NAME

       provider-decoder - The OSSL_DECODER library <-> provider functions

SYNOPSIS

        #include <openssl/core_dispatch.h>

        /*
         * None of these are actual functions, but are displayed like this for
         * the function signatures for functions that are offered as function
         * pointers in OSSL_DISPATCH arrays.
         */

        /* Decoder parameter accessor and descriptor */
        const OSSL_PARAM *OSSL_FUNC_decoder_gettable_params(void *provctx);
        int OSSL_FUNC_decoder_get_params(OSSL_PARAM params[]);

        /* Functions to construct / destruct / manipulate the decoder context */
        void *OSSL_FUNC_decoder_newctx(void *provctx);
        void OSSL_FUNC_decoder_freectx(void *ctx);
        const OSSL_PARAM *OSSL_FUNC_decoder_settable_ctx_params(void *provctx);
        int OSSL_FUNC_decoder_set_ctx_params(void *ctx, const OSSL_PARAM params[]);

        /* Functions to check selection support */
        int OSSL_FUNC_decoder_does_selection(void *provctx, int selection);

        /* Functions to decode object data */
        int OSSL_FUNC_decoder_decode(void *ctx, OSSL_CORE_BIO *in,
                                     int selection,
                                     OSSL_CALLBACK *data_cb, void *data_cbarg,
                                     OSSL_PASSPHRASE_CALLBACK *cb, void *cbarg);

        /* Functions to export a decoded object */
        void *OSSL_FUNC_decoder_export_object(void *ctx,
                                              const void *objref, size_t objref_sz,
                                              OSSL_CALLBACK *export_cb,
                                              void *export_cbarg);

DESCRIPTION

       The term "decode" is used throughout this manual.  This includes but is not limited to
       deserialization as individual decoders can also do decoding into intermediate data
       formats.

       The DECODER operation is a generic method to create a provider-native object reference or
       intermediate decoded data from an encoded form read from the given OSSL_CORE_BIO. If the
       caller wants to decode data from memory, it should provide a BIO_s_mem(3) BIO. The decoded
       data or object reference is passed along with eventual metadata to the metadata_cb as
       OSSL_PARAM parameters.

       The decoder doesn't need to know more about the OSSL_CORE_BIO pointer than being able to
       pass it to the appropriate BIO upcalls (see "Core functions" in provider-base(7)).

       The DECODER implementation may be part of a chain, where data is passed from one to the
       next.  For example, there may be an implementation to decode an object from PEM to DER,
       and another one that decodes DER to a provider-native object.

       The last decoding step in the decoding chain is usually supposed to create a provider-
       native object referenced by an object reference. To import that object into a different
       provider the OSSL_FUNC_decoder_export_object() can be called as the final step of the
       decoding process.

       All "functions" mentioned here are passed as function pointers between libcrypto and the
       provider in OSSL_DISPATCH arrays via OSSL_ALGORITHM arrays that are returned by the
       provider's provider_query_operation() function (see "Provider Functions" in
       provider-base(7)).

       All these "functions" have a corresponding function type definition named
       OSSL_FUNC_{name}_fn, and a helper function to retrieve the function pointer from an
       OSSL_DISPATCH element named OSSL_FUNC_{name}.  For example, the "function"
       OSSL_FUNC_decoder_decode() has these:

        typedef int
            (OSSL_FUNC_decoder_decode_fn)(void *ctx, OSSL_CORE_BIO *in,
                                          int selection,
                                          OSSL_CALLBACK *data_cb, void *data_cbarg,
                                          OSSL_PASSPHRASE_CALLBACK *cb, void *cbarg);
        static ossl_inline OSSL_FUNC_decoder_decode_fn
            OSSL_FUNC_decoder_decode(const OSSL_DISPATCH *opf);

       OSSL_DISPATCH arrays are indexed by numbers that are provided as macros in
       openssl-core_dispatch.h(7), as follows:

        OSSL_FUNC_decoder_get_params          OSSL_FUNC_DECODER_GET_PARAMS
        OSSL_FUNC_decoder_gettable_params     OSSL_FUNC_DECODER_GETTABLE_PARAMS

        OSSL_FUNC_decoder_newctx              OSSL_FUNC_DECODER_NEWCTX
        OSSL_FUNC_decoder_freectx             OSSL_FUNC_DECODER_FREECTX
        OSSL_FUNC_decoder_set_ctx_params      OSSL_FUNC_DECODER_SET_CTX_PARAMS
        OSSL_FUNC_decoder_settable_ctx_params OSSL_FUNC_DECODER_SETTABLE_CTX_PARAMS

        OSSL_FUNC_decoder_does_selection      OSSL_FUNC_DECODER_DOES_SELECTION

        OSSL_FUNC_decoder_decode              OSSL_FUNC_DECODER_DECODE

        OSSL_FUNC_decoder_export_object       OSSL_FUNC_DECODER_EXPORT_OBJECT

   Names and properties
       The name of an implementation should match the target type of object it decodes. For
       example, an implementation that decodes an RSA key should be named "RSA". Likewise, an
       implementation that decodes DER data from PEM input should be named "DER".

       Properties can be used to further specify details about an implementation:

       input
           This property is used to specify what format of input the implementation can decode.

