crypto.7ssl(7)
SECCIÓN: 7 - Miscelánea
CRYPTO(7SSL) OpenSSL CRYPTO(7SSL)
NAME
crypto - OpenSSL cryptographic library
SYNOPSIS
See the individual manual pages for details.
DESCRIPTION
The OpenSSL crypto library ("libcrypto") implements a wide range of
cryptographic algorithms used in various Internet standards. The
services provided by this library are used by the OpenSSL
implementations of TLS and CMS, and they have also been used to
implement many other third party products and protocols.
The functionality includes symmetric encryption, public key
cryptography, key agreement, certificate handling, cryptographic hash
functions, cryptographic pseudo-random number generators, message
authentication codes (MACs), key derivation functions (KDFs), and
various utilities.
Algorithms
Cryptographic primitives such as the SHA256 digest, or AES encryption
are referred to in OpenSSL as "algorithms". Each algorithm may have
multiple implementations available for use. For example the RSA
algorithm is available as a "default" implementation suitable for
general use, and a "fips" implementation which has been validated to
FIPS standards for situations where that is important. It is also
possible that a third party could add additional implementations such
as in a hardware security module (HSM).
Operations
Different algorithms can be grouped together by their purpose. For
example there are algorithms for encryption, and different algorithms
for digesting data. These different groups are known as "operations"
in OpenSSL. Each operation has a different set of functions associated
with it. For example to perform an encryption operation using AES (or
any other encryption algorithm) you would use the encryption functions
detailed on the EVP_EncryptInit(3) page. Or to perform a digest
operation using SHA256 then you would use the digesting functions on
the EVP_DigestInit(3) page.
Providers
A provider in OpenSSL is a component that collects together algorithm
implementations. In order to use an algorithm you must have at least
one provider loaded that contains an implementation of it. OpenSSL
comes with a number of providers and they may also be obtained from
third parties. If you don't load a provider explicitly (either in
program code or via config) then the OpenSSL built-in "default"
provider will be automatically loaded.
Library contexts
A library context can be thought of as a "scope" within which
configuration options take effect. When a provider is loaded, it is
only loaded within the scope of a given library context. In this way it
is possible for different components of a complex application to each
use a different library context and have different providers loaded
with different configuration settings.
If an application does not explicitly create a library context then the
"default" library context will be used.
Library contexts are represented by the OSSL_LIB_CTX type. Many OpenSSL
API functions take a library context as a parameter. Applications can
always pass NULL for this parameter to just use the default library
context.
The default library context is automatically created the first time it
is needed. This will automatically load any available configuration
file and will initialise OpenSSL for use. Unlike in earlier versions of
OpenSSL (prior to 1.1.0) no explicit initialisation steps need to be
taken.
Similarly when the application exits the default library context is
automatically destroyed. No explicit de-initialisation steps need to be
taken.
See OSSL_LIB_CTX(3) for more information about library contexts. See
also "ALGORITHM FETCHING".
Multi-threaded applications
As long as OpenSSL has been built with support for threads (the default
case on most platforms) then most OpenSSL functions are thread-safe in
the sense that it is safe to call the same function from multiple
threads at the same time. However most OpenSSL data structures are not
thread-safe. For example the BIO_write(3) and BIO_read(3) functions are
thread safe. However it would not be thread safe to call BIO_write()
from one thread while calling BIO_read() in another where both
functions are passed the same BIO object since both of them may attempt
to make changes to the same BIO object.
There are exceptions to these rules. A small number of functions are
not thread safe at all. Where this is the case this restriction should
be noted in the documentation for the function. Similarly some data
structures may be partially or fully thread safe. For example it is
safe to use an OSSL_LIB_CTX in multiple threads.
See openssl-threads(7) for a more detailed discussion on OpenSSL
threading support.
ALGORITHM FETCHING
In order to use an algorithm an implementation for it must first be
"fetched". Fetching is the process of looking through the available
implementations, applying selection criteria (via a property query
string), and finally choosing the implementation that will be used.
