[SystemSafety] Modelling and coding guidelines: "Unambiguous Graphical Representation"

[paul_e.bennett at topmail.co.uk]

On 27/02/2016 at 9:11 AM, "Steve Tockey" <Steve.Tockey at construx.com> wrote:
>
>One only needs to look at how difficult it is for a computer to 
>generate a
>"truly random number" to understand how difficult it is to make a 
>computer
>behave truly non-deterministically. There's a difference between 
>"truly
>random/non-deterministic" and "so computationally complex that 
>mimicking
>the behavior is next to impossible". Authentication, encryption, 
>etc
>depend on computational complexity, not true randomness. Again, 
>someone
>with a sufficiently powerful computer can break the encryption /
>authentication. Deterministically.
>
>The game is for the computational complexity of a nonce to be out 
>of reach
>of the unauthorized entity. As computers become more and more 
>powerful
>(Moore's Law), it calls for constantly increasing computational
>complexity. 
>
>In a computer, nothing is truly random.

In developing truly dependable computing elements, one is always trying 
to construct a system from a collective of very dependable components.
The simplest possible components that have been thoroughly tested
would stand a much better chance of producing that totally dependable 
system, especially if any individual component failings can be fully 
mitigated.

Software does not run without hardware to support it and even the best
and most robust hardware is prone to random failings. The failings of 
software will tend to be more systematic and a product of failings during
the development process.

As for modelling. I use a variety of ways to model and describe the ways
a system is expected to behave (which will include the wetware as well).
Some of it is in diagram form, some in pseudo-code, a lot of it in plain
English description as a formalised procedure. Whatever you use,ensure
all the threads describing expectation match the real intent.

Regards

Paul E. Bennett IEng MIET
Systems Engineer

-- 
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Paul E. Bennett IEng MIET.....<email://Paul_E.Bennett@topmail.co.uk>
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