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.

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".

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>.