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FROST

Note. Originally hosted on GitLab at XAIN Foundation.

Experimental implementation of the FROST access control policy language.

Getting Started

To build the project from source, GHC (Glasgow Haskell Compiler) is required. The recommended way to install it is via Stack.

The command

$ stack build

compiles the project, including the frostc executable.

$ frostc --help

frostc - a compiler for FROST

Usage: frostc ((-i|--ifile INFILE) | (-x|--iexpr EXPRESSION)) [-p|--pretty]
              [-c|--oconsole] [-o|--ofile OUTFILE]
  Synthesize a circuit-pair from a policy EXPRESSION or INFILE

Available options:
  -i,--ifile INFILE        Input file
  -x,--iexpr EXPRESSION    FROST expression
  -p,--pretty              Whether to pretty-print output
  -c,--oconsole            Whether to only output to console
  -o,--ofile OUTFILE       Output file (default: "out.json")
  -h,--help                Show this help text

Compiling Policies

The tools in the project allow two modes of operation for building FROST policies: using frostc, or as an EDSL (embedded domain-specific language) library. The former is easier to start with.

Take this simple example of a policy:

grant if subject == user
      && age > 18
      && action == "drink"

Here subject, user, age and action are named entities that may be resolved to values later (e.g. subject, object and action typically feature in access requests). 18 is an integer literal and "drink" a string literal.

Assume these are the contents of a file policy.fro. Compile this with frostc using the option --ifile / -i:

$ frostc -i policy.fro

This produces a file out.json in an intermediate format, essentially a pair of Boolean circuits equivalent to the policy.

  • To specify a different output file name use the --ofile / -o option.
  • To pretty-print the JSON output, use the --pretty / -p flag.
  • To just output to console rather than to file, use the --oconsole / -c flag.
  • Policy expressions can also be passed directly on the command line (rather than by file) using the --iexpr / -x option