1CRYPTO(7ossl)                       OpenSSL                      CRYPTO(7ossl)
2
3
4

NAME

6       crypto - OpenSSL cryptographic library
7

SYNOPSIS

9       See the individual manual pages for details.
10

DESCRIPTION

12       The OpenSSL crypto library ("libcrypto") implements a wide range of
13       cryptographic algorithms used in various Internet standards. The
14       services provided by this library are used by the OpenSSL
15       implementations of TLS and CMS, and they have also been used to
16       implement many other third party products and protocols.
17
18       The functionality includes symmetric encryption, public key
19       cryptography, key agreement, certificate handling, cryptographic hash
20       functions, cryptographic pseudo-random number generators, message
21       authentication codes (MACs), key derivation functions (KDFs), and
22       various utilities.
23
24   Algorithms
25       Cryptographic primitives such as the SHA256 digest, or AES encryption
26       are referred to in OpenSSL as "algorithms". Each algorithm may have
27       multiple implementations available for use. For example the RSA
28       algorithm is available as a "default" implementation suitable for
29       general use, and a "fips" implementation which has been validated to
30       FIPS standards for situations where that is important. It is also
31       possible that a third party could add additional implementations such
32       as in a hardware security module (HSM).
33
34   Operations
35       Different algorithms can be grouped together by their purpose. For
36       example there are algorithms for encryption, and different algorithms
37       for digesting data.  These different groups are known as "operations"
38       in OpenSSL. Each operation has a different set of functions associated
39       with it. For example to perform an encryption operation using AES (or
40       any other encryption algorithm) you would use the encryption functions
41       detailed on the EVP_EncryptInit(3) page. Or to perform a digest
42       operation using SHA256 then you would use the digesting functions on
43       the EVP_DigestInit(3) page.
44
45   Providers
46       A provider in OpenSSL is a component that collects together algorithm
47       implementations. In order to use an algorithm you must have at least
48       one provider loaded that contains an implementation of it. OpenSSL
49       comes with a number of providers and they may also be obtained from
50       third parties. If you don't load a provider explicitly (either in
51       program code or via config) then the OpenSSL built-in "default"
52       provider will be automatically loaded.
53
54   Library contexts
55       A library context can be thought of as a "scope" within which
56       configuration options take effect. When a provider is loaded, it is
57       only loaded within the scope of a given library context. In this way it
58       is possible for different components of a complex application to each
59       use a different library context and have different providers loaded
60       with different configuration settings.
61
62       If an application does not explicitly create a library context then the
63       "default" library context will be used.
64
65       Library contexts are represented by the OSSL_LIB_CTX type. Many OpenSSL
66       API functions take a library context as a parameter. Applications can
67       always pass NULL for this parameter to just use the default library
68       context.
69
70       The default library context is automatically created the first time it
71       is needed. This will automatically load any available configuration
72       file and will initialise OpenSSL for use. Unlike in earlier versions of
73       OpenSSL (prior to 1.1.0) no explicit initialisation steps need to be
74       taken.
75
76       Similarly when the application exits the default library context is
77       automatically destroyed. No explicit de-initialisation steps need to be
78       taken.
79
80       See OSSL_LIB_CTX(3) for more information about library contexts.  See
81       also "ALGORITHM FETCHING".
82
83   Multi-threaded applications
84       As long as OpenSSL has been built with support for threads (the default
85       case on most platforms) then most OpenSSL functions are thread-safe in
86       the sense that it is safe to call the same function from multiple
87       threads at the same time. However most OpenSSL data structures are not
88       thread-safe. For example the BIO_write(3) and BIO_read(3) functions are
89       thread safe. However it would not be thread safe to call BIO_write()
90       from one thread while calling BIO_read() in another where both
91       functions are passed the same BIO object since both of them may attempt
92       to make changes to the same BIO object.
93
94       There are exceptions to these rules. A small number of functions are
95       not thread safe at all. Where this is the case this restriction should
96       be noted in the documentation for the function. Similarly some data
97       structures may be partially or fully thread safe. For example it is
98       safe to use an OSSL_LIB_CTX in multiple threads.
99
100       See openssl-threads(7) for a more detailed discussion on OpenSSL
101       threading support.
102

