6 - Functions to explicitly fetch algorithm implementations
10 #include <openssl/evp.h>
12 EVP_MD *EVP_MD_fetch(OPENSSL_CTX *ctx, const char *algorithm,
13 const char *properties);
17 The B<EVP_MD> object is used for representing a digest method implementation.
19 Having obtained a digest implementation as an B<EVP_MD> type it can be used to
20 calculate the digest of input data using functions such as
21 L<EVP_DigestInit_ex(3)>, L<EVP_DigestUpdate(3)> and L<EVP_DigestFinal_ex(3)>.
23 Digest implementations may be obtained in one of three ways, i.e. implicit
24 fetch, explicit fetch or user defined.
30 With implicit fetch an application can use functions such as L<EVP_sha256(3)>,
31 L<EVP_sha512(3)> or L<EVP_blake2b512(3)> to obtain an B<EVP_MD> object. When
32 used in a function like L<EVP_DigestInit_ex(3)> the actual implementation to
33 be used will be fetched implicitly using default search criteria. Typically,
34 (unless the default search criteria have been changed and/or different providers
35 have been loaded), this will return an implementation of the appropriate
36 algorithm from the default provider.
40 With explicit fetch an application uses the EVP_MD_fetch() function to obtain
41 an algorithm implementation. An implementation with the given name and
42 satisfying the search criteria specified in the B<properties> parameter
43 combined with the default search criteria will be looked for within the
44 available providers and returned.
45 See L<EVP_set_default_properties(3)> for information on default search criteria
46 and L<OSSL_PROVIDER(3)> for information about providers.
50 Using the user defined approach an application constructs its own EVP_MD object.
51 See L<EVP_MD_meth_new(3)> for details.
55 The EVP_MD_fetch() function will look for an algorithm within the providers that
56 have been loaded into the B<OPENSSL_CTX> given in the B<ctx> parameter. This
57 parameter may be NULL in which case the default B<OPENSSL_CTX> will be used. See
58 L<OPENSSL_CTX_new(3)> and L<OSSL_PROVIDER_load(3)> for further details.
60 The B<algorithm> parameter gives the name of the algorithm to be looked up.
61 Different algorithms can be made available by loading different providers. The
62 built-in default provider algorithm implementation names are: SHA1, SHA224,
63 SHA256, SHA384, SHA512, SHA512-224, SHA512-256,SHA3-224, SHA3-256, SHA3-384,
64 SHA3-512, SHAKE128, SHAKE256, SM3, BLAKE2b512, BLAKE2s256 and MD5-SHA1.
66 Additional algorithm implementations may be obtained by loading the "legacy"
67 provider. The names of these algorithms are: RIPEMD160, MD2, MD4, MD5, MDC2 and
70 The B<properties> parameter specifies the search criteria that will be used to
71 look for an algorithm implementation. Properties are given as a comma delimited
72 string of name value pairs. In order for an implementation to match, all the
73 properties in the query string must match those defined for that implementation.
74 Any properties defined by an implementation but not given in the query string
75 are ignored. All algorithm implementations in the default provider have the
76 property "default=yes". All algorithm implementations in the legacy provider have
77 the property "legacy=yes". All algorithm implementations in the FIPS provider
78 have the property "fips=yes". In the event that more than one implementation
79 of the given algorithm name matches the specified properties then an unspecified
80 one of those implementations may be returned. The B<properties> parameter may be
81 NULL in which case any implementation from the available providers with the
82 given algorithm name will be returned.
84 The return value from a call to EVP_MD_fetch() must be freed by the caller using
85 L<EVP_MD_meth_free(3)>. Note that EVP_MD objects are reference counted. See
90 Where an application that previously used implicit fetch is converted to use
91 explicit fetch care should be taken with the L<EVP_MD_CTX_md(3)> function.
92 Specifically, this function returns the EVP_MD object orginally passed to
93 EVP_DigestInit_ex() (or other similar function). With implicit fetch the
94 returned EVP_MD object is guaranteed to be available throughout the application
95 lifetime. However, with explicit fetch EVP_MD objects are reference counted.
96 EVP_MD_CTX_md does not increment the reference count and so the returned EVP_MD
97 object may not be accessible beyond the lifetime of the EVP_MD_CTX it is
102 EVP_MD_fetch() returns a pointer to the algorithm implementation represented by
103 an EVP_MD object, or NULL on error.
107 Fetch any available implementation of SHA256 in the default context:
109 EVP_MD *md = EVP_MD_fetch(NULL, "SHA256", NULL);
111 Fetch an implementation of SHA256 from the default provider in the default
114 EVP_MD *md = EVP_MD_fetch(NULL, "SHA256", "default=yes");
116 EVP_MD_meth_free(md);
118 Fetch an implementation of SHA256 that is not from the default provider in the
121 EVP_MD *md = EVP_MD_fetch(NULL, "SHA256", "default=no");
123 EVP_MD_meth_free(md);
125 Fetch an implementation of SHA256 from the default provider in the specified
128 EVP_MD *md = EVP_MD_fetch(ctx, "SHA256", "default=yes");
130 EVP_MD_meth_free(md);
132 Load the legacy provider into the default context and then fetch an
133 implementation of whirlpool from it:
135 /* This only needs to be done once - usually at application start up */
136 OSSL_PROVIDER *legacy = OSSL_PROVIDER_load(NULL, "legacy");
138 EVP_MD *md = EVP_MD_fetch(NULL, "whirlpool", "legacy=yes");
140 EVP_MD_meth_free(md);
142 Note that in the above example the property string "legacy=yes" is optional
143 since, assuming no other providers have been loaded, the only implmentation of
144 the "whirlpool" algorithm is in the "legacy" provider. Also note that the
145 default provider should be explicitly loaded if it is required in addition to
148 /* This only needs to be done once - usually at application start up */
149 OSSL_PROVIDER *legacy = OSSL_PROVIDER_load(NULL, "legacy");
150 OSSL_PROVIDER *default = OSSL_PROVIDER_load(NULL, "default");
152 EVP_MD *md_whirlpool = EVP_MD_fetch(NULL, "whirlpool", NULL);
153 EVP_MD *md_sha256 = EVP_MD_fetch(NULL, "SHA256", NULL);
155 EVP_MD_meth_free(md_whirlpool);
156 EVP_MD_meth_free(md_sha256);
160 L<EVP_DigestInit(3)>, L<EVP_MD_meth_new(3)>, L<EVP_MD_meth_free(3)>,
161 L<EVP_MD_upref(3)>, L<OSSL_PROVIDER_load(3)>, L<OPENSSL_CTX(3)>,
162 L<EVP_set_default_properties(3)>
166 The functions described here were added in OpenSSL 3.0.
170 Copyright 2019 The OpenSSL Project Authors. All Rights Reserved.
172 Licensed under the Apache License 2.0 (the "License"). You may not use
173 this file except in compliance with the License. You can obtain a copy
174 in the file LICENSE in the source distribution or at
175 L<https://www.openssl.org/source/license.html>.