OSSL-GUIDE-L...-INTRODUCTION(7SSL) OpenSSL OSSL-GUIDE-L...-INTRODUCTION(7SSL)
NAME
ossl-guide-libcrypto-introduction, crypto - OpenSSL Guide: An
introduction to libcrypto
INTRODUCTION
The OpenSSL cryptography library ("libcrypto") enables access to 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 140 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).
Algorithms are implemented in providers. See
ossl-guide-libraries-introduction(7) for information about providers.
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.
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".
Explicit fetching
Explicit fetching involves directly calling a specific API to fetch an
algorithm implementation from a provider. This fetched object can then
be passed to other APIs. These explicit fetching 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. Each provider supports a list of algorithm
implementations. See the provider specific documentation for
information on the algorithm implementations available in each
provider: "OPERATIONS AND ALGORITHMS" in OSSL_PROVIDER-default(7),
"OPERATIONS AND ALGORITHMS" in OSSL_PROVIDER-FIPS(7), "OPERATIONS
AND ALGORITHMS" in OSSL_PROVIDER-legacy(7) and "OPERATIONS AND
ALGORITHMS" in OSSL_PROVIDER-base(7).
Note, while providers may register algorithms against a list of
names using a string with a colon separated list of names, fetching
algorithms using that format is currently unsupported.
A property query string
The property query string used to guide selection of the algorithm
implementation. See "PROPERTY QUERY STRINGS" in
ossl-guide-libraries-introduction(7).
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 (which uses NULL for the
library context and property query string).
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 use an EVP_PKEY_CTX or an EVP_PKEY(3), such as
EVP_DigestSignInit(3), all fetch the implementations implicitly. Usually
the algorithm to fetch is determined based on the type of key that is
being used and the function that has been called.
Performance
If you perform the same operation many times with the same algorithm
then it is recommended to use a single explicit fetch of the algorithm
and then reuse the explicitly fetched algorithm each subsequent time.
This will typically be faster than implicitly fetching the algorithm
every time you use it. See an example of Explicit fetching in "USING
ALGORITHMS IN APPLICATIONS".
Prior to OpenSSL 3.0, functions such as EVP_sha256() which return a
"const" object were used directly to indicate the algorithm to use in
various function calls. If you pass the return value of one of these
convenience functions to an operation then you are using implicit
fetching. If you are converting an application that worked with an
OpenSSL version prior to OpenSSL 3.0 then consider changing instances of
implicit fetching to explicit fetching instead.
If an explicitly fetched object is not passed to an operation, then any
implicit fetch will use an internally cached prefetched object, but it
will still be slower than passing the explicitly fetched object
directly.
The following functions can be used for explicit fetching:
EVP_MD_fetch(3)
Fetch a message digest/hashing algorithm implementation.
EVP_CIPHER_fetch(3)
Fetch a symmetric cipher algorithm implementation.
EVP_KDF_fetch(3)
Fetch a Key Derivation Function (KDF) algorithm implementation.
EVP_MAC_fetch(3)
Fetch a Message Authentication Code (MAC) algorithm implementation.
EVP_KEM_fetch(3)
Fetch a Key Encapsulation Mechanism (KEM) algorithm implementation
OSSL_ENCODER_fetch(3)
Fetch an encoder algorithm implementation (e.g. to encode keys to a
specified format).
OSSL_DECODER_fetch(3)
Fetch a decoder algorithm implementation (e.g. to decode keys from a
specified format).
EVP_RAND_fetch(3)
Fetch a Pseudo Random Number Generator (PRNG) algorithm
implementation.
See "OPERATIONS AND ALGORITHMS" in OSSL_PROVIDER-default(7), "OPERATIONS
AND ALGORITHMS" in OSSL_PROVIDER-FIPS(7), "OPERATIONS AND ALGORITHMS" in
OSSL_PROVIDER-legacy(7) and "OPERATIONS AND ALGORITHMS" in
OSSL_PROVIDER-base(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 that is preferably from the FIPS
provider in the default context:
EVP_MD *md = EVP_MD_fetch(NULL, "SHA2-256", "provider=?fips");
...
EVP_MD_free(md);
Fetch an implementation of SHA2-256 from the default provider in the
specified library context:
EVP_MD *md = EVP_MD_fetch(libctx, "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);
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. Additional examples can be found in the OpenSSL demos (see "DEMO
APPLICATIONS" in ossl-guide-libraries-introduction(7)).
#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;
}
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).
As well as using encoders/decoders directly there are also some helper
functions that can be used for certain well known and commonly used
formats. For example see PEM_read_PrivateKey(3) and
PEM_write_PrivateKey(3) for information about reading and writing key
data from PEM encoded files.
FURTHER READING
See ossl-guide-libssl-introduction(7) for an introduction to using
"libssl".
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 2000-2024 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>.
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