Cryptography Reference
In-Depth Information
Herman Munster</a>
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21.3.2 Rice's Theorem
One of the more interesting theorems from computer science theory
suggests that it may be theoretically impossible for anyone to exam-
ine a program and determine whether it is packing extra information
in a data file. This theorem is worth mentioning even if it may not
have much practical use.
A casual version of the theorem, due to Henry Gordon Rice, states
that a software program can't be counted on to detect whether an-
other program is conforming to some standard for a file format.
The theorem itself says that the problem is
undecidable
, a term that
means that the program is guaranteed to halt and give a definitive
answer. If it doesn't halt, it could go on checking, rechecking or look-
ing for some complex answer.
This theorem may help establish a theoretical limit to checking
for secret messages in file formats, but it may not be of practical value
because the theoretical result is based on asking a computer program
to examine itself, a sort of logical tongue-twister that can have odd
