import hashlib
import smtplib

def forge(pubkey):
    ... hashlib.sha256(m0) ... smtplib.sendmail(m1) ...
    return (forged_msg, forged_sig)

def forge0(hash, sign, pubkey):
    ... hash(m0) ... sign(m1) ...
    return (forged_msg, forged_sig)

def forge(pubkey):
    import hashlib
    import smtplib
    def hash(m): return hashlib.sha256(m).digest()
    def sign(m): smtplib.sendmail(...) ... return signature
    return forge0(hash, sign, pubkey)

def cdh(gx, gy):
    def hash(m): ...
    def sign(m): ...
    ... (m1, s1) = forge(hash, sign, gx) ...
    ... (m2, s2) = forge(hash, sign, gy) ...
    return gxy