Two types of fetching are supported by OpenSSL - explicit fetching and
implicit fetching.
Property query strings
When fetching an algorithm it is possible to specify a property query
string to guide the selection process. For example a property query
string of "provider=default" could be used to force the selection to
only consider algorithm implementations in the default provider.
Property query strings can be specified explicitly as an argument to a
function. It is also possible to specify a default property query
string for the whole library context using the
EVP_set_default_properties(3) or EVP_default_properties_enable_fips(3)
functions. Where both default properties and function specific
properties are specified then they are combined. Function specific
properties will override default properties where there is a conflict.
See property(7) for more information about properties.
Explicit fetching
Users of the OpenSSL libraries never query a provider directly for an
algorithm implementation. Instead, the diverse OpenSSL APIs often have
explicit fetching functions that do the work, and they return an
appropriate algorithm object back to the user. These functions usually
have the name "APINAME_fetch", where "APINAME" is the name of the
operation. For example EVP_MD_fetch(3) can be used to explicitly fetch
a digest algorithm implementation. The user is responsible for freeing
the object returned from the "APINAME_fetch" function using
"APINAME_free" when it is no longer needed.
These fetching functions follow a fairly common pattern, where three
arguments are passed:
The library context
See OSSL_LIB_CTX(3) for a more detailed description. This may be
NULL to signify the default (global) library context, or a context
created by the user. Only providers loaded in this library context
(see OSSL_PROVIDER_load(3)) will be considered by the fetching
function. In case no provider has been loaded in this library
context then the default provider will be loaded as a fallback (see
OSSL_PROVIDER-default(7)).
An identifier
For all currently implemented fetching functions this is the
algorithm name.
A property query string
The property query string used to guide selection of the algorithm
implementation.
The algorithm implementation that is fetched can then be used with
other diverse functions that use them. For example the
EVP_DigestInit_ex(3) function takes as a parameter an EVP_MD object
which may have been returned from an earlier call to EVP_MD_fetch(3).
Implicit fetching
OpenSSL has a number of functions that return an algorithm object with
no associated implementation, such as EVP_sha256(3),
EVP_aes_128_cbc(3), EVP_get_cipherbyname(3) or EVP_get_digestbyname(3).
These are present for compatibility with OpenSSL before version 3.0
where explicit fetching was not available.
When they are used with functions like EVP_DigestInit_ex(3) or
EVP_CipherInit_ex(3), the actual implementation to be used is fetched
implicitly using default search criteria.
In some cases implicit fetching can also occur when a NULL algorithm
parameter is supplied. In this case an algorithm implementation is
implicitly fetched using default search criteria and an algorithm name
that is consistent with the context in which it is being used.
Functions that revolve around EVP_PKEY_CTX and EVP_PKEY(3), such as
EVP_DigestSignInit(3) and friends, all fetch the implementations
implicitly. Because these functions involve both an operation type
(such as EVP_SIGNATURE(3)) and an EVP_KEYMGMT(3) for the EVP_PKEY(3),
they try the following:
1. Fetch the operation type implementation from any provider given a
library context and property string stored in the EVP_PKEY_CTX.
If the provider of the operation type implementation is different
from the provider of the EVP_PKEY(3)'s EVP_KEYMGMT(3)
implementation, try to fetch a EVP_KEYMGMT(3) implementation in the
same provider as the operation type implementation and export the
EVP_PKEY(3) to it (effectively making a temporary copy of the
original key).
If anything in this step fails, the next step is used as a
fallback.
2. As a fallback, try to fetch the operation type implementation from
the same provider as the original EVP_PKEY(3)'s EVP_KEYMGMT(3),
still using the property string from the EVP_PKEY_CTX.
