text: # This file is dual licensed under the terms of the Apache License, Version
  # 2.0, and the BSD License. See the LICENSE file in the root of this repository
  # for complete details.
  
  from __future__ import absolute_import, division, print_function
  
  
  from cryptography.exceptions import InvalidSignature
  from cryptography.hazmat.primitives import constant_time
  
  
  _POLY1305_TAG_SIZE = 16
  _POLY1305_KEY_SIZE = 32
  
  
  class _Poly1305Context(object):
      def __init__(self, backend, key):
          self._backend = backend
  
          key_ptr = self._backend._ffi.from_buffer(key)
          # This function copies the key into OpenSSL-owned memory so we don't
          # need to retain it ourselves
          evp_pkey = self._backend._lib.EVP_PKEY_new_raw_private_key(
              self._backend._lib.NID_poly1305,
              self._backend._ffi.NULL,
              key_ptr,
              len(key),
          )
          self._backend.openssl_assert(evp_pkey != self._backend._ffi.NULL)
          self._evp_pkey = self._backend._ffi.gc(
              evp_pkey, self._backend._lib.EVP_PKEY_free
          )
          ctx = self._backend._lib.EVP_MD_CTX_new()
          self._backend.openssl_assert(ctx != self._backend._ffi.NULL)
          self._ctx = self._backend._ffi.gc(
              ctx, self._backend._lib.EVP_MD_CTX_free
          )
          res = self._backend._lib.EVP_DigestSignInit(
              self._ctx,
              self._backend._ffi.NULL,
              self._backend._ffi.NULL,
              self._backend._ffi.NULL,
              self._evp_pkey,
          )
          self._backend.openssl_assert(res == 1)
  
      def update(self, data):
          data_ptr = self._backend._ffi.from_buffer(data)
          res = self._backend._lib.EVP_DigestSignUpdate(
              self._ctx, data_ptr, len(data)
          )
          self._backend.openssl_assert(res != 0)
  
      def finalize(self):
          buf = self._backend._ffi.new("unsigned char[]", _POLY1305_TAG_SIZE)
          outlen = self._backend._ffi.new("size_t *")
          res = self._backend._lib.EVP_DigestSignFinal(self._ctx, buf, outlen)
          self._backend.openssl_assert(res != 0)
          self._backend.openssl_assert(outlen[0] == _POLY1305_TAG_SIZE)
          return self._backend._ffi.buffer(buf)[: outlen[0]]
  
      def verify(self, tag):
          mac = self.finalize()
          if not constant_time.bytes_eq(mac, tag):
              raise InvalidSignature("Value did not match computed tag.")
  