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Re: Stupid hash question?



Ben Laurie writes:
> I've been working on signatures recently, and I'm puzzled. As I
> understand it, the form of a decrypted signature is:
>
> 01 FF FF ... FF FF 00 <ASN.1 nonsense> <hash>
>
> However, every signature I look at decrypts to:
>
> 00 01 FF FF ... FF FF 00 <ASN.1 nonsense> <hash>
>
> Before I hurt my head trying to figure out why, I wonder if there's
> something obvious I missed?


Actually if you look at PKCS-1 v1.5 you will find that in fact the
MSB is a 0 and the next byte is a 1 for signatures, a 2 for encryption.
Generally the MSB may not be a whole octet, depending on the size of
the modulus, so they put a zero there.

Hal Finney