OSSL_PARAM.3ossl - Man Page

a structure to pass or request object parameters

Synopsis

 #include <openssl/core.h>

 typedef struct ossl_param_st OSSL_PARAM;
 struct ossl_param_st {
     const char *key;             /* the name of the parameter */
     unsigned char data_type;     /* declare what kind of content is in data */
     void *data;                  /* value being passed in or out */
     size_t data_size;            /* data size */
     size_t return_size;          /* returned size */
 };

Description

OSSL_PARAM is a type that allows passing arbitrary data for some object between two parties that have no or very little shared knowledge about their respective internal structures for that object.

A typical usage example could be an application that wants to set some parameters for an object, or wants to find out some parameters of an object.

Arrays of this type can be used for the following purposes:

Normally, the order of the an OSSL_PARAM array is not relevant. However, if the responder can handle multiple elements with the same key, those elements must be handled in the order they are in.

An OSSL_PARAM array must have a terminating element, where key is NULL.  The usual full terminating template is:

    { NULL, 0, NULL, 0, 0 }

This can also be specified using OSSL_PARAM_END(3).

Functional support

Libcrypto offers a limited set of helper functions to handle OSSL_PARAM items and arrays, please see OSSL_PARAM_get_int(3). Developers are free to extend or replace those as they see fit.

OSSL_PARAM fields

key

The identity of the parameter in the form of a string.

In an OSSL_PARAM array, an item with this field set to NULL is considered a terminating item.

data_type

The data_type is a value that describes the type and organization of the data. See “Supported types” below for a description of the types.

data
data_size

data is a pointer to the memory where the parameter data is (when setting parameters) or shall (when requesting parameters) be stored, and data_size is its size in bytes. The organization of the data depends on the parameter type and flag.

The data_size needs special attention with the parameter type OSSL_PARAM_UTF8_STRING in relation to C strings.  When setting parameters, the size should be set to the length of the string, not counting the terminating NUL byte.  When requesting parameters, the size should be set to the size of the buffer to be populated, which should accomodate enough space for a terminating NUL byte.

When requesting parameters, it's acceptable for data to be NULL. This can be used by the requestor to figure out dynamically exactly how much buffer space is needed to store the parameter data. In this case, data_size is ignored.

When the OSSL_PARAM is used as a parameter descriptor, data should be ignored. If data_size is zero, it means that an arbitrary data size is accepted, otherwise it specifies the maximum size allowed.

return_size

When an array of OSSL_PARAM is used to request data, the responder must set this field to indicate size of the parameter data, including padding as the case may be. In case the data_size is an unsuitable size for the data, the responder must still set this field to indicate the minimum data size required. (further notes on this in “Notes” below).

When the OSSL_PARAM is used as a parameter descriptor, return_size should be ignored.

NOTE:

The key names and associated types are defined by the entity that offers these parameters, i.e. names for parameters provided by the OpenSSL libraries are defined by the libraries, and names for parameters provided by providers are defined by those providers, except for the pointer form of strings (see data type descriptions below). Entities that want to set or request parameters need to know what those keys are and of what type, any functionality between those two entities should remain oblivious and just pass the OSSL_PARAM array along.

Supported types

The data_type field can be one of the following types:

OSSL_PARAM_INTEGER
OSSL_PARAM_UNSIGNED_INTEGER

The parameter data is an integer (signed or unsigned) of arbitrary length, organized in native form, i.e. most significant byte first on Big-Endian systems, and least significant byte first on Little-Endian systems.

OSSL_PARAM_REAL

The parameter data is a floating point value in native form.

OSSL_PARAM_UTF8_STRING

The parameter data is a printable string.

OSSL_PARAM_OCTET_STRING

The parameter data is an arbitrary string of bytes.

OSSL_PARAM_UTF8_PTR

The parameter data is a pointer to a printable string.

The difference between this and OSSL_PARAM_UTF8_STRING is that data doesn't point directly at the data, but to a pointer that points to the data.

If there is any uncertainty about which to use, OSSL_PARAM_UTF8_STRING is almost certainly the correct choice.

This is used to indicate that constant data is or will be passed, and there is therefore no need to copy the data that is passed, just the pointer to it.

data_size must be set to the size of the data, not the size of the pointer to the data. If this is used in a parameter request, data_size is not relevant.  However, the responder will set return_size to the size of the data.

Note that the use of this type is fragile and can only be safely used for data that remains constant and in a constant location for a long enough duration (such as the life-time of the entity that offers these parameters).

OSSL_PARAM_OCTET_PTR

The parameter data is a pointer to an arbitrary string of bytes.

The difference between this and OSSL_PARAM_OCTET_STRING is that data doesn't point directly at the data, but to a pointer that points to the data.

If there is any uncertainty about which to use, OSSL_PARAM_OCTET_STRING is almost certainly the correct choice.

