Implement import of custom external keys

Implement import of custom external keys

  Our key object retains info about the external
  key as an opaque handle to the backend. We also
  need the public key as an EVP_PKEY *.

  For native keys we use OpenSSL API to import
  data into the key. The 'handle' representing the
  private key in that case is the OpenSSL EVP_PKEY
  object itself.

  For importing custom keys, we define custom
  parameters describing the key using OSSL_PARAM
  structure. We define 4 required and 1 optional
  parameters for loading the key:

  Required params of type OSSL_PARAM:

  {.key="xkey-origin", .data_type = OSSL_PARAM_UTF8_STRING
   .data = "foobar", .data_size = 0 }

  Note: data_size = 0 refer to NUL terminated string in OpenSSL.
  This parameter is only used to identify that the key as non-native
  with an opaque handle. We really do not check the content of
  the string. Should not be NULL.

  {.key="handle", .data_type = OSSL_PARAM_OCTET_PTR,
   .data = &handle, .data_size = sizeof(handle)}

  {.key="pubkey", .data_type = OSSL_PARAM_OCTET_STRING,
   .data = &pubkey, .data_size = sizeof(pubkey)}

  {.key="sign_op", .data_type = OSSL_PARAM_OCTET_PTR,
   .data = &sign_op_ptr, .data_size = sizeof(sign_op_ptr)}

  Optional param:

  {.key="free_op", .data_type = OSSL_PARAM_OCTET_PTR,
   .data = &free_op_ptr, .data_size = sizeof(free_op_ptr)}

  The 'handle' is opaque to us and is retained. The caller
  should not free it. We will free it when no longer required
  by calling 'free_op()', if provided. The 'handle' should
  not be NULL as that indicates missing private key.

  The 'pubkey' must be an 'EVP_PKEY *' variable, and is duplicated
  by us. The caller may free it after return from import.

  The 'sign_op' and 'free_op' function pointers should be of type
  'XKEY_EXTERNAL_SIGN_fn' and 'XKEY_PRIVKEY_FREE_fn' defined
  in xkey_common.h

For example, for management-external-key, we really do not
need any 'handle'. Pass anything that will live long and
won't dereference to NULL. We do not use it for any other
purpose. Pointer to a const string could be a choice.
In this case, free_op = NULL is the safest choice.

For a usage of keymgmt_import(), see the helper function
implemented using it to load the management key in the next commit.

v2 changes: "origin" --> "xkey-origin"
            This was 5/9 in v1

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20211214165928.30676-5-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23439.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>