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
  
  import warnings
  
  from cryptography import utils
  from cryptography.hazmat.primitives import hashes
  from cryptography.hazmat.primitives.asymmetric.utils import Prehashed
  
  
  def _evp_pkey_derive(backend, evp_pkey, peer_public_key):
      ctx = backend._lib.EVP_PKEY_CTX_new(evp_pkey, backend._ffi.NULL)
      backend.openssl_assert(ctx != backend._ffi.NULL)
      ctx = backend._ffi.gc(ctx, backend._lib.EVP_PKEY_CTX_free)
      res = backend._lib.EVP_PKEY_derive_init(ctx)
      backend.openssl_assert(res == 1)
      res = backend._lib.EVP_PKEY_derive_set_peer(
          ctx, peer_public_key._evp_pkey
      )
      backend.openssl_assert(res == 1)
      keylen = backend._ffi.new("size_t *")
      res = backend._lib.EVP_PKEY_derive(ctx, backend._ffi.NULL, keylen)
      backend.openssl_assert(res == 1)
      backend.openssl_assert(keylen[0] > 0)
      buf = backend._ffi.new("unsigned char[]", keylen[0])
      res = backend._lib.EVP_PKEY_derive(ctx, buf, keylen)
      if res != 1:
          raise ValueError(
              "Null shared key derived from public/private pair."
          )
  
      return backend._ffi.buffer(buf, keylen[0])[:]
  
  
  def _calculate_digest_and_algorithm(backend, data, algorithm):
      if not isinstance(algorithm, Prehashed):
          hash_ctx = hashes.Hash(algorithm, backend)
          hash_ctx.update(data)
          data = hash_ctx.finalize()
      else:
          algorithm = algorithm._algorithm
  
      if len(data) != algorithm.digest_size:
          raise ValueError(
              "The provided data must be the same length as the hash "
              "algorithm's digest size."
          )
  
      return (data, algorithm)
  
  
  def _check_not_prehashed(signature_algorithm):
      if isinstance(signature_algorithm, Prehashed):
          raise TypeError(
              "Prehashed is only supported in the sign and verify methods. "
              "It cannot be used with signer or verifier."
          )
  
  
  def _warn_sign_verify_deprecated():
      warnings.warn(
          "signer and verifier have been deprecated. Please use sign "
          "and verify instead.",
          utils.PersistentlyDeprecated2017,
          stacklevel=3
      )
  