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.
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.
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.
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.
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>.