- NAME
- SYNOPSIS
- DESCRIPTION
  - Context Management Functions
  - Encryption Functions
  - Decryption Functions
  - Asymmetric Cipher Parameters
- RETURN VALUES
- SEE ALSO
- HISTORY
- COPYRIGHT

# NAME

provider-asym_cipher - The asym_cipher library <-> provider functions

# SYNOPSIS

    #include <openssl/core_dispatch.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 *OSSL_FUNC_asym_cipher_newctx(void *provctx);
    void OSSL_FUNC_asym_cipher_freectx(void *ctx);
    void *OSSL_FUNC_asym_cipher_dupctx(void *ctx);

    /* Encryption */
    int OSSL_FUNC_asym_cipher_encrypt_init(void *ctx, void *provkey,
                                           const OSSL_PARAM params[]);
    int OSSL_FUNC_asym_cipher_encrypt(void *ctx, unsigned char *out, size_t *outlen,
                                      size_t outsize, const unsigned char *in,
                                      size_t inlen);

    /* Decryption */
    int OSSL_FUNC_asym_cipher_decrypt_init(void *ctx, void *provkey,
                                           const OSSL_PARAM params[]);
    int OSSL_FUNC_asym_cipher_decrypt(void *ctx, unsigned char *out, size_t *outlen,
                                      size_t outsize, const unsigned char *in,
                                      size_t inlen);

    /* Asymmetric Cipher parameters */
    int OSSL_FUNC_asym_cipher_get_ctx_params(void *ctx, OSSL_PARAM params[]);
    const OSSL_PARAM *OSSL_FUNC_asym_cipher_gettable_ctx_params(void *provctx);
    int OSSL_FUNC_asym_cipher_set_ctx_params(void *ctx, const OSSL_PARAM params[]);
    const OSSL_PARAM *OSSL_FUNC_asym_cipher_settable_ctx_params(void *provctx);

# DESCRIPTION

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

The asymmetric cipher (OSSL_OP_ASYM_CIPHER) operation enables providers to implement asymmetric cipher algorithms and make them available to applications via the API functions EVP_PKEY_encrypt(3), EVP_PKEY_decrypt(3) and other related functions).

All "functions" mentioned here are passed as function pointers between libcrypto and the provider in OSSL_DISPATCH(3) arrays via OSSL_ALGORITHM(3) 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(3) element named OSSL_FUNC_{name}. For example, the "function" OSSL_FUNC_asym_cipher_newctx() has these:

    typedef void *(OSSL_FUNC_asym_cipher_newctx_fn)(void *provctx);
    static ossl_inline OSSL_FUNC_asym_cipher_newctx_fn
        OSSL_FUNC_asym_cipher_newctx(const OSSL_DISPATCH *opf);

OSSL_DISPATCH(3) arrays are indexed by numbers that are provided as macros in openssl-core_dispatch.h(7), as follows:

    OSSL_FUNC_asym_cipher_newctx               OSSL_FUNC_ASYM_CIPHER_NEWCTX
    OSSL_FUNC_asym_cipher_freectx              OSSL_FUNC_ASYM_CIPHER_FREECTX
    OSSL_FUNC_asym_cipher_dupctx               OSSL_FUNC_ASYM_CIPHER_DUPCTX

    OSSL_FUNC_asym_cipher_encrypt_init         OSSL_FUNC_ASYM_CIPHER_ENCRYPT_INIT
    OSSL_FUNC_asym_cipher_encrypt              OSSL_FUNC_ASYM_CIPHER_ENCRYPT

    OSSL_FUNC_asym_cipher_decrypt_init         OSSL_FUNC_ASYM_CIPHER_DECRYPT_INIT
    OSSL_FUNC_asym_cipher_decrypt              OSSL_FUNC_ASYM_CIPHER_DECRYPT

    OSSL_FUNC_asym_cipher_get_ctx_params       OSSL_FUNC_ASYM_CIPHER_GET_CTX_PARAMS
    OSSL_FUNC_asym_cipher_gettable_ctx_params  OSSL_FUNC_ASYM_CIPHER_GETTABLE_CTX_PARAMS
    OSSL_FUNC_asym_cipher_set_ctx_params       OSSL_FUNC_ASYM_CIPHER_SET_CTX_PARAMS
    OSSL_FUNC_asym_cipher_settable_ctx_params  OSSL_FUNC_ASYM_CIPHER_SETTABLE_CTX_PARAMS

