- NAME
- SYNOPSIS
- DESCRIPTION

- Context Management Functions
- Shared Secret Derivation Functions
- Key Exchange Parameters Functions
- Key Exchange Parameters

- RETURN VALUES
- SEE ALSO
- HISTORY
- COPYRIGHT

# NAME

provider-keyexch - The keyexch library <-> provider functions

# SYNOPSIS

     #include <openssl/core_numbers.h>
     #include <openssl/core_names.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.
      */

     /* Context management */
     void *OP_keyexch_newctx(void *provctx);
     void OP_keyexch_freectx(void *ctx);
     void *OP_keyexch_dupctx(void *ctx);

     /* Shared secret derivation */
     int OP_keyexch_init(void *ctx, void *provkey);
     int OP_keyexch_set_peer(void *ctx, void *provkey);
     int OP_keyexch_derive(void *ctx, unsigned char *secret, size_t *secretlen,
                           size_t outlen);

     /* Key Exchange parameters */
     int OP_keyexch_set_ctx_params(void *ctx, const OSSL_PARAM params[]);
     const OSSL_PARAM *OP_keyexch_settable_ctx_params(void);
     int OP_keyexch_get_ctx_params(void *ctx, OSSL_PARAM params[]);
     const OSSL_PARAM *OP_keyexch_gettable_ctx_params(void);

# DESCRIPTION

This documentation is primarily aimed at provider authors. See provider(7) for further information.

The key exchange (OSSL_OP_KEYEXCH) operation enables providers to implement key exchange algorithms and make them available to applications via EVP_PKEY_derive(3) and other related functions).

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-base(7)/Provider Functions).

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

     typedef void *(OSSL_OP_keyexch_newctx_fn)(void *provctx);
     static ossl_inline OSSL_OP_keyexch_newctx_fn
         OSSL_get_OP_keyexch_newctx(const OSSL_DISPATCH *opf);

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

     OP_keyexch_newctx                OSSL_FUNC_KEYEXCH_NEWCTX
     OP_keyexch_freectx               OSSL_FUNC_KEYEXCH_FREECTX
     OP_keyexch_dupctx                OSSL_FUNC_KEYEXCH_DUPCTX

     OP_keyexch_init                  OSSL_FUNC_KEYEXCH_INIT
     OP_keyexch_set_peer              OSSL_FUNC_KEYEXCH_SET_PEER
     OP_keyexch_derive                OSSL_FUNC_KEYEXCH_DERIVE

     OP_keyexch_set_ctx_params        OSSL_FUNC_KEYEXCH_SET_CTX_PARAMS
     OP_keyexch_settable_ctx_params   OSSL_FUNC_KEYEXCH_SETTABLE_CTX_PARAMS
     OP_keyexch_get_ctx_params        OSSL_FUNC_KEYEXCH_GET_CTX_PARAMS
     OP_keyexch_gettable_ctx_params   OSSL_FUNC_KEYEXCH_GETTABLE_CTX_PARAMS

A key exchange algorithm implementation may not implement all of these functions. In order to be a consistent set of functions a provider must implement OP_keyexch_newctx, OP_keyexch_freectx, OP_keyexch_init and OP_keyexch_derive. All other functions are optional.

A key exchange algorithm must also implement some mechanism for generating, loading or importing keys via the key management (OSSL_OP_KEYMGMT) operation. See provider-keymgmt(7) for further details.

## Context Management Functions

`OP_keyexch_newctx()` should create and return a pointer to a provider side structure for holding context information during a key exchange operation. A pointer to this context will be passed back in a number of the other key exchange operation function calls. The parameter provctx is the provider context generated during provider initialisation (see provider(7)).

`OP_keyexch_freectx()` is passed a pointer to the provider side key exchange context in the ctx parameter. This function should free any resources associated with that context.

`OP_keyexch_dupctx()` should duplicate the provider side key exchange context in the ctx parameter and return the duplicate copy.

## Shared Secret Derivation Functions

`OP_keyexch_init()` initialises a key exchange operation given a provider side key exchange context in the ctx parameter, and a pointer to a provider key object in the provkey parameter. The key object should have been previously generated, loaded or imported into the provider using the key management (OSSL_OP_KEYMGMT) operation (see provider-keymgmt(7)>.

`OP_keyexch_set_peer()` is called to supply the peer's public key (in the provkey parameter) to be used when deriving the shared secret. It is also passed a previously initialised key exchange context in the ctx parameter. The key object should have been previously generated, loaded or imported into the provider using the key management (OSSL_OP_KEYMGMT) operation (see provider-keymgmt(7)>.

`OP_keyexch_derive()` performs the actual key exchange itself by deriving a shared secret. A previously initialised key exchange context is passed in the ctx parameter. The derived secret should be written to the location secret which should not exceed outlen bytes. The length of the shared secret should be written to *secretlen. If secret is NULL then the maximum length of the shared secret should be written to *secretlen.

