5 OSSL_PARAM - a structure to pass or request object parameters
9 #include <openssl/core.h>
11 typedef struct ossl_param_st OSSL_PARAM;
12 struct ossl_param_st {
13 const char *key; /* the name of the parameter */
14 unsigned char data_type; /* declare what kind of content is in data */
15 void *data; /* value being passed in or out */
16 size_t data_size; /* data size */
17 size_t return_size; /* returned size */
22 B<OSSL_PARAM> is a type that allows passing arbitrary data for some
23 object between two parties that have no or very little shared
24 knowledge about their respective internal structures for that object.
26 A typical usage example could be an application that wants to set some
27 parameters for an object, or wants to find out some parameters of an
30 Arrays of this type can be used for the following purposes:
34 =item * Setting parameters for some object
36 The caller sets up the B<OSSL_PARAM> array and calls some function
37 (the I<setter>) that has intimate knowledge about the object that can
38 take the data from the B<OSSL_PARAM> array and assign them in a
39 suitable form for the internal structure of the object.
41 =item * Request parameters of some object
43 The caller (the I<requestor>) sets up the B<OSSL_PARAM> array and
44 calls some function (the I<responder>) that has intimate knowledge
45 about the object, which can take the internal data of the object and
46 copy (possibly convert) that to the memory prepared by the
47 I<requestor> and pointed at with the B<OSSL_PARAM> I<data>.
49 =item * Request parameter descriptors
51 The caller gets an array of constant B<OSSL_PARAM>, which describe
52 available parameters and some of their properties; name, data type and
54 For a detailed description of each field for this use, see the field
57 The caller may then use the information from this descriptor array to
58 build up its own B<OSSL_PARAM> array to pass down to a I<setter> or
63 Normally, the order of the an B<OSSL_PARAM> array is not relevant.
64 However, if the I<responder> can handle multiple elements with the
65 same key, those elements must be handled in the order they are in.
67 =head2 B<OSSL_PARAM> fields
73 The identity of the parameter in the form of a string.
77 The I<data_type> is a value that describes the type and organization of
79 See L</Supported types> below for a description of the types.
85 I<data> is a pointer to the memory where the parameter data is (when
86 setting parameters) or shall (when requesting parameters) be stored,
87 and I<data_size> is its size in bytes.
88 The organization of the data depends on the parameter type and flag.
90 When I<requesting parameters>, it's acceptable for I<data> to be NULL.
91 This can be used by the I<requestor> to figure out dynamically exactly
92 how much buffer space is needed to store the parameter data.
93 In this case, I<data_size> is ignored.
95 When the B<OSSL_PARAM> is used as a parameter descriptor, I<data>
97 If I<data_size> is zero, it means that an arbitrary data size is
98 accepted, otherwise it specifies the maximum size allowed.
102 When an array of B<OSSL_PARAM> is used to request data, the
103 I<responder> must set this field to indicate size of the parameter
104 data, including padding as the case may be.
105 In case the I<data_size> is an unsuitable size for the data, the
106 I<responder> must still set this field to indicate the minimum data
108 (further notes on this in L</NOTES> below).
110 When the B<OSSL_PARAM> is used as a parameter descriptor,
111 I<return_size> should be ignored.
117 The key names and associated types are defined by the entity that
118 offers these parameters, i.e. names for parameters provided by the
119 OpenSSL libraries are defined by the libraries, and names for
120 parameters provided by providers are defined by those providers,
121 except for the pointer form of strings (see data type descriptions
123 Entities that want to set or request parameters need to know what
124 those keys are and of what type, any functionality between those two
125 entities should remain oblivious and just pass the B<OSSL_PARAM> array
128 =head2 Supported types
130 The I<data_type> field can be one of the following types:
134 =item B<OSSL_PARAM_INTEGER>
136 =item B<OSSL_PARAM_UNSIGNED_INTEGER>
138 The parameter data is an integer (signed or unsigned) of arbitrary
139 length, organized in native form, i.e. most significant byte first on
140 Big-Endian systems, and least significant byte first on Little-Endian
143 =item B<OSSL_PARAM_REAL>
145 The parameter data is a floating point value in native form.
147 =item B<OSSL_PARAM_UTF8_STRING>
149 The parameter data is a printable string.
151 =item B<OSSL_PARAM_OCTET_STRING>
153 The parameter data is an arbitrary string of bytes.
155 =item B<OSSL_PARAM_UTF8_PTR>
157 The parameter data is a pointer to a printable string.
159 The difference between this and B<OSSL_PARAM_UTF8_STRING> is that I<data>
160 doesn't point directly at the data, but to a pointer that points to the data.
