Provided by: openssl_3.0.13-0ubuntu3.6_amd64 
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 one of the following OpenSSL providers will be
loaded automatically:
• The built-in "default" provider when running on a system that is not operating in FIPS-approved mode.
• The "fips" provider when running on a system that is operating in FIPS-approved mode.
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 and the system is not running in FIPS-approved mode, 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".
If you don't load any providers at all and the system is running in FIPS-approved mode, then the FIPS
provider will be automatically loaded. If you explicitly load any provider then the FIPS provider would
also need to be explicitly loaded if it is required.
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>.
3.0.13 2025-09-18 CRYPTO(7SSL)