=pod =head1 NAME EVP_MD_fetch - Functions to explicitly fetch algorithm implementations =head1 SYNOPSIS #include EVP_MD *EVP_MD_fetch(OPENSSL_CTX *ctx, const char *algorithm, const char *properties); =head1 DESCRIPTION The B object is used for representing a digest method implementation. Having obtained a digest implementation as an B type it can be used to calculate the digest of input data using functions such as L, L and L. Digest implementations may be obtained in one of three ways, i.e. implicit lookup, explicit lookup or user defined. =over 4 =item Implicit Lookup With implicit lookup an application can use functions such as L, L or L to obtain an B object. When used in a function like L the actual implementation to be used will be fetched implicitly using default search criteria. Typically, (unless the default search criteria have been changed and/or different providers have been loaded), this will return an implementation of the appropriate algorithm from the default provider. =item Explicit Lookup With explicit lookup an application uses the EVP_MD_fetch() function to obtain an algorithm implementation. An implementation with the given name and satisfying the search criteria specified in the B parameter will be looked for within the available providers and returned. See L for information about providers. =item User defined Using the user defined approach an application constructs its own EVP_MD object. See L for details. =back The EVP_MD_fetch() function will look for an algorithm within the providers that have been loaded into the B given in the B parameter. This parameter may be NULL in which case the default B will be used. See L and L for further details. The B parameter gives the name of the algorithm to be looked up. Different algorithms can be made available by loading different providers. The built-in default provider algorithm implementation names are: SHA1, SHA224, SHA256, SHA384, SHA512, SHA512-224, SHA512-256,SHA3-224, SHA3-256, SHA3-384, SHA3-512, SHAKE128, SHAKE256, SM3, BLAKE2b512, BLAKE2s256 and MD5-SHA1. Additional algorithm implementations may be obtained by loading the "legacy" provider. The names of these algorithms are: RIPEMD160, MD2, MD4, MD5, MDC2 and whirlpool. The B parameter specifies the search criteria that will be used to look for an algorithm implementation. Properties are given as a comma delimited string of name value pairs. In order for an implementation to match, all the properties in the query string must match those defined for that implementation. Any properties defined by an implementation but not given in the query string are ignored. All algorithm implementations in the default provider have the property "default=yes". All algorithm implementations in the legacy provider have the property "legacy=yes". All algorithm implementations in the FIPS provider have the property "fips=yes". In the event that more than one implementation of the given algorithm name matches the specified properties then an unspecified one of those implementations may be returned. The B parameter may be NULL in which case any implementation from the available providers with the given algorithm name will be returned. The return value from a call to EVP_MD_fetch() must be freed by the caller using L. Note that EVP_MD objects are reference counted. See L. =head1 RETURN VALUES EVP_MD_fetch() returns a pointer to the algorithm implementation represented by an EVP_MD object, or NULL on error. =head1 EXAMPLES Fetch any available implementation of SHA256 in the default context: EVP_MD *md = EVP_MD_fetch(NULL, "SHA256", NULL); Fetch an implementation of SHA256 from the default provider in the default context: EVP_MD *md = EVP_MD_fetch(NULL, "SHA256", "default=yes"); ... EVP_MD_meth_free(md); Fetch an implementation of SHA256 that is not from the default provider in the default context: EVP_MD *md = EVP_MD_fetch(NULL, "SHA256", "default=no"); ... EVP_MD_meth_free(md); Fetch an implementation of SHA256 from the default provider in the specified context: EVP_MD *md = EVP_MD_fetch(ctx, "SHA256", "default=yes"); ... EVP_MD_meth_free(md); Load the legacy provider into the default context and then fetch an implementation of whirlpool from it: /* This only needs to be done once - usually at application start up */ OSSL_PROVIDER *legacy = OSSL_PROVIDER_load(NULL, "legacy"); EVP_MD *md = EVP_MD_fetch(NULL, "whirlpool", "legacy=yes"); ... EVP_MD_meth_free(md); Note that in the above example the property string "legacy=yes" is optional since, assuming no other providers have been loaded, the only implmentation of the "whirlpool" algorithm is in the "legacy" provider. Also note that the default provider should be explicitly loaded if it is required in addition to other providers: /* This only needs to be done once - usually at application start up */ OSSL_PROVIDER *legacy = OSSL_PROVIDER_load(NULL, "legacy"); OSSL_PROVIDER *default = OSSL_PROVIDER_load(NULL, "default"); EVP_MD *md_whirlpool = EVP_MD_fetch(NULL, "whirlpool", NULL); EVP_MD *md_sha256 = EVP_MD_fetch(NULL, "SHA256", NULL); ... EVP_MD_meth_free(md_whirlpool); EVP_MD_meth_free(md_sha256); =head1 SEE ALSO L, L, L, L, L, L =head1 HISTORY The functions described here were added in OpenSSL 3.0. =head1 COPYRIGHT Copyright 2019 The OpenSSL Project Authors. All Rights Reserved. Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at L. =cut