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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def step1(self, username, password):
 """First authentication step."""

        self._check_initialized()
        context = AtvSRPContext(
            str(username), str(password),
            prime=constants.PRIME_2048,
            generator=constants.PRIME_2048_GEN)
        self.session = SRPClientSession(
            context, binascii.hexlify(self._auth_private).decode())