So, I'm trying to consult with some RNG experts because I'm by no means an
expert and anything I say should be taken with huge grain of salt. When I get
the experts take, I'll share it.
Basically, the way I read this code is that it by no means tries to enforce any
entropy requirement outside of what you ask for.
My understanding is the 256 Bit Entropy requirements comes from when you are
using a DRNG algorithm such as:
```
#define EFI_RNG_ALGORITHM_SP800_90_HASH_256_GUID \
{0xa7af67cb, 0x603b, 0x4d42,\
{0xba, 0x21, 0x70, 0xbf, 0xb6, 0x29, 0x3f, 0x96}}
#define EFI_RNG_ALGORITHM_SP800_90_HMAC_256_GUID \
{0xc5149b43, 0xae85, 0x4f53,\
{0x99, 0x82, 0xb9, 0x43, 0x35, 0xd3, 0xa9, 0xe7}}
#define EFI_RNG_ALGORITHM_SP800_90_CTR_256_GUID \
{0x44f0de6e, 0x4d8c, 0x4045, \
{0xa8, 0xc7, 0x4d, 0xd1, 0x68, 0x85, 0x6b, 0x9e}}
```
> "When a Deterministic Random Bit Generator (DRBG) is used on the output of a
> (raw) entropy source, its security level must be at least 256 bits."
https://uefi.org/specs/UEFI/2.10/37_Secure_Technologies.html#random-number-generator-protocol
That is, the seed of these algorithms must be at a minimum 256 bits from your
entropy source.
Now when you call for instance EFI_RNG_ALGORITHM_SP800_90_CTR_256_GUID
On an INTEL CPU it uses the Intel RDRAND Instruction
https://github.com/tianocore/edk2/blob/4b6ee06a090d956f80b4a92fb9bf03098a372f39/SecurityPkg/RandomNumberGenerator/RngDxe/Rand/RngDxe.c#L108C45-L108C51
Which from what I can tell the generator takes pairs of 256-bit raw entropy
samples generated by the hardware entropy source and applies them to an
Advanced Encryption Standard (AES) (in CBC-MAC mode) conditioner which reduces
them to a single 256-bit conditioned entropy sample.
https://en.wikipedia.org/wiki/RDRAND
https://www.intel.com/content/www/us/en/developer/articles/guide/intel-digital-random-number-generator-drng-software-implementation-guide.html
Which means, if you are implementing these algorithms in software, you must
comply with the 256 bit entropy requirement for your source. However in our
case the CPU is performing that requirement for us.
Again I'm no expert. So if an expert is reading this and I'm completely wrong
please let me know :)
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