Re: RSA moduli sizes

On Mar 27, 10:42�am, Paulo Marques <pmarq...@xxxxxxxxxxxx> wrote:
Stan Lee wrote:

--
Paulo Marques
Software Development Department - Grupo PIE, S.A.
Phone: +351 252 290600, Fax: +351 252 290601
Web:www.grupopie.com





Thomas Pornin wrote:
According to K. Jennings <kjenni...@xxxxxxxxxxxxxx>:
� �That's fair enough. Let me please rephrase my question: Is it
your opinion that an RSA key generation code should guarantee that, when
generating N-bit keys, the leading bit of the modulus must always be set
to 1?
My own opinion is that if a device or function is supposed to generate
an N-bit key, and returns an (N-1)-bit key, then the device or function
did not follow its specification. I asked for 1024 bits, if I get only
1023 then I feel defrauded.

� � � �--Thomas Pornin

What I don't understand is, why is a bit only considered as being a bit,
if it is set to 1?

I think you'd still be getting a 1024-bit key. It just so happens the
most significant bit is set to 0.

I think the confusion here arises from the fact that when you have a 256
bit AES key (for instance) it doesn't matter what bits are 0 or 1, as
the AES will always work, but in RSA if the key is shorter in the sense
that the most significant bits are 0, then the messages you can sign or
encrypt are also shorter, i.e., size matters ;)


AES keys are bit strings, not numbers.
.

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