## Key Exchange Parameters Functions

`OP_keyexch_set_ctx_params()` sets key exchange parameters associated with the given provider side key exchange context ctx to params, see Key Exchange Parameters. Any parameter settings are additional to any that were previously set.

`OP_keyexch_get_ctx_params()` gets key exchange parameters associated with the given provider side key exchange context ctx into params, see Key Exchange Parameters.

`OP_keyexch_settable_ctx_params()` yields a constant OSSL_PARAM array that describes the settable parameters, i.e. parameters that can be used with `OP_signature_set_ctx_params()`. If `OP_keyexch_settable_ctx_params()` is present, `OP_keyexch_set_ctx_params()` must also be present, and vice versa. Similarly, `OP_keyexch_gettable_ctx_params()` yields a constant OSSL_PARAM array that describes the gettable parameters, i.e. parameters that can be handled by `OP_signature_get_ctx_params()`. If `OP_keyexch_gettable_ctx_params()` is present, `OP_keyexch_get_ctx_params()` must also be present, and vice versa. See OSSL_PARAM(3) for the use of OSSL_PARAM as parameter descriptor.

Notice that not all settable parameters are also gettable, and vice versa.

## Key Exchange Parameters

See OSSL_PARAM(3) for further details on the parameters structure used by the `OP_keyexch_set_ctx_params()` and `OP_keyexch_get_ctx_params()` functions.

Parameters currently recognised by built-in key exchange algorithms are as follows. Not all parameters are relevant to, or are understood by all key exchange algorithms:

"pad" (OSSL_EXCHANGE_PARAM_PAD) <unsigned integer>

  Sets the padding mode for the associated key exchange ctx. Setting a value of 1 will turn padding on. Setting a vlue of 0 will turn padding off. If padding is off then the derived shared secret may be smaller than the largest possible secret size. If padding is on then the derived shared secret will have its first bytes filled with 0s where necessary to make the shared secret the same size as the largest possible secret size.

"ecdh-cofactor-mode" (OSSL_EXCHANGE_PARAM_EC_ECDH_COFACTOR_MODE) <integer>

  Sets/gets the ECDH mode of operation for the associated key exchange ctx.

  In the context of an Elliptic Curve Diffie-Hellman key exchange, this parameter can be used to select between the plain Diffie-Hellman (DH) or Cofactor Diffie-Hellman (CDH) variants of the key exchange algorithm.

  When setting, the value should be 1, 0 or -1, respectively forcing cofactor mode on, off, or resetting it to the default for the private key associated with the given key exchange ctx.

  When getting, the value should be either 1 or 0, respectively signaling if the cofactor mode is on or off.

  See also provider-keymgmt(7) for the related OSSL_PKEY_PARAM_USE_COFACTOR_ECDH parameter that can be set on a per-key basis.

"kdf-type" (OSSL_EXCHANGE_PARAM_KDF_TYPE) <utf8_string>

  Sets/gets the Key Derivation Function type to apply within the associated key exchange ctx.

"kdf-digest" (OSSL_EXCHANGE_PARAM_KDF_DIGEST) <utf8_string>

  Sets/gets the Digest algorithm to be used as part of the Key Derivation Function associated with the given key exchange ctx.

"kdf-digest-props" (OSSL_EXCHANGE_PARAM_KDF_DIGEST_PROPS) <utf8_string>

  Sets properties to be used upon look up of the implementation for the selected Digest algorithm for the Key Derivation Function associated with the given key exchange ctx.

"kdf-outlen" (OSSL_EXCHANGE_PARAM_KDF_OUTLEN) <size_t>

  Sets/gets the desired size for the output of the chosen Key Derivation Function associated with the given key exchange ctx.

"kdf-ukm" (OSSL_EXCHANGE_PARAM_KDF_UKM) <octet_string>

  Sets/gets User Key Material to be used as part of the selected Key Derivation Function associated with the given key exchange ctx.

"kdf-ukm-len" (OSSL_EXCHANGE_PARAM_KDF_UKM_LEN) <size_t>

  Sets/gets the size of the User Key Material to be used as part of the selected Key Derivation Function associated with the given key exchange ctx.

# RETURN VALUES

`OP_keyexch_newctx()` and `OP_keyexch_dupctx()` should return the newly created provider side key exchange context, or NULL on failure.

`OP_keyexch_init()`, `OP_keyexch_set_peer()`, `OP_keyexch_derive()`, `OP_keyexch_set_params()`, and `OP_keyexch_get_params()` should return 1 for success or 0 on error.

`OP_keyexch_settable_ctx_params()` and `OP_keyexch_gettable_ctx_params()` should always return a constant OSSL_PARAM array.

# SEE ALSO

provider(7)

# HISTORY

The provider KEYEXCH interface was introduced in OpenSSL 3.0.

# COPYRIGHT

Copyright 2019 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.