pycrypto rsa inverse of p modulo q

[jt]

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)

Thx,

 

[mensanator]

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 */