ALGORITHM FETCHING

104       In order to use an algorithm an implementation for it must first be
105       "fetched".  Fetching is the process of looking through the available
106       implementations, applying selection criteria (via a property query
107       string), and finally choosing the implementation that will be used.
108
109       Two types of fetching are supported by OpenSSL - explicit fetching and
110       implicit fetching.
111
112   Property query strings
113       When fetching an algorithm it is possible to specify a property query
114       string to guide the selection process. For example a property query
115       string of "provider=default" could be used to force the selection to
116       only consider algorithm implementations in the default provider.
117
118       Property query strings can be specified explicitly as an argument to a
119       function.  It is also possible to specify a default property query
120       string for the whole library context using the
121       EVP_set_default_properties(3) function. Where both default properties
122       and function specific properties are specified then they are combined.
123       Function specific properties will override default properties where
124       there is a conflict.
125
126       See property(7) for more information about properties.
127
128   Explicit fetching
129       Users of the OpenSSL libraries never query a provider directly for an
130       algorithm implementation. Instead, the diverse OpenSSL APIs often have
131       explicit fetching functions that do the work, and they return an
132       appropriate algorithm object back to the user. These functions usually
133       have the name "APINAME_fetch", where "APINAME" is the name of the
134       operation. For example EVP_MD_fetch(3) can be used to explicitly fetch
135       a digest algorithm implementation. The user is responsible for freeing
136       the object returned from the "APINAME_fetch" function using
137       "APINAME_free" when it is no longer needed.
138
139       These fetching functions follow a fairly common pattern, where three
140       arguments are passed:
141
142       The library context
143           See OSSL_LIB_CTX(3) for a more detailed description.  This may be
144           NULL to signify the default (global) library context, or a context
145           created by the user. Only providers loaded in this library context
146           (see OSSL_PROVIDER_load(3)) will be considered by the fetching
147           function. In case no provider has been loaded in this library
148           context then the default provider will be loaded as a fallback (see
149           OSSL_PROVIDER-default(7)).
150
151       An identifier
152           For all currently implemented fetching functions this is the
153           algorithm name.
154
155       A property query string
156           The property query string used to guide selection of the algorithm
157           implementation.
158
159       The algorithm implementation that is fetched can then be used with
160       other diverse functions that use them. For example the
161       EVP_DigestInit_ex(3) function takes as a parameter an EVP_MD object
162       which may have been returned from an earlier call to EVP_MD_fetch(3).
163
164   Implicit fetch
165       OpenSSL has a number of functions that return an algorithm object with
166       no associated implementation, such as EVP_sha256(3),
167       EVP_aes_128_cbc(3), EVP_get_cipherbyname(3) or EVP_get_digestbyname(3).
168       These are present for compatibility with OpenSSL before version 3.0
169       where explicit fetching was not available.
170
171       When they are used with functions like EVP_DigestInit_ex(3) or
172       EVP_CipherInit_ex(3), the actual implementation to be used is fetched
173       implicitly using default search criteria.
174
175       In some cases implicit fetching can also occur when a NULL algorithm
176       parameter is supplied. In this case an algorithm implementation is
177       implicitly fetched using default search criteria and an algorithm name
178       that is consistent with the context in which it is being used.
179
180       Functions that revolve around EVP_PKEY_CTX and EVP_PKEY(3), such as
181       EVP_DigestSignInit(3) and friends, all fetch the implementations
182       implicitly.  Because these functions involve both an operation type
183       (such as EVP_SIGNATURE(3)) and an EVP_KEYMGMT(3) for the EVP_PKEY(3),
184       they try the following:
185
186       1.  Fetch the operation type implementation from any provider given a
187           library context and property string stored in the EVP_PKEY_CTX.
188
189           If the provider of the operation type implementation is different
190           from the provider of the EVP_PKEY(3)'s EVP_KEYMGMT(3)
191           implementation, try to fetch a EVP_KEYMGMT(3) implementation in the
192           same provider as the operation type implementation and export the
193           EVP_PKEY(3) to it (effectively making a temporary copy of the
194           original key).
195
196           If anything in this step fails, the next step is used as a
197           fallback.
198
199       2.  As a fallback, try to fetch the operation type implementation from
200           the same provider as the original EVP_PKEY(3)'s EVP_KEYMGMT(3),
201           still using the propery string from the EVP_PKEY_CTX.
202

