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 An B<OSSL_PARAM> array must have a terminating element, where I<key>
68 is NULL. The usual full terminating template is:
70 { NULL, 0, NULL, 0, 0 }
72 This can also be specified using L<OSSL_PARAM_END(3)>.
74 =head2 B<OSSL_PARAM> fields
80 The identity of the parameter in the form of a string.
82 In an B<OSSL_PARAM> array, an item with this field set to NULL is
83 considered a terminating item.
87 The I<data_type> is a value that describes the type and organization of
89 See L</Supported types> below for a description of the types.
95 I<data> is a pointer to the memory where the parameter data is (when
96 setting parameters) or shall (when requesting parameters) be stored,
97 and I<data_size> is its size in bytes.
98 The organization of the data depends on the parameter type and flag.
100 When I<requesting parameters>, it's acceptable for I<data> to be NULL.
101 This can be used by the I<requestor> to figure out dynamically exactly
102 how much buffer space is needed to store the parameter data.
103 In this case, I<data_size> is ignored.
105 When the B<OSSL_PARAM> is used as a parameter descriptor, I<data>
107 If I<data_size> is zero, it means that an arbitrary data size is
108 accepted, otherwise it specifies the maximum size allowed.
112 When an array of B<OSSL_PARAM> is used to request data, the
113 I<responder> must set this field to indicate size of the parameter
114 data, including padding as the case may be.
115 In case the I<data_size> is an unsuitable size for the data, the
116 I<responder> must still set this field to indicate the minimum data
118 (further notes on this in L</NOTES> below).
120 When the B<OSSL_PARAM> is used as a parameter descriptor,
121 I<return_size> should be ignored.
127 The key names and associated types are defined by the entity that
128 offers these parameters, i.e. names for parameters provided by the
129 OpenSSL libraries are defined by the libraries, and names for
130 parameters provided by providers are defined by those providers,
131 except for the pointer form of strings (see data type descriptions
133 Entities that want to set or request parameters need to know what
134 those keys are and of what type, any functionality between those two
135 entities should remain oblivious and just pass the B<OSSL_PARAM> array
138 =head2 Supported types
140 The I<data_type> field can be one of the following types:
144 =item B<OSSL_PARAM_INTEGER>
146 =item B<OSSL_PARAM_UNSIGNED_INTEGER>
148 The parameter data is an integer (signed or unsigned) of arbitrary
149 length, organized in native form, i.e. most significant byte first on
150 Big-Endian systems, and least significant byte first on Little-Endian
153 =item B<OSSL_PARAM_REAL>
155 The parameter data is a floating point value in native form.
157 =item B<OSSL_PARAM_UTF8_STRING>
159 The parameter data is a printable string.
161 =item B<OSSL_PARAM_OCTET_STRING>
163 The parameter data is an arbitrary string of bytes.
165 =item B<OSSL_PARAM_UTF8_PTR>
167 The parameter data is a pointer to a printable string.
169 The difference between this and B<OSSL_PARAM_UTF8_STRING> is that I<data>
170 doesn't point directly at the data, but to a pointer that points to the data.
172 If there is any uncertainty about which to use, B<OSSL_PARAM_UTF8_STRING> is
173 almost certainly the correct choice.
175 This is used to indicate that constant data is or will be passed,
176 and there is therefore no need to copy the data that is passed, just
179 I<data_size> must be set to the size of the data, not the size of the
181 If this is used in a parameter request,
182 I<data_size> is not relevant. However, the I<responder> will set
183 I<return_size> to the size of the data.
185 Note that the use of this type is B<fragile> and can only be safely
186 used for data that remains constant and in a constant location for a
187 long enough duration (such as the life-time of the entity that
188 offers these parameters).
190 =item B<OSSL_PARAM_OCTET_PTR>
192 The parameter data is a pointer to an arbitrary string of bytes.
194 The difference between this and B<OSSL_PARAM_OCTET_STRING> is that
195 I<data> doesn't point directly at the data, but to a pointer that
198 If there is any uncertainty about which to use, B<OSSL_PARAM_OCTET_STRING> is
199 almost certainly the correct choice.
