Source code for chamhash_adm05

''' Ateneise-Medeiros (Schnorr group-based)
 
 | From: "Ateneise-Breno de Medeiros On the Key Exposure Problem in Chameleon Hashes", Section 4.
 | Published in: SCN 2004
 | Available from:
 | Notes: 

 * type:         hash function (chameleon)
 * setting:      Schnorr groups
 * assumption:   DL-Hard

:Authors: J Ayo Akinyele
:Date:    4/2011
'''
from charm.toolbox.Hash import ChamHash,Hash
from charm.toolbox.integergroup import IntegerGroupQ

debug = False
class ChamHash_Adm05(ChamHash):
    """
    >>> from charm.core.math.integer import integer
    >>> p = integer(141660875619984104245410764464185421040193281776686085728248762539241852738181649330509191671665849071206347515263344232662465937366909502530516774705282764748558934610432918614104329009095808618770549804432868118610669336907161081169097403439689930233383598055540343198389409225338204714777812724565461351567)
    >>> q = integer(70830437809992052122705382232092710520096640888343042864124381269620926369090824665254595835832924535603173757631672116331232968683454751265258387352641382374279467305216459307052164504547904309385274902216434059305334668453580540584548701719844965116691799027770171599194704612669102357388906362282730675783)
    >>> chamHash = ChamHash_Adm05(p, q)
    >>> (public_key, secret_key) = chamHash.paramgen()
    >>> msg = "hello world this is the message"
    >>> c = chamHash.hash(public_key, msg)
    >>> c == chamHash.hash(public_key, msg, c[1], c[2])
    True
    """

    def __init__(self, p=0, q=0):
        ChamHash.__init__(self)
        global group; 
        group = IntegerGroupQ(0)
        # if p and q parameters have already been selected
        group.p, group.q, group.r = p, q, 2
        self.group = group        
    
    def paramgen(self, secparam=1024):
        if group.p == 0 or group.q == 0:
            group.paramgen(secparam)
        g, x = group.randomGen(), group.random() # g, [1,q-1]
        y = g ** x
        
        if debug:
            print("Public params")
            print("g =>", g); print("y =>", y)
        
        pk = { 'g':g, 'y':y }
        sk = { 'x':x }        
        return (pk, sk)
    
    def hash(self, pk, m, r = 0, s = 0):
        p,q = group.p, group.q
        if r == 0: r = group.random()
        if s == 0: s = group.random()        
        e = group.hash(m, r)
        
        C = r - (((pk['y'] ** e) * (pk['g'] ** s)) % p) % q
        return (C, r, s)