FETCHING EXAMPLES

204       The following section provides a series of examples of fetching
205       algorithm implementations.
206
207       Fetch any available implementation of SHA2-256 in the default context.
208       Note that some algorithms have aliases. So "SHA256" and "SHA2-256" are
209       synonymous:
210
211        EVP_MD *md = EVP_MD_fetch(NULL, "SHA2-256", NULL);
212        ...
213        EVP_MD_free(md);
214
215       Fetch any available implementation of AES-128-CBC in the default
216       context:
217
218        EVP_CIPHER *cipher = EVP_CIPHER_fetch(NULL, "AES-128-CBC", NULL);
219        ...
220        EVP_CIPHER_free(cipher);
221
222       Fetch an implementation of SHA2-256 from the default provider in the
223       default context:
224
225        EVP_MD *md = EVP_MD_fetch(NULL, "SHA2-256", "provider=default");
226        ...
227        EVP_MD_free(md);
228
229       Fetch an implementation of SHA2-256 that is not from the default
230       provider in the default context:
231
232        EVP_MD *md = EVP_MD_fetch(NULL, "SHA2-256", "provider!=default");
233        ...
234        EVP_MD_free(md);
235
236       Fetch an implementation of SHA2-256 from the default provider in the
237       specified context:
238
239        EVP_MD *md = EVP_MD_fetch(ctx, "SHA2-256", "provider=default");
240        ...
241        EVP_MD_free(md);
242
243       Load the legacy provider into the default context and then fetch an
244       implementation of WHIRLPOOL from it:
245
246        /* This only needs to be done once - usually at application start up */
247        OSSL_PROVIDER *legacy = OSSL_PROVIDER_load(NULL, "legacy");
248
249        EVP_MD *md = EVP_MD_fetch(NULL, "WHIRLPOOL", "provider=legacy");
250        ...
251        EVP_MD_free(md);
252
253       Note that in the above example the property string "provider=legacy" is
254       optional since, assuming no other providers have been loaded, the only
255       implementation of the "whirlpool" algorithm is in the "legacy"
256       provider. Also note that the default provider should be explicitly
257       loaded if it is required in addition to other providers:
258
259        /* This only needs to be done once - usually at application start up */
260        OSSL_PROVIDER *legacy = OSSL_PROVIDER_load(NULL, "legacy");
261        OSSL_PROVIDER *default = OSSL_PROVIDER_load(NULL, "default");
262
263        EVP_MD *md_whirlpool = EVP_MD_fetch(NULL, "whirlpool", NULL);
264        EVP_MD *md_sha256 = EVP_MD_fetch(NULL, "SHA2-256", NULL);
265        ...
266        EVP_MD_free(md_whirlpool);
267        EVP_MD_free(md_sha256);
268