Performance
If you perform the same operation many times then it is recommended to
use "Explicit fetching" to prefetch an algorithm once initially, and
then pass this created object to any operations that are currently
using "Implicit fetching". See an example of Explicit fetching in
"USING ALGORITHMS IN APPLICATIONS".
Prior to OpenSSL 3.0, constant method tables (such as EVP_sha256())
were used directly to access methods. If you pass one of these
convenience functions to an operation the fixed methods are ignored,
and only the name is used to internally fetch methods from a provider.
If the prefetched object is not passed to operations, then any implicit
fetch will use the internally cached prefetched object, but it will
still be slower than passing the prefetched object directly.
Fetching via a provider offers more flexibility, but it is slower than
the old method, since it must search for the algorithm in all loaded
providers, and then populate the method table using provider supplied
methods. Internally OpenSSL caches similar algorithms on the first
fetch (so loading a digest caches all digests).
The following methods can be used for prefetching:
EVP_MD_fetch(3)
EVP_CIPHER_fetch(3)
EVP_KDF_fetch(3)
EVP_MAC_fetch(3)
EVP_KEM_fetch(3)
OSSL_ENCODER_fetch(3)
OSSL_DECODER_fetch(3)
EVP_RAND_fetch(3)
The following methods are used internally when performing operations:
EVP_KEYMGMT_fetch(3)
EVP_KEYEXCH_fetch(3)
EVP_SIGNATURE_fetch(3)
OSSL_STORE_LOADER_fetch(3)
See OSSL_PROVIDER-default(7), <OSSL_PROVIDER-fips(7)> and
<OSSL_PROVIDER-legacy(7)>for a list of algorithm names that can be
fetched.
FETCHING EXAMPLES
The following section provides a series of examples of fetching
algorithm implementations.
Fetch any available implementation of SHA2-256 in the default context.
Note that some algorithms have aliases. So "SHA256" and "SHA2-256" are
synonymous:
EVP_MD *md = EVP_MD_fetch(NULL, "SHA2-256", NULL);
...
EVP_MD_free(md);
Fetch any available implementation of AES-128-CBC in the default
context:
EVP_CIPHER *cipher = EVP_CIPHER_fetch(NULL, "AES-128-CBC", NULL);
...
EVP_CIPHER_free(cipher);
Fetch an implementation of SHA2-256 from the default provider in the
default context:
EVP_MD *md = EVP_MD_fetch(NULL, "SHA2-256", "provider=default");
...
EVP_MD_free(md);
Fetch an implementation of SHA2-256 that is not from the default
provider in the default context:
EVP_MD *md = EVP_MD_fetch(NULL, "SHA2-256", "provider!=default");
...
EVP_MD_free(md);
Fetch an implementation of SHA2-256 from the default provider in the
specified context:
EVP_MD *md = EVP_MD_fetch(ctx, "SHA2-256", "provider=default");
...
EVP_MD_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", "provider=legacy");
...
EVP_MD_free(md);
Note that in the above example the property string "provider=legacy" is
optional since, assuming no other providers have been loaded, the only
implementation 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, "SHA2-256", NULL);
...
EVP_MD_free(md_whirlpool);
EVP_MD_free(md_sha256);
OPENSSL PROVIDERS
OpenSSL comes with a set of providers.
The algorithms available in each of these providers may vary due to
build time configuration options. The openssl-list(1) command can be
used to list the currently available algorithms.
The names of the algorithms shown from openssl-list(1) can be used as
an algorithm identifier to the appropriate fetching function. Also see
the provider specific manual pages linked below for further details
about using the algorithms available in each of the providers.
As well as the OpenSSL providers third parties can also implement
providers. For information on writing a provider see provider(7).
Default provider
The default provider is built in as part of the libcrypto library and
contains all of the most commonly used algorithm implementations.
Should it be needed (if other providers are loaded and offer
implementations of the same algorithms), the property query string
"provider=default" can be used as a search criterion for these
implementations. The default provider includes all of the
functionality in the base provider below.