162 This is used to indicate that constant data is or will be passed,
163 and there is therefore no need to copy the data that is passed, just
166 I<data_size> must be set to the size of the data, not the size of the
168 If this is used in a parameter request,
169 I<data_size> is not relevant. However, the I<responder> will set
170 I<return_size> to the size of the data.
172 Note that the use of this type is B<fragile> and can only be safely
173 used for data that remains constant and in a constant location for a
174 long enough duration (such as the life-time of the entity that
175 offers these parameters).
177 =item B<OSSL_PARAM_OCTET_PTR>
179 The parameter data is a pointer to an arbitrary string of bytes.
181 The difference between this and B<OSSL_PARAM_OCTET_STRING> is that
182 I<data> doesn't point directly at the data, but to a pointer that
185 This is used to indicate that constant data is or will be passed, and
186 there is therefore no need to copy the data that is passed, just the
189 I<data_size> must be set to the size of the data, not the size of the
191 If this is used in a parameter request,
192 I<data_size> is not relevant. However, the I<responder> will set
193 I<return_size> to the size of the data.
195 Note that the use of this type is B<fragile> and can only be safely
196 used for data that remains constant and in a constant location for a
197 long enough duration (such as the life-time of the entity that
198 offers these parameters).
204 Both when setting and requesting parameters, the functions that are
205 called will have to decide what is and what is not an error.
206 The recommended behaviour is:
212 Keys that a I<setter> or I<responder> doesn't recognise should simply
214 That in itself isn't an error.
218 If the keys that a called I<setter> recognises form a consistent
219 enough set of data, that call should succeed.
223 Apart from the I<return_size>, a I<responder> must never change the fields
225 To return a value, it should change the contents of the memory that
230 If the data type for a key that it's associated with is incorrect,
231 the called function may return an error.
233 The called function may also try to convert the data to a suitable
234 form (for example, it's plausible to pass a large number as an octet
235 string, so even though a given key is defined as an
236 B<OSSL_PARAM_UNSIGNED_INTEGER>, is plausible to pass the value as an
237 B<OSSL_PARAM_OCTET_STRING>), but this is in no way mandatory.
241 If a I<responder> finds that some data sizes are too small for the
242 requested data, it must set I<return_size> for each such
243 B<OSSL_PARAM> item to the minimum required size, and eventually return
248 For the integer type parameters (B<OSSL_PARAM_UNSIGNED_INTEGER> and
249 B<OSSL_PARAM_INTEGER>), a I<responder> may choose to return an error
250 if the I<data_size> isn't a suitable size (even if I<data_size> is
251 bigger than needed). If the I<responder> finds the size suitable, it
252 must fill all I<data_size> bytes and ensure correct padding for the
253 native endianness, and set I<return_size> to the same value as
258 =begin comment RETURN VALUES doesn't make sense for a manual that only
259 describes a type, but document checkers still want that section, and
260 to have more than just the section title.
270 A couple of examples to just show how B<OSSL_PARAM> arrays could be
275 This example is for setting parameters on some object:
277 #include <openssl/core.h>
279 const char *foo = "some string";
280 size_t foo_l = strlen(foo) + 1;
281 const char bar[] = "some other string";
283 { "foo", OSSL_PARAM_UTF8_STRING_PTR, &foo, foo_l, 0 },
284 { "bar", OSSL_PARAM_UTF8_STRING, &bar, sizeof(bar), 0 },
285 { NULL, 0, NULL, 0, NULL }
290 This example is for requesting parameters on some object:
292 const char *foo = NULL;
296 OSSL_PARAM request[] = {
297 { "foo", OSSL_PARAM_UTF8_STRING_PTR, &foo, 0 /*irrelevant*/, 0 },
298 { "bar", OSSL_PARAM_UTF8_STRING, &bar, sizeof(bar), 0 },
299 { NULL, 0, NULL, 0, NULL }
302 A I<responder> that receives this array (as I<params> in this example)
303 could fill in the parameters like this:
305 /* OSSL_PARAM *params */
309 for (i = 0; params[i].key != NULL; i++) {
310 if (strcmp(params[i].key, "foo") == 0) {
311 *(char **)params[i].data = "foo value";
312 params[i].return_size = 10; /* size of "foo value" */
313 } else if (strcmp(params[i].key, "bar") == 0) {
314 memcpy(params[i].data, "bar value", 10);
315 params[i].return_size = 10; /* size of "bar value" */
317 /* Ignore stuff we don't know */
322 L<openssl-core.h(7)>, L<OSSL_PARAM_get_int(3)>
326 B<OSSL_PARAM> was added in OpenSSL 3.0.
330 Copyright 2019 The OpenSSL Project Authors. All Rights Reserved.
332 Licensed under the Apache License 2.0 (the "License"). You may not use
333 this file except in compliance with the License. You can obtain a copy
334 in the file LICENSE in the source distribution or at
335 L<https://www.openssl.org/source/license.html>.