OPENSSL PROVIDERS

270       OpenSSL comes with a set of providers.
271
272       The algorithms available in each of these providers may vary due to
273       build time configuration options. The openssl-list(1) command can be
274       used to list the currently available algorithms.
275
276       The names of the algorithms shown from openssl-list(1) can be used as
277       an algorithm identifier to the appropriate fetching function. Also see
278       the provider specific manual pages linked below for further details
279       about using the algorithms available in each of the providers.
280
281       As well as the OpenSSL providers third parties can also implement
282       providers.  For information on writing a provider see provider(7).
283
284   Default provider
285       The default provider is built in as part of the libcrypto library and
286       contains all of the most commonly used algorithm implementations.
287       Should it be needed (if other providers are loaded and offer
288       implementations of the same algorithms), the property query string
289       "provider=default" can be used as a search criterion for these
290       implementations.  The default provider includes all of the
291       functionality in the base provider below.
292
293       If you don't load any providers at all then the "default" provider will
294       be automatically loaded. If you explicitly load any provider then the
295       "default" provider would also need to be explicitly loaded if it is
296       required.
297
298       See OSSL_PROVIDER-default(7).
299
300   Base provider
301       The base provider is built in as part of the libcrypto library and
302       contains algorithm implementations for encoding and decoding for
303       OpenSSL keys.  Should it be needed (if other providers are loaded and
304       offer implementations of the same algorithms), the property query
305       string "provider=base" can be used as a search criterion for these
306       implementations.  Some encoding and decoding algorithm implementations
307       are not FIPS algorithm implementations in themselves but support
308       algorithms from the FIPS provider and are allowed for use in "FIPS
309       mode". The property query string "fips=yes" can be used to select such
310       algorithms.
311
312       See OSSL_PROVIDER-base(7).
313
314   FIPS provider
315       The FIPS provider is a dynamically loadable module, and must therefore
316       be loaded explicitly, either in code or through OpenSSL configuration
317       (see config(5)). It contains algorithm implementations that have been
318       validated according to the FIPS 140-2 standard. Should it be needed (if
319       other providers are loaded and offer implementations of the same
320       algorithms), the property query string "provider=fips" can be used as a
321       search criterion for these implementations. All approved algorithm
322       implementations in the FIPS provider can also be selected with the
323       property "fips=yes". The FIPS provider may also contain non-approved
324       algorithm implementations and these can be selected with the property
325       "fips=no".
326
327       See OSSL_PROVIDER-FIPS(7) and fips_module(7).
328
329   Legacy provider
330       The legacy provider is a dynamically loadable module, and must
331       therefore be loaded explicitly, either in code or through OpenSSL
332       configuration (see config(5)). It contains algorithm implementations
333       that are considered insecure, or are no longer in common use such as
334       MD2 or RC4. Should it be needed (if other providers are loaded and
335       offer implementations of the same algorithms), the property
336       "provider=legacy" can be used as a search criterion for these
337       implementations.
338
339       See OSSL_PROVIDER-legacy(7).
340
341   Null provider
342       The null provider is built in as part of the libcrypto library. It
343       contains no algorithms in it at all. When fetching algorithms the
344       default provider will be automatically loaded if no other provider has
345       been explicitly loaded. To prevent that from happening you can
346       explicitly load the null provider.
347
348       See OSSL_PROVIDER-null(7).
349

USING ALGORITHMS IN APPLICATIONS

351       Cryptographic algorithms are made available to applications through use
352       of the "EVP" APIs. Each of the various operations such as encryption,
353       digesting, message authentication codes, etc., have a set of EVP
354       function calls that can be invoked to use them. See the evp(7) page for
355       further details.
356
357       Most of these follow a common pattern. A "context" object is first
358       created. For example for a digest operation you would use an
359       EVP_MD_CTX, and for an encryption/decryption operation you would use an
360       EVP_CIPHER_CTX. The operation is then initialised ready for use via an
361       "init" function - optionally passing in a set of parameters (using the
362       OSSL_PARAM type) to configure how the operation should behave. Next
363       data is fed into the operation in a series of "update" calls. The
364       operation is finalised using a "final" call which will typically
365       provide some kind of output. Finally the context is cleaned up and
366       freed.
367
368       The following shows a complete example for doing this process for
369       digesting data using SHA256. The process is similar for other
370       operations such as encryption/decryption, signatures, message
371       authentication codes, etc.
372
373        #include <stdio.h>
374        #include <openssl/evp.h>
375        #include <openssl/bio.h>
376        #include <openssl/err.h>
377
378        int main(void)
379        {
380            EVP_MD_CTX *ctx = NULL;
381            EVP_MD *sha256 = NULL;
382            const unsigned char msg[] = {
383                0x00, 0x01, 0x02, 0x03
384            };
385            unsigned int len = 0;
386            unsigned char *outdigest = NULL;
387            int ret = 1;
388
389            /* Create a context for the digest operation */
390            ctx = EVP_MD_CTX_new();
391            if (ctx == NULL)
392                goto err;
393
394            /*
395             * Fetch the SHA256 algorithm implementation for doing the digest. We're
396             * using the "default" library context here (first NULL parameter), and
397             * we're not supplying any particular search criteria for our SHA256
398             * implementation (second NULL parameter). Any SHA256 implementation will
399             * do.
400             */
401            sha256 = EVP_MD_fetch(NULL, "SHA256", NULL);
402            if (sha256 == NULL)
403                goto err;
404
405           /* Initialise the digest operation */
406           if (!EVP_DigestInit_ex(ctx, sha256, NULL))
407               goto err;
408
409            /*
410             * Pass the message to be digested. This can be passed in over multiple
411             * EVP_DigestUpdate calls if necessary
412             */
413            if (!EVP_DigestUpdate(ctx, msg, sizeof(msg)))
414                goto err;
415
416            /* Allocate the output buffer */
417            outdigest = OPENSSL_malloc(EVP_MD_get_size(sha256));
418            if (outdigest == NULL)
419                goto err;
420
421            /* Now calculate the digest itself */
422            if (!EVP_DigestFinal_ex(ctx, outdigest, &len))
423                goto err;
424
425            /* Print out the digest result */
426            BIO_dump_fp(stdout, outdigest, len);
427
428            ret = 0;
429
430         err:
431            /* Clean up all the resources we allocated */
432            OPENSSL_free(outdigest);
433            EVP_MD_free(sha256);
434            EVP_MD_CTX_free(ctx);
435            if (ret != 0)
436               ERR_print_errors_fp(stderr);
437            return ret;
438        }
439

