pycrypto rsa inverse of p modulo q

mensanator at aol.com mensanator at aol.com
Sat Nov 5 00:03:00 CET 2005


jt wrote:
> Looking up into Crypto.PublicKey.RSA, I see there is a computed value
> named "u" for which I can't see the use. The value of "u" is the
> inverse of p modulo q, in the code:
>     obj.u = pubkey.inverse(obj.p, obj.q)
> Can someone tell me where this value could be used in the RSA scheme?
> (it is not used in the code anyway)

Yes, it is. Look in _fastmath.c. It's used in rsaDecrypt:

        /* fast path */
        /* m1 = c ^ (d mod (p-1)) mod p */
        /* m2 = c ^ (d mod (q-1)) mod q */
        /* h = u * ( m2 - m1 ) mod q */          <-- right here
        /* m = m2 + h * p */


> 
> Thx,
> -- 
> jt




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