An asymmetric cipher algorithm implementation may not implement all of these functions. In order to be a consistent set of functions a provider must implement OSSL_FUNC_asym_cipher_newctx and OSSL_FUNC_asym_cipher_freectx. It must also implement both of OSSL_FUNC_asym_cipher_encrypt_init and OSSL_FUNC_asym_cipher_encrypt, or both of OSSL_FUNC_asym_cipher_decrypt_init and OSSL_FUNC_asym_cipher_decrypt. OSSL_FUNC_asym_cipher_get_ctx_params is optional but if it is present then so must OSSL_FUNC_asym_cipher_gettable_ctx_params. Similarly, OSSL_FUNC_asym_cipher_set_ctx_params is optional but if it is present then so must OSSL_FUNC_asym_cipher_settable_ctx_params.

An asymmetric cipher 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

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

OSSL_FUNC_asym_cipher_freectx() is passed a pointer to the provider side asymmetric cipher context in the ctx parameter. This function should free any resources associated with that context.

OSSL_FUNC_asym_cipher_dupctx() should duplicate the provider side asymmetric cipher context in the ctx parameter and return the duplicate copy.

## Encryption Functions

OSSL_FUNC_asym_cipher_encrypt_init() initialises a context for an asymmetric encryption given a provider side asymmetric cipher context in the ctx parameter, and a pointer to a provider key object in the provkey parameter. The params, if not NULL, should be set on the context in a manner similar to using OSSL_FUNC_asym_cipher_set_ctx_params(). 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)). OSSL_FUNC_asym_cipher_encrypt() performs the actual encryption itself. A previously initialised asymmetric cipher context is passed in the ctx parameter. The data to be encrypted is pointed to by the in parameter which is inlen bytes long. Unless out is NULL, the encrypted data should be written to the location pointed to by the out parameter and it should not exceed outsize bytes in length. The length of the encrypted data should be written to *outlen. If out is NULL then the maximum length of the encrypted data should be written to *outlen.

## Decryption Functions

OSSL_FUNC_asym_cipher_decrypt_init() initialises a context for an asymmetric decryption given a provider side asymmetric cipher context in the ctx parameter, and a pointer to a provider key object in the provkey parameter. The params, if not NULL, should be set on the context in a manner similar to using OSSL_FUNC_asym_cipher_set_ctx_params(). 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)).

OSSL_FUNC_asym_cipher_decrypt() performs the actual decryption itself. A previously initialised asymmetric cipher context is passed in the ctx parameter. The data to be decrypted is pointed to by the in parameter which is inlen bytes long. Unless out is NULL, the decrypted data should be written to the location pointed to by the out parameter and it should not exceed outsize bytes in length. The length of the decrypted data should be written to *outlen. If out is NULL then the maximum length of the decrypted data should be written to *outlen.

## Asymmetric Cipher Parameters

See OSSL_PARAM(3) for further details on the parameters structure used by the OSSL_FUNC_asym_cipher_get_ctx_params() and OSSL_FUNC_asym_cipher_set_ctx_params() functions.

OSSL_FUNC_asym_cipher_get_ctx_params() gets asymmetric cipher parameters associated with the given provider side asymmetric cipher context ctx and stores them in params. Passing NULL for params should return true.

OSSL_FUNC_asym_cipher_set_ctx_params() sets the asymmetric cipher parameters associated with the given provider side asymmetric cipher context ctx to params. Any parameter settings are additional to any that were previously set. Passing NULL for params should return true.

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

"pad-mode" (OSSL_ASYM_CIPHER_PARAM_PAD_MODE) <UTF8 string> OR <integer>

  The type of padding to be used. The interpretation of this value will depend on the algorithm in use.

"digest" (OSSL_ASYM_CIPHER_PARAM_OAEP_DIGEST) <UTF8 string>

  Gets or sets the name of the OAEP digest algorithm used when OAEP padding is in use.

