def encrypt_message(cls, message, pubkey):
pk = ser_to_point(pubkey)
if not ecdsa.ecdsa.point_is_valid(generator_secp256k1, pk.x(), pk.y()):
raise Exception('invalid pubkey')
ephemeral_exponent = number_to_string(ecdsa.util.randrange(pow(2, 256)),
generator_secp256k1.order())
ephemeral = EC_KEY(ephemeral_exponent)
ecdh_key = point_to_ser(pk * ephemeral.privkey.secret_multiplier)
key = hashlib.sha512(ecdh_key).digest()
iv, key_e, key_m = key[0:16], key[16:32], key[32:]
ciphertext = aes_encrypt_with_iv(key_e, iv, message)
ephemeral_pubkey = ephemeral.get_public_key(compressed=True).decode('hex')
encrypted = 'BIE1' + ephemeral_pubkey + ciphertext
mac = hmac.new(key_m, encrypted, hashlib.sha256).digest()
return base64.b64encode(encrypted + mac)
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