def AffineEnc(a, b, PlainText,size=26):
    PlainText=PlainText.upper()
    if gcd(a,26)!=1:
        print "warning GCD of a and {0} is not 1".format(size)
    outstr = ""
    for c in PlainText:
        outstr+= chr(((ord(c)-65)*a+b)%size+65)
    print outstr

AffineEnc(9,2,"AFFINE")

def AffineDec(a,b,CipherText,size=26):
    CipherText = CipherText.upper()
    if gcd(a,26)!=1:
        print "warning GCD of a and {0} is not 1".format(size)
        return
    ainv = inverse_mod(a,size)
    outstr=""
    for c in CipherText:
        outstr+= chr(((ord(c)-65-b)*ainv)%size+65)
    print outstr

AffineDec(3,2,"HOEH")

(11^(-1)*7)%26