           This property is mandatory.

           OpenSSL providers recognize the following input types:

           pem An implementation with that input type decodes PEM formatted data.

           der An implementation with that input type decodes DER formatted data.

           msblob
               An implementation with that input type decodes MSBLOB formatted data.

           pvk An implementation with that input type decodes PVK formatted data.

       structure
           This property is used to specify the structure that the decoded data is expected to
           have.

           This property is optional.

           Structures currently recognised by built-in decoders:

           "type-specific"
               Type specific structure.

           "pkcs8"
               Structure according to the PKCS#8 specification.

           "SubjectPublicKeyInfo"
               Encoding of public keys according to the Subject Public Key Info of RFC 5280.

       The possible values of both these properties is open ended.  A provider may very well
       specify input types and structures that libcrypto doesn't know anything about.

   Subset selections
       Sometimes, an object has more than one subset of data that is interesting to treat
       separately or together.  It's possible to specify what subsets are to be decoded, with a
       set of bits selection that are passed in an int.

       This set of bits depend entirely on what kind of provider-side object is to be decoded.
       For example, those bits are assumed to be the same as those used with provider-keymgmt(7)
       (see "Key Objects" in provider-keymgmt(7)) when the object is an asymmetric keypair -
       e.g., OSSL_KEYMGMT_SELECT_PRIVATE_KEY if the object to be decoded is supposed to contain
       private key components.

       OSSL_FUNC_decoder_does_selection() should tell if a particular implementation supports any
       of the combinations given by selection.

   Context functions
       OSSL_FUNC_decoder_newctx() returns a context to be used with the rest of the functions.

       OSSL_FUNC_decoder_freectx() frees the given ctx as created by OSSL_FUNC_decoder_newctx().

       OSSL_FUNC_decoder_set_ctx_params() sets context data according to parameters from params
       that it recognises.  Unrecognised parameters should be ignored.  Passing NULL for params
       should return true.

       OSSL_FUNC_decoder_settable_ctx_params() returns a constant OSSL_PARAM array describing the
       parameters that OSSL_FUNC_decoder_set_ctx_params() can handle.

       See OSSL_PARAM(3) for further details on the parameters structure used by
       OSSL_FUNC_decoder_set_ctx_params() and OSSL_FUNC_decoder_settable_ctx_params().

   Export function
       When a provider-native object is created by a decoder it would be unsuitable for direct
       use with a foreign provider. The export function allows for exporting the object into that
       foreign provider if the foreign provider supports the type of the object and provides an
       import function.

       OSSL_FUNC_decoder_export_object() should export the object of size objref_sz referenced by
       objref as an OSSL_PARAM array and pass that into the export_cb as well as the given
       export_cbarg.

   Decoding functions
       OSSL_FUNC_decoder_decode() should decode the data as read from the OSSL_CORE_BIO in to
       produce decoded data or an object to be passed as reference in an OSSL_PARAM array along
       with possible other metadata that was decoded from the input. This OSSL_PARAM array is
       then passed to the data_cb callback.  The selection bits, if relevant, should determine
       what the input data should contain.  The decoding functions also take an
       OSSL_PASSPHRASE_CALLBACK function pointer along with a pointer to application data cbarg,
       which should be used when a pass phrase prompt is needed.

       It's important to understand that the return value from this function is interpreted as
       follows:

       True (1)
           This means "carry on the decoding process", and is meaningful even though this
           function couldn't decode the input into anything, because there may be another decoder
           implementation that can decode it into something.

           The data_cb callback should never be called when this function can't decode the input
           into anything.

       False (0)
           This means "stop the decoding process", and is meaningful when the input could be
           decoded into some sort of object that this function understands, but further treatment
           of that object results into errors that won't be possible for some other decoder
           implementation to get a different result.

       The conditions to stop the decoding process are at the discretion of the implementation.

   Decoder operation parameters
       There are currently no operation parameters currently recognised by the built-in decoders.

       Parameters currently recognised by the built-in pass phrase callback:

       "info" (OSSL_PASSPHRASE_PARAM_INFO) <UTF8 string>
           A string of information that will become part of the pass phrase prompt.  This could
           be used to give the user information on what kind of object it's being prompted for.

RETURN VALUES

       OSSL_FUNC_decoder_newctx() returns a pointer to a context, or NULL on failure.

       OSSL_FUNC_decoder_set_ctx_params() returns 1, unless a recognised parameter was invalid or
       caused an error, for which 0 is returned.

       OSSL_FUNC_decoder_settable_ctx_params() returns a pointer to an array of constant
       OSSL_PARAM elements.

       OSSL_FUNC_decoder_does_selection() returns 1 if the decoder implementation supports any of
       the selection bits, otherwise 0.

       OSSL_FUNC_decoder_decode() returns 1 to signal that the decoding process should continue,
       or 0 to signal that it should stop.

SEE ALSO

       provider(7)

HISTORY

       The DECODER interface was introduced in OpenSSL 3.0.

COPYRIGHT

       Copyright 2019-2021 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the Apache License 2.0 (the "License").  You may not use this file except
       in compliance with the License.  You can obtain a copy in the file LICENSE in the source
       distribution or at <https://www.openssl.org/source/license.html>.