[pycrypto] Bug in PyCrypto 2.6.1

[Lorenz Quack]

Hi Luis,

Thanks for reporting!
I agree that this seems like strange/wrong behaviour.
Especially when realise that the docs seem to contradict the behaviour.
_RSA.RSAobj.size.__doc__ says: Return the maximum number of bits that can be handled by this key.
But this works fine:
 >>> key.encrypt(1<<key.size(), "")
Note that 1 << x has x+1 bits so that the above key happily encrypts key.size()+1 bits.

The only thing holding me back from strait out calling it a bug is that
  1) this is very old code dating back to 2003.
  and 2) the -1 is actively coded there but with out explanation

As a pointer to others the relevant places are:
/lib/Crypto/PublicKey/_RSA.py:80
/src/_fastmath.c:949

If it is decided that this is a bug and should be fixed one should also look at other keys, e.g. DSA which seems to do 
the same -1 calculation.

Cheers,
Lorenz



On 01/12/14 07:35, Luis González Fernández wrote:
> Hello All:
>
>
> I found a strange bug in size() function that return a wrong key size after creating or importing an RSA key. The return
> value for the size funciont seems to be keysize-1.
>
> Here a demo of the bug:
>
>
> luisgf at NCC1701B:~$ python3
> Python 3.4.0 (default, Apr 11 2014, 13:05:11)
> [GCC 4.8.2] on linux
> Type "help", "copyright", "credits" or "license" for more information.
>  >>> from Crypto.PublicKey import RSA
>  >>> key = RSA.generate(2048)
>  >>> key.size()
> 2047
>  >>>
>
>
> Regards.
>
> --
>
> --
> Luis González Fernández
> https://www.luisgf.es
> PGP ID: C918B80F (DD6F BFC1 FC14 4C81 34F8 EA1E 6BCB C27F C918 B80F)
> Twitter: @luisgf_2001 / Jabber:luisgf at mijabber.es
>
>
>
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