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Add more info about the ROSC TRNG

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Signed-off-by: Joachim Strömbergson <joachim@assured.se>
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Joachim Strömbergson 2022-11-29 14:43:35 +01:00
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doc/system_description/fpga.md
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@ -100,15 +100,15 @@ The UART contain a 512 but Rx-FIFO with status (data available).
The timer is available to use by firmware and applications. The timer is available to use by firmware and applications.
#### TRNG #### ROSC
The TRNG is a ring oscillator based internal entropy source. The ROSC is a ring oscillator based internal entropy source, or
By default the TRNG use 32 free running digital oscillators. By True Random Number Generator (TRNG). By default the TRNG use 32
default, the oscillators are sampled after 4096 cycles. The free running digital oscillators. By default, the oscillators are
states are XOR combined to create a single bit. After another sampled after 4096 cycles. The states are XOR combined to create
4096 cycles a second bit is created, and the two bits are a single bit. After another 4096 cycles a second bit is created,
XOR combined to a single entropy bit. The entropy bit is and the two bits are XOR combined to a single entropy bit. The
added to a 32 bit entropy word. entropy bit is added to a 32 bit entropy word.
After 32 bits has been collected, the data ready flag is set, After 32 bits has been collected, the data ready flag is set,
indicating that an entropy word is ready for consumption. Note indicating that an entropy word is ready for consumption. Note
@ -119,7 +119,15 @@ been added.
If a data word has been read from the TRNG, by default at least If a data word has been read from the TRNG, by default at least
32 bits will collected before new data will be available. 32 bits will collected before new data will be available.
The timer is available to use by firmware and applications. The ROSC TRNG is available to use by firmware and applications.
Note: The ROSC generates entropy with a fairly good quality.
However for security related use cases, for example keys, the ROSC
should not be used directly. Instead use it to create a seed
for a Digital Random Bit Generator (DRBG), also known as a
Cryptographically Safe Pseudo Random Number Generator (CSPRNG).
Examples of such generators are Hash_DRGG, CTR_DRBG, HKDF.
#### Touch sensor #### Touch sensor