201 This is used to indicate that constant data is or will be passed, and
202 there is therefore no need to copy the data that is passed, just the
205 I<data_size> must be set to the size of the data, not the size of the
207 If this is used in a parameter request,
208 I<data_size> is not relevant. However, the I<responder> will set
209 I<return_size> to the size of the data.
211 Note that the use of this type is B<fragile> and can only be safely
212 used for data that remains constant and in a constant location for a
213 long enough duration (such as the life-time of the entity that
214 offers these parameters).
220 Both when setting and requesting parameters, the functions that are
221 called will have to decide what is and what is not an error.
222 The recommended behaviour is:
228 Keys that a I<setter> or I<responder> doesn't recognise should simply
230 That in itself isn't an error.
234 If the keys that a called I<setter> recognises form a consistent
235 enough set of data, that call should succeed.
239 Apart from the I<return_size>, a I<responder> must never change the fields
241 To return a value, it should change the contents of the memory that
246 If the data type for a key that it's associated with is incorrect,
247 the called function may return an error.
249 The called function may also try to convert the data to a suitable
250 form (for example, it's plausible to pass a large number as an octet
251 string, so even though a given key is defined as an
252 B<OSSL_PARAM_UNSIGNED_INTEGER>, is plausible to pass the value as an
253 B<OSSL_PARAM_OCTET_STRING>), but this is in no way mandatory.
257 If a I<responder> finds that some data sizes are too small for the
258 requested data, it must set I<return_size> for each such
259 B<OSSL_PARAM> item to the minimum required size, and eventually return
264 For the integer type parameters (B<OSSL_PARAM_UNSIGNED_INTEGER> and
265 B<OSSL_PARAM_INTEGER>), a I<responder> may choose to return an error
266 if the I<data_size> isn't a suitable size (even if I<data_size> is
267 bigger than needed). If the I<responder> finds the size suitable, it
268 must fill all I<data_size> bytes and ensure correct padding for the
269 native endianness, and set I<return_size> to the same value as
274 =begin comment RETURN VALUES doesn't make sense for a manual that only
275 describes a type, but document checkers still want that section, and
276 to have more than just the section title.
286 A couple of examples to just show how B<OSSL_PARAM> arrays could be
291 This example is for setting parameters on some object:
293 #include <openssl/core.h>
295 const char *foo = "some string";
296 size_t foo_l = strlen(foo) + 1;
297 const char bar[] = "some other string";
299 { "foo", OSSL_PARAM_UTF8_STRING_PTR, &foo, foo_l, 0 },
300 { "bar", OSSL_PARAM_UTF8_STRING, &bar, sizeof(bar), 0 },
301 { NULL, 0, NULL, 0, 0 }
306 This example is for requesting parameters on some object:
308 const char *foo = NULL;
312 OSSL_PARAM request[] = {
313 { "foo", OSSL_PARAM_UTF8_STRING_PTR, &foo, 0 /*irrelevant*/, 0 },
314 { "bar", OSSL_PARAM_UTF8_STRING, &bar, sizeof(bar), 0 },
315 { NULL, 0, NULL, 0, 0 }
318 A I<responder> that receives this array (as I<params> in this example)
319 could fill in the parameters like this:
321 /* OSSL_PARAM *params */
325 for (i = 0; params[i].key != NULL; i++) {
326 if (strcmp(params[i].key, "foo") == 0) {
327 *(char **)params[i].data = "foo value";
328 params[i].return_size = 10; /* size of "foo value" */
329 } else if (strcmp(params[i].key, "bar") == 0) {
330 memcpy(params[i].data, "bar value", 10);
331 params[i].return_size = 10; /* size of "bar value" */
333 /* Ignore stuff we don't know */
338 L<openssl-core.h(7)>, L<OSSL_PARAM_get_int(3)>
342 B<OSSL_PARAM> was added in OpenSSL 3.0.
346 Copyright 2019-2020 The OpenSSL Project Authors. All Rights Reserved.
348 Licensed under the Apache License 2.0 (the "License"). You may not use
349 this file except in compliance with the License. You can obtain a copy
350 in the file LICENSE in the source distribution or at
351 L<https://www.openssl.org/source/license.html>.