Re: RNG algo question

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 16 September 2016 at 22:33, Palmer, Thomas <thomas.palmer@hpe.com> wrote:
>
> EDK2 community
>
> Why is the RngGetRNG  sending requests for "gEfiRngAlgorithmRaw"  to "RdRandGenerateEntropy",  which does AES operations on RDRAND output, whereas the requests for "gEfiRngAlgorithmSp80090Ctr256Guid" get sent to RdRandGetBytes which simply reads the rdrand source without modification.
>
> Shouldn't the processing be switched, so that "Raw" goes to RdRandGetBytes and "gEfiRngAlgorithmSp80090Ctr256Guid" goes to RdRandGenerateEntropy?  I did not see anything in the UEFI 2.5 spec indicating why this was the case.
>

The RDRAND instruction does not give you raw entropy, but the output
of a DRBG. So the 'raw' entropy is being emulated by taking multiple
rounds of RDRAND output and shuffling it around to make it 'raw'

>From the top of RngDxe.c:

  RNG Algoritnms defined in UEFI 2.4:
   - EFI_RNG_ALGORITHM_SP800_90_CTR_256_GUID  - Supported
     (RDRAND implements a hardware NIST SP800-90 AES-CTR-256 based DRBG)
   - EFI_RNG_ALGORITHM_RAW                    - Supported
     (Structuring RDRAND invocation can be guaranteed as high-quality
entropy source)