If you don't load any providers at all then the "default" provider will
be automatically loaded. If you explicitly load any provider then the
"default" provider would also need to be explicitly loaded if it is
required.
See OSSL_PROVIDER-default(7).
Base provider
The base provider is built in as part of the libcrypto library and
contains algorithm implementations for encoding and decoding for
OpenSSL keys. Should it be needed (if other providers are loaded and
offer implementations of the same algorithms), the property query
string "provider=base" can be used as a search criterion for these
implementations. Some encoding and decoding algorithm implementations
are not FIPS algorithm implementations in themselves but support
algorithms from the FIPS provider and are allowed for use in "FIPS
mode". The property query string "fips=yes" can be used to select such
algorithms.
See OSSL_PROVIDER-base(7).
FIPS provider
The FIPS provider is a dynamically loadable module, and must therefore
be loaded explicitly, either in code or through OpenSSL configuration
(see config(5)). It contains algorithm implementations that have been
validated according to the FIPS 140-2 standard. Should it be needed (if
other providers are loaded and offer implementations of the same
algorithms), the property query string "provider=fips" can be used as a
search criterion for these implementations. All approved algorithm
implementations in the FIPS provider can also be selected with the
property "fips=yes". The FIPS provider may also contain non-approved
algorithm implementations and these can be selected with the property
"fips=no".
See OSSL_PROVIDER-FIPS(7) and fips_module(7).
Legacy provider
The legacy provider is a dynamically loadable module, and must
therefore be loaded explicitly, either in code or through OpenSSL
configuration (see config(5)). It contains algorithm implementations
that are considered insecure, or are no longer in common use such as
MD2 or RC4. Should it be needed (if other providers are loaded and
offer implementations of the same algorithms), the property
"provider=legacy" can be used as a search criterion for these
implementations.
See OSSL_PROVIDER-legacy(7).
Null provider
The null provider is built in as part of the libcrypto library. It
contains no algorithms in it at all. When fetching algorithms the
default provider will be automatically loaded if no other provider has
been explicitly loaded. To prevent that from happening you can
explicitly load the null provider.
See OSSL_PROVIDER-null(7).
USING ALGORITHMS IN APPLICATIONS
Cryptographic algorithms are made available to applications through use
of the "EVP" APIs. Each of the various operations such as encryption,
digesting, message authentication codes, etc., have a set of EVP
function calls that can be invoked to use them. See the evp(7) page for
further details.
Most of these follow a common pattern. A "context" object is first
created. For example for a digest operation you would use an
EVP_MD_CTX, and for an encryption/decryption operation you would use an
EVP_CIPHER_CTX. The operation is then initialised ready for use via an
"init" function - optionally passing in a set of parameters (using the
OSSL_PARAM(3) type) to configure how the operation should behave. Next
data is fed into the operation in a series of "update" calls. The
operation is finalised using a "final" call which will typically
provide some kind of output. Finally the context is cleaned up and
freed.
The following shows a complete example for doing this process for
digesting data using SHA256. The process is similar for other
operations such as encryption/decryption, signatures, message
authentication codes, etc.
#include <stdio.h>
#include <openssl/evp.h>
#include <openssl/bio.h>
#include <openssl/err.h>
int main(void)
{
EVP_MD_CTX *ctx = NULL;
EVP_MD *sha256 = NULL;
const unsigned char msg[] = {
0x00, 0x01, 0x02, 0x03
};
unsigned int len = 0;
unsigned char *outdigest = NULL;
int ret = 1;
/* Create a context for the digest operation */
ctx = EVP_MD_CTX_new();
if (ctx == NULL)
goto err;
/*
* Fetch the SHA256 algorithm implementation for doing the digest. We're
* using the "default" library context here (first NULL parameter), and
* we're not supplying any particular search criteria for our SHA256
* implementation (second NULL parameter). Any SHA256 implementation will
* do.