This is used to indicate that constant data is or will be passed, and there is therefore no need to copy the data that is passed, just the pointer to it.

data_size must be set to the size of the data, not the size of the pointer to the data. If this is used in a parameter request, data_size is not relevant.  However, the responder will set return_size to the size of the data.

Note that the use of this type is fragile and can only be safely used for data that remains constant and in a constant location for a long enough duration (such as the life-time of the entity that offers these parameters).

Notes

Both when setting and requesting parameters, the functions that are called will have to decide what is and what is not an error. The recommended behaviour is:

Examples

A couple of examples to just show how OSSL_PARAM arrays could be set up.

Example 1

This example is for setting parameters on some object:

    #include <openssl/core.h>

    const char *foo = "some string";
    size_t foo_l = strlen(foo);
    const char bar[] = "some other string";
    OSSL_PARAM set[] = {
        { "foo", OSSL_PARAM_UTF8_STRING_PTR, &foo, foo_l, 0 },
        { "bar", OSSL_PARAM_UTF8_STRING, &bar, sizeof(bar) - 1, 0 },
        { NULL, 0, NULL, 0, 0 }
    };

Example 2

This example is for requesting parameters on some object:

    const char *foo = NULL;
    size_t foo_l;
    char bar[1024];
    size_t bar_l;
    OSSL_PARAM request[] = {
        { "foo", OSSL_PARAM_UTF8_STRING_PTR, &foo, 0 /*irrelevant*/, 0 },
        { "bar", OSSL_PARAM_UTF8_STRING, &bar, sizeof(bar), 0 },
        { NULL, 0, NULL, 0, 0 }
    };

A responder that receives this array (as params in this example) could fill in the parameters like this:

    /* OSSL_PARAM *params */

    int i;

    for (i = 0; params[i].key != NULL; i++) {
        if (strcmp(params[i].key, "foo") == 0) {
            *(char **)params[i].data = "foo value";
            params[i].return_size = 9; /* length of "foo value" string */
        } else if (strcmp(params[i].key, "bar") == 0) {
            memcpy(params[i].data, "bar value", 10);
            params[i].return_size = 9; /* length of "bar value" string */
        }
        /* Ignore stuff we don't know */
    }

See Also

openssl-core.h(7), OSSL_PARAM_get_int(3), OSSL_PARAM_dup(3)

History

OSSL_PARAM was added in OpenSSL 3.0.

Referenced By

evp.7ossl(7), EVP_DigestInit.3ossl(3), EVP_EncryptInit.3ossl(3), EVP_KDF.3ossl(3), EVP_KEYMGMT.3ossl(3), EVP_MAC.3ossl(3), EVP_MAC-BLAKE2.7ossl(7), EVP_MAC-CMAC.7ossl(7), EVP_MAC-GMAC.7ossl(7), EVP_MAC-HMAC.7ossl(7), EVP_MAC-KMAC.7ossl(7), EVP_MAC-Poly1305.7ossl(7), EVP_MAC-Siphash.7ossl(7), EVP_MD-common.7ossl(7), EVP_MD-MD5-SHA1.7ossl(7), EVP_MD-MDC2.7ossl(7), EVP_MD-SHA1.7ossl(7), EVP_MD-SHAKE.7ossl(7), EVP_PKEY_CTX_set_params.3ossl(3), EVP_PKEY_fromdata.3ossl(3), EVP_PKEY_gettable_params.3ossl(3), EVP_PKEY_settable_params.3ossl(3), EVP_PKEY_todata.3ossl(3), EVP_RAND.3ossl(3), migration_guide.7ossl(7), openssl-core.h.7ossl(7), openssl-core_names.h.7ossl(7), OSSL_DECODER.3ossl(3), OSSL_DECODER_CTX.3ossl(3), OSSL_ENCODER.3ossl(3), OSSL_ENCODER_CTX.3ossl(3), OSSL_PARAM_allocate_from_text.3ossl(3), OSSL_PARAM_BLD.3ossl(3), OSSL_PARAM_dup.3ossl(3), OSSL_PARAM_int.3ossl(3), OSSL_PROVIDER.3ossl(3), OSSL_PROVIDER-FIPS.7ossl(7), OSSL_PROVIDER-legacy.7ossl(7), provider-asym_cipher.7ossl(7), provider-base.7ossl(7), provider-cipher.7ossl(7), provider-decoder.7ossl(7), provider-digest.7ossl(7), provider-encoder.7ossl(7), provider-kdf.7ossl(7), provider-kem.7ossl(7), provider-keyexch.7ossl(7), provider-keymgmt.7ossl(7), provider-mac.7ossl(7), provider-object.7ossl(7), provider-rand.7ossl(7), provider-signature.7ossl(7).

2021-09-09 3.0.0 OpenSSL