Handling Real Numbers

A DP algorithm (a single Measurement) may be constructed using the Tumult Core library by composing arbitrarily many transformations and measurements; and calling the privacy_function() or stability_function() requires calling the corresponding privacy/stability functions on all the constituent components, each of which may involve multiple arbitrary arithmetic operations (+,-,*,/,radicals, powers) on user-supplied numbers. Any approximation/precision errors on these user-supplied numbers may be arbitrarily accentuated during these arithmetic operations & comparisons – causing significant differences in expected outputs and actual outputs of the privacy/stability functions.

One approach to control the extent of these errors is to carefully control the precision of these numbers during these operations using arbitrary precision arithmetic. While this approach might control the amount of imprecision, the computation is inevitably inexact – so, there can be significant differences in the analysis performed by hand and what the core library outputs.

The approach adopted in the Tumult Core library is to represent any privacy-sensitive number and perform any privacy-sensitive arithmetic operation symbolically. Instead of trying to control errors caused by finite precision arithmetic, we avoid this class of errors altogether. Users supply values that represent real numbers exactly and can be manipulated with the usual set of arithmetic operators.

In order to support symbolic representation and computation, we internally use SymPy - a Python library for symbolic mathematics, but allow users to provide inputs as ExactNumberInput’s. These are values which can be unambiguously interpreted as specific real numbers or +/- infinity. We also provide a class ExactNumber for parsing these values which also supports arithmetic operators. All privacy-sensitive numeric inputs use these exact representations, and all downstream operations on these inputs are performed symbolically.

See tmlt.core.utils.exact_number for more information.