understand and enforce. Unfortunately, real programs need to declassify results and endorse inputs, which noninterference disallows, while preventing attackers from controlling leakage, including through progress channels, which progress-insensitivity ignores. This work combines ideas for progress-sensitive security with secure downgrading (declassification and endorsement) to identify a notion of securely downgrading progress information. We use hyperproperties to distill the separation between progress-sensitive and progress-insensitive noninterference and combine it with nonmalleable information flow, an existing (progress-insensitive) definition of secure downgrading, to define nonmalleable progress leakage (NMPL). We present the first information-flow type system to allow some progress leakage while enforcing NMPL, and we show how to infer the location of secure progress downgrades. All theorems are verified in Rocq.