CONFIGURATION

441       By default OpenSSL will load a configuration file when it is first
442       used. This will set up various configuration settings within the
443       default library context.  Applications that create their own library
444       contexts may optionally configure them with a config file using the
445       OSSL_LIB_CTX_load_config(3) function.
446
447       The configuration file can be used to automatically load providers and
448       set up default property query strings.
449
450       For information on the OpenSSL configuration file format see config(5).
451

ENCODING AND DECODING KEYS

453       Many algorithms require the use of a key. Keys can be generated
454       dynamically using the EVP APIs (for example see EVP_PKEY_Q_keygen(3)).
455       However it is often necessary to save or load keys (or their associated
456       parameters) to or from some external format such as PEM or DER (see
457       openssl-glossary(7)). OpenSSL uses encoders and decoders to perform
458       this task.
459
460       Encoders and decoders are just algorithm implementations in the same
461       way as any other algorithm implementation in OpenSSL. They are
462       implemented by providers. The OpenSSL encoders and decoders are
463       available in the default provider. They are also duplicated in the base
464       provider.
465
466       For information about encoders see OSSL_ENCODER_CTX_new_for_pkey(3).
467       For information about decoders see OSSL_DECODER_CTX_new_for_pkey(3).
468

LIBRARY CONVENTIONS

470       Many OpenSSL functions that "get" or "set" a value follow a naming
471       convention using the numbers 0 and 1, i.e. "get0", "get1", "set0" and
472       "set1". This can also apply to some functions that "add" a value to an
473       existing set, i.e.  "add0" and "add1".
474
475       For example the functions:
476
477        int X509_CRL_add0_revoked(X509_CRL *crl, X509_REVOKED *rev);
478        int X509_add1_trust_object(X509 *x, const ASN1_OBJECT *obj);
479
480       In the 0 version the ownership of the object is passed to (for an add
481       or set) or retained by (for a get) the parent object. For example after
482       calling the X509_CRL_add0_revoked() function above, ownership of the
483       rev object is passed to the crl object. Therefore, after calling this
484       function rev should not be freed directly. It will be freed implicitly
485       when crl is freed.
486
487       In the 1 version the ownership of the object is not passed to or
488       retained by the parent object. Instead a copy or "up ref" of the object
489       is performed. So after calling the X509_add1_trust_object() function
490       above the application will still be responsible for freeing the obj
491       value where appropriate.
492

SEE ALSO

494       openssl(1), ssl(7), evp(7), OSSL_LIB_CTX(3), openssl-threads(7),
495       property(7), OSSL_PROVIDER-default(7), OSSL_PROVIDER-base(7),
496       OSSL_PROVIDER-FIPS(7), OSSL_PROVIDER-legacy(7), OSSL_PROVIDER-null(7),
497       openssl-glossary(7), provider(7)
498
500       Copyright 2000-2022 The OpenSSL Project Authors. All Rights Reserved.
501
502       Licensed under the Apache License 2.0 (the "License").  You may not use
503       this file except in compliance with the License.  You can obtain a copy
504       in the file LICENSE in the source distribution or at
505       <https://www.openssl.org/source/license.html>.
506
507
508
5093.0.5                             2022-11-01                     CRYPTO(7ossl)
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