Re: [belenios-discuss] Computing a fingerprint of a trustee's public key

[Pierrick Gaudry <pierrick.gaudry AT loria.fr>]

On Wed, Jan 06, 2021 at 05:27:24PM +0100, Stéphane Glondu wrote:
> There is no easy way to compute the public key from the private key. One
> has to do arithmetic using some third-party tool.

Here is a small python3 script that allows to recompute the public key
from the private key.

Under linux (and probably also MacOs), you can save the file check-key.py
somewhere, that make it executable with:

  chmod 755 check-key.py

and then run it with:

  ./check-key.py

Regards,
Pierrick
#!/usr/bin/env python3

## Belenios default group parameters:
p = 20694785691422546401013643657505008064922989295751104097100884787057374219242717401922237254497684338129066633138078958404960054389636289796393038773905722803605973749427671376777618898589872735865049081167099310535867780980030790491654063777173764198678527273474476341835600035698305193144284561701911000786737307333564123971732897913240474578834468260652327974647951137672658693582180046317922073668860052627186363386088796882120769432366149491002923444346373222145884100586421050242120365433561201320481118852408731077014151666200162313177169372189248078507711827842317498073276598828825169183103125680162072880719
g = 2402352677501852209227687703532399932712287657378364916510075318787663274146353219320285676155269678799694668298749389095083896573425601900601068477164491735474137283104610458681314511781646755400527402889846139864532661215055797097162016168270312886432456663834863635782106154918419982534315189740658186868651151358576410138882215396016043228843603930989333662772848406593138406010231675095763777982665103606822406635076697764025346253773085133173495194248967754052573659049492477631475991575198775177711481490920456600205478127054728238140972518639858334115700568353695553423781475582491896050296680037745308460627
q = 78571733251071885079927659812671450121821421258408794611510081919805623223441
assert pow(g, q, p) == 1

## Read private key from user
inp = input("Enter your private key, without the double-quotes: ")
s = int(inp)
assert s < q

## The corresponding public key is g^s mod p
## This is the value that should be present in the field "public_key" in
## the "trustees.json" file.
pk = pow(g, s, p)
print("Your public key is: " + str(pk))