Messages in this thread |  | | Date | Tue, 28 Apr 2026 15:24:48 +0200 | | From | Marek Behún <> | | Subject | Re: [PATCH v2] crypto: atmel-sha204a - drop hwrng quality reduction for ATSHA204A |
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Hi Thorsten,
Bill also wrote about ATSHA204A [1]
My best guess as to what's going on here is that the device has a ring-oscillator based entropy source, but that it generates only a few bits of entropy for each use. It seems to be called before generating each 32-byte "random" value, which is why the second set of 32-bit values have more possible values, and the 3rd has even more. However, the number of unique values in the final column of 32*N byte values is always equal to the number of unique values of the entire string of bytes.
If this is true that the device generates <256 true random bits and then mixes in non-volatile pseudorandom number generator to produce 256 bits, then the quality should not be set to full 1024.
Marek
[1] https://www.metzdowd.com/pipermail/cryptography/2014-December/023858.html
On Tue, Apr 28, 2026 at 02:32:52PM +0200, Thorsten Blum wrote: > > Bill wrote in his review: > > "If I made no mistake (and I do make a lot), the "random" data from > the Atmel ATSHA204A is highly predictable when you disable the seed > update to EEPROM." > > However, the atmel-sha204a driver doesn't operate the device in that > mode. It uses the Random command with seed updates enabled, which is > also what the datasheet recommends for highest security: > > "Microchip recommends that the EEPROM seed always be updated." > > So the reported behavior doesn't reflect how the driver uses the device. > > Thanks, > Thorsten
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