String Encoding: ASCII

def STR(a):
    a = str(a)
    # Yes, this is a little bit silly :-)
    for i in range(0, len(a) - 1, 2):
            print(chr(int(a[i:i+2]))),

e.g. STR(7269767679) = "HELLO"

Cryptosystem:
Elliptic Curve: y^2 ≡ x^3 + ax + b mod n
a = 0
b = ?
n = 928669833265826932708591

Encryption: C = e * M mod n
Decryption: M = d * C mod n
e = 141597355687225811174313
d = 87441340171043308346177
C = (236857987845294655469221, 12418605208975891779391)
STR(M.x) + STR(M.y) = ?

It seems that the author has written out the parameters for an elliptic curve defined over the
integers modulo n, where the numbers "wrap around" back to zero upon reaching the modulus, like
with a clock. However, one of the elliptic curve parameters, b, has been smudged out and is no
longer visible. Nevertheless, since you know that the given point C satisfies the elliptic curve
equation, you can substitute it into the elliptic curve equation to determine the value of b.

Although the author has also devised a rudimentary elliptic curve encryption scheme by encoding a
message as a point using ASCII and multiplying it (repeated addition) by a secret encryption key,
they apparently forgot to erase both the encryption and decryption keys! To break the encryption,
you only need to multiply the encrypted message by the decryption key to obtain the original
message, and then convert the result back into a string.