"digest" (OSSL_ASYM_CIPHER_PARAM_DIGEST) <UTF8 string>

  Gets or sets the name of the digest algorithm used by the algorithm (where applicable).

"digest-props" (OSSL_ASYM_CIPHER_PARAM_OAEP_DIGEST_PROPS) <UTF8 string>

  Gets or sets the properties to use when fetching the OAEP digest algorithm.

"digest-props" (OSSL_ASYM_CIPHER_PARAM_DIGEST_PROPS) <UTF8 string>

  Gets or sets the properties to use when fetching the cipher digest algorithm.

"mgf1-digest" (OSSL_ASYM_CIPHER_PARAM_MGF1_DIGEST) <UTF8 string>

  Gets or sets the name of the MGF1 digest algorithm used when OAEP or PSS padding is in use.

"mgf1-digest-props" (OSSL_ASYM_CIPHER_PARAM_MGF1_DIGEST_PROPS) <UTF8 string>

  Gets or sets the properties to use when fetching the MGF1 digest algorithm.

"oaep-label" (OSSL_ASYM_CIPHER_PARAM_OAEP_LABEL) <octet string ptr>

  Gets the OAEP label used when OAEP padding is in use.

"oaep-label" (OSSL_ASYM_CIPHER_PARAM_OAEP_LABEL) <octet string>

  Sets the OAEP label used when OAEP padding is in use.

"tls-client-version" (OSSL_ASYM_CIPHER_PARAM_TLS_CLIENT_VERSION) <unsigned integer>

  The TLS protocol version first requested by the client.

"tls-negotiated-version" (OSSL_ASYM_CIPHER_PARAM_TLS_CLIENT_VERSION) <unsigned integer>

  The negotiated TLS protocol version.

"implicit-rejection" (OSSL_ASYM_CIPHER_PARAM_IMPLICIT_REJECTION) <unsigned integer>

  Gets or sets the use of the implicit rejection mechanism for RSA PKCS#1 v1.5 decryption. When set (non zero value), the decryption API will return a deterministically random value if the PKCS#1 v1.5 padding check fails. This makes exploitation of the Bleichenbacher significantly harder, even if the code using the RSA decryption API is not implemented in side-channel free manner. Set by default in OpenSSL providers.

The OpenSSL FIPS provider also supports the following parameters:

"fips-indicator" (OSSL_ASYM_CIPHER_PARAM_FIPS_APPROVED_INDICATOR) <integer>

  A getter that returns 1 if the operation is FIPS approved, or 0 otherwise. This may be used after calling either OSSL_FUNC_asym_cipher_encrypt() or OSSL_FUNC_asym_cipher_decrypt(). It may return 0 if "key-check" is set to 0.

"key-check" (OSSL_ASYM_CIPHER_PARAM_FIPS_KEY_CHECK) <integer>

  If required this parameter should be set using either OSSL_FUNC_asym_cipher_encrypt_init() or OSSL_FUNC_asym_cipher_decrypt_init(). The default value of 1 causes an error during the init if the key is not FIPS approved (e.g. The key has a security strength of less than 112 bits). Setting this to 0 will ignore the error and set the approved "fips-indicator" to 0. This option breaks FIPS compliance if it causes the approved "fips-indicator" to return 0.

OSSL_FUNC_asym_cipher_gettable_ctx_params() and OSSL_FUNC_asym_cipher_settable_ctx_params() get a constant OSSL_PARAM(3) array that describes the gettable and settable parameters, i.e. parameters that can be used with OSSL_FUNC_asym_cipherget_ctx_params() and OSSL_FUNC_asym_cipher_set_ctx_params() respectively.

# RETURN VALUES

OSSL_FUNC_asym_cipher_newctx() and OSSL_FUNC_asym_cipher_dupctx() should return the newly created provider side asymmetric cipher context, or NULL on failure.

All other functions should return 1 for success or 0 on error.

# SEE ALSO

provider(7)

# HISTORY

The provider ASYM_CIPHER interface was introduced in OpenSSL 3.0. The Asymmetric Cipher Parameters "fips-indicator" and "key-check" were added in OpenSSL 3.4.

# COPYRIGHT

Copyright 2019-2025 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.