crypto.diffieHellman padding issue #5239
Description
here is a special case I encountered:
var df = require('crypto').createDiffieHellman('90f8cb2b', 'hex');
df.setPrivateKey('4e953092', 'hex');
df.setPublicKey('479fd00c', 'hex'); // comment below
console.log(df.computeSecret('8e366f02', 'hex', 'hex'));
the output is:
694f8500
the correct secret should be 00694f85, which can be verified by a small python script:
class DiffieHellman(object):
def __init__(self, p, g):
self.p = p
self.g = g
def generateKeys(self, a):
self.a = a
return pow(self.g, self.a, self.p)
def computeSecret(self, B):
return pow(B, self.a, self.p)
df = DiffieHellman(0x90f8cb2b, 2)
print '%08x' % df.generateKeys(0x4e953092)
print '%08x' % df.computeSecret(0x8e366f02)
the output is:
479fd00c
00694f85
And another verification snippet using jsbn has the result 00694f85 too, the code is short but jsbn needs a little work to be happy with node.js so it's not pasted here.
It looks like a padding issue to me.
Using setPublicKey after setPrivateKey looks less convincing but sadly setPrivateKey won't generate the corresponding public key and generateKeys does not support specifying a private key so using generateKeys will need a loop to get such a special case and make the code longer (and less obvious).
Please do not discuss about "one shall not use Diffie-Hellman on his own" like topics here, I know that :(