SilcSKRStatus silc_skr_add_public_key(SilcSKR skr,
                                          SilcPublicKey public_key,
                                          SilcSKRKeyUsage usage,
                                          void *key_context,
                                          SilcSKRKey *return_key);

    Add a public key to repository.  The repository will steal `public_key'
    and caller must not free it.  The `key_context' is optional key specific
    context that will be saved in the repository with the key, and can be
    retrieved with the key.  Public key can be added only once to the
    repository.  To add same key more than once to repository different
    `key_context' must be used each time.

    Returns an entry of the added public key in the repository to the
    `return_key' pointer, if it is non-NULL.  The returned entry remains
    valid as long as the public key is in the repository, however a
    reference may be taken with silc_skr_ref_public_key to assure the
    entry remains valid.

    Returns SILC_SKR_OK if the key was added successfully, and error
    status if key could not be added, or has been added already.

    // Add a key to repository
    if (silc_skr_add_public_key(repository, pubkey, SILC_SKR_USAGE_ANY,
                                NULL, NULL) != SILC_SKR_OK)
      goto error;