[Cryptech Tech] counter data

[Bernd Paysan]

Am Donnerstag, 21. August 2014, 22:06:49 schrieb Fredrik Thulin:
> On Thursday, August 21, 2014 09:52:55 PM Bernd Paysan wrote:
> > Am Donnerstag, 21. August 2014, 17:51:28 schrieb Fredrik Thulin:
> > > Here is the private e-mail with raw counter data from my entropy source.
> > 
> > Thanks.
> > 
> > > Sorry for the 14MB e-mail.
> > 
> > Don't worry, that's the e-mail server I maintain myself, so it can handle
> > that stuff.
> 
> Good =).
> 
> I changed the counter to run up to 0xff before resetting, and have been
> gathering about 5.9M values of the timer when the rising edge occurs over
> the last 2.5 hours.
> 
> I'm going to leave it running but I think there might be an
> overrepresentation of about 16 groups of digits. Look at the attached
> histogram - looks suspect to me. I wonder if it is the noise source or the
> MCU that introduces this, or if it will go away with more samples.

I've processed the data you sent me, and there's clearly some pattern to be 
seen.  I've plotted this y axis logarithmic - it's the deltas between 
consecutive counters converted to a histogram.

The 114 is much overrepresented, and that's because that's the bucket for all 
the transitions which are faster than you can measure them.  This also causes 
your low output bandwidth, because repeated 114 will get filtered out.

The remaining pattern looks like the MCU has counter values which are much 
more likely to be detected than others.

This means: You will get much better results if you use a predictable capture 
counter in an FPGA (or a higher end fast microcontroller), i.e. count until 
the intput changes, then store the counter in RAM, increment the pointer and 
reset the counter - until the RAM is full, and you can wait for the external 
readout to take the sampled data.

Apart from these artefacts (controller too slow, high likely/low likely timer 
values), the data looks good; an exponential cutoff for higher delays is 
expected.

Sent also on tech at cryptech.is, so that others can comment.  This is now data 
in a form that doesn't clutter up the mailing list archive.

-- 
Bernd Paysan
"If you want it done right, you have to do it yourself"
http://bernd-paysan.de/
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114 1042259 
115 54665 
116 22403 
117 109308 
118 109237 
119 22523 
120 56162 
121 164091 
122 54259 
123 21975 
124 105728 
125 105582 
126 21076 
127 52173 
128 154483 
129 50850 
130 20465 
131 99260 
132 97830 
133 19659 
134 47915 
135 140859 
136 46019 
137 18732 
138 90451 
139 88372 
140 18024 
141 43992 
142 128133 
143 42097 
144 17061 
145 81292 
146 80138 
147 16297 
148 39483 
149 114901 
150 37664 
151 91251 
152 74070 
153 72046 
154 16877 
155 37869 
156 103974 
157 34639 
158 17212 
159 66365 
160 64016 
161 14943 
162 33467 
163 91554 
164 30670 
165 15259 
166 58752 
167 56632 
168 13418 
169 29559 
170 80906 
171 27038 
172 13410 
173 51152 
174 49773 
175 11788 
176 26168 
177 71015 
178 23510 
179 11830 
180 44701 
181 43578 
182 10339 
183 23152 
184 61654 
185 53433 
186 10764 
187 39322 
188 38161 
189 9584 
190 19971 
191 54294 
192 18735 
193 9526 
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198 47066 
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296 5806 
297 2014 
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301 1036 
302 1948 
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413 107 
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417 104 
418 275 
419 289 
420 88 
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424 103 
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427 82 
428 137 
429 306 
430 111 
431 87 
432 204 
433 213 
434 77 
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441 54 
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