* In a larger application this fetch would just be done once, and could
* be used for multiple calls to other operations such as EVP_DigestInit_ex().
*/
sha256 = EVP_MD_fetch(NULL, "SHA256", NULL);
if (sha256 == NULL)
goto err;
/* Initialise the digest operation */
if (!EVP_DigestInit_ex(ctx, sha256, NULL))
goto err;
/*
* Pass the message to be digested. This can be passed in over multiple
* EVP_DigestUpdate calls if necessary
*/
if (!EVP_DigestUpdate(ctx, msg, sizeof(msg)))
goto err;
/* Allocate the output buffer */
outdigest = OPENSSL_malloc(EVP_MD_get_size(sha256));
if (outdigest == NULL)
goto err;
/* Now calculate the digest itself */
if (!EVP_DigestFinal_ex(ctx, outdigest, &len))
goto err;
/* Print out the digest result */
BIO_dump_fp(stdout, outdigest, len);
ret = 0;
err:
/* Clean up all the resources we allocated */
OPENSSL_free(outdigest);
EVP_MD_free(sha256);
EVP_MD_CTX_free(ctx);
if (ret != 0)
ERR_print_errors_fp(stderr);
return ret;
}
CONFIGURATION
By default OpenSSL will load a configuration file when it is first
used. This will set up various configuration settings within the
default library context. Applications that create their own library
contexts may optionally configure them with a config file using the
OSSL_LIB_CTX_load_config(3) function.
The configuration file can be used to automatically load providers and
set up default property query strings.
For information on the OpenSSL configuration file format see config(5).
ENCODING AND DECODING KEYS
Many algorithms require the use of a key. Keys can be generated
dynamically using the EVP APIs (for example see EVP_PKEY_Q_keygen(3)).
However it is often necessary to save or load keys (or their associated
parameters) to or from some external format such as PEM or DER (see
openssl-glossary(7)). OpenSSL uses encoders and decoders to perform
this task.
Encoders and decoders are just algorithm implementations in the same
way as any other algorithm implementation in OpenSSL. They are
implemented by providers. The OpenSSL encoders and decoders are
available in the default provider. They are also duplicated in the base
provider.
For information about encoders see OSSL_ENCODER_CTX_new_for_pkey(3).
For information about decoders see OSSL_DECODER_CTX_new_for_pkey(3).
LIBRARY CONVENTIONS
Many OpenSSL functions that "get" or "set" a value follow a naming
convention using the numbers 0 and 1, i.e. "get0", "get1", "set0" and
"set1". This can also apply to some functions that "add" a value to an
existing set, i.e. "add0" and "add1".
For example the functions:
int X509_CRL_add0_revoked(X509_CRL *crl, X509_REVOKED *rev);
int X509_add1_trust_object(X509 *x, const ASN1_OBJECT *obj);
In the 0 version the ownership of the object is passed to (for an add
or set) or retained by (for a get) the parent object. For example after
calling the X509_CRL_add0_revoked() function above, ownership of the
rev object is passed to the crl object. Therefore, after calling this
function rev should not be freed directly. It will be freed implicitly
when crl is freed.
In the 1 version the ownership of the object is not passed to or
retained by the parent object. Instead a copy or "up ref" of the object
is performed. So after calling the X509_add1_trust_object() function
above the application will still be responsible for freeing the obj
value where appropriate.
SEE ALSO
openssl(1), ssl(7), evp(7), OSSL_LIB_CTX(3), openssl-threads(7),
property(7), OSSL_PROVIDER-default(7), OSSL_PROVIDER-base(7),
OSSL_PROVIDER-FIPS(7), OSSL_PROVIDER-legacy(7), OSSL_PROVIDER-null(7),
openssl-glossary(7), provider(7)
COPYRIGHT
Copyright 2000-2023 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
<https://www.openssl.org/source/license.html>.
3.0.17 2025-09-26 CRYPTO(7SSL)
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