Re: [PATCH v2] X.509: unpack RSA signatureValue field from BIT STRING

[Kamil Konieczny]

On 17.04.2018 15:39, Maciej S. Szmigiero wrote:
> The signatureValue field of a X.509 certificate is encoded as a BIT STRING.
> For RSA signatures this BIT STRING is of so-called primitive subtype, which
> contains a u8 prefix indicating a count of unused bits in the encoding.
> 
> We have to strip this prefix from signature data, just as we already do for
> key data in x509_extract_key_data() function.
> 
> This wasn't noticed earlier because this prefix byte is zero for RSA key
> sizes divisible by 8. Since BIT STRING is a big-endian encoding adding zero
> prefixes has no bearing on its value.
> 
> The signature length, however was incorrect, which is a problem for RSA
> implementations that need it to be exactly correct (like AMD CCP).
> 
> Signed-off-by: Maciej S. Szmigiero <mail@xxxxxxxxxxxxxxxxxxxxx>

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-- 
Best regards,
Kamil Konieczny
Samsung R&D Institute Poland