OSSL-GUIDE-L...-INTRODUCTION(7ossl)

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💖 OSSL-GUIDE-L...-INTRODUCTION(7ossl) 💖

okay bestie, welcome to the OpenSSL Guide: An introduction to libcrypto — manual name ossl-guide-libcrypto-introduction, crypto. This is giving "crypto library lore, but make it friendly," and we're getting through it together. 🫶

🌸 NAME 🌸

ossl-guide-libcrypto-introduction, crypto — OpenSSL Guide: An introduction to libcrypto

✨ INTRODUCTION ✨

The OpenSSL cryptography library (aka "libcrypto") is the icon that gives you access to a wide range of cryptographic algorithms used in various Internet standards. No cap, the services this library provides are used by the OpenSSL implementations of TLS and CMS, and they've also been used to implement many other third party products and protocols. 💅

The functionality list is lowkey stacked: 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. Obsessed. ✨

🧚 Algorithms 🧚

Cryptographic primitives like the SHA256 digest or AES encryption are called "algorithms" in OpenSSL. Each algorithm can have multiple implementations ready to use. For example, the RSA algorithm comes as a "default" implementation that's suitable for general use, and a "fips" implementation that's been validated to FIPS 140 standards for when that matters. It's also possible for a third party to add extra implementations, like in a hardware security module (HSM). 🦋

Algorithms are implemented in providers. See ossl-guide-libraries-introduction(7) for the provider tea. 🍓

🦋 Operations 🦋

Different algorithms can be grouped by purpose — like encryption algorithms vs digesting-data algorithms. These groups are called "operations" in OpenSSL. Each operation has its own set of functions. For example, to perform an encryption operation using AES (or any other encryption algorithm) you'd use the encryption functions detailed on the EVP_EncryptInit(3) page. Or to perform a digest operation using SHA256, you'd use the digesting functions on the EVP_DigestInit(3) page. Periodt. 💅

🔥 ALGORITHM FETCHING 🔥

To use an algorithm, an implementation for it must first be "fetched." Fetching is literally 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. 🔍

OpenSSL supports two types of fetching — "Explicit fetching" and "Implicit fetching."

💅 Explicit fetching 💅

Explicit fetching is when you directly call a specific API to fetch an algorithm implementation from a provider. That 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. You — yes, you, bestie — are responsible for freeing the object returned from the APINAME_fetch function using APINAME_free when it's no longer needed. 🫶

These fetching functions follow a pretty 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).
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).

side note, bestie: providers may register algorithms against a list of names using a string with a colon separated list of names, but fetching algorithms using that format is currently unsupported. Don't try it. 🙈

The algorithm implementation that gets fetched can then be used with other diverse functions that use them. For example, EVP_DigestInit_ex(3) takes as a parameter an EVP_MD object that may have been returned from an earlier call to EVP_MD_fetch(3). ✨

🌈 Implicit fetching 🌈

OpenSSL has a bunch of functions that return an algorithm object with no associated implementation, like EVP_sha256(3), EVP_aes_128_cbc(3), EVP_get_cipherbyname(3) or EVP_get_digestbyname(3). These exist for compatibility with OpenSSL before version 3.0, back when explicit fetching wasn't available. 🌸

When they're 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 happen when a NULL algorithm parameter is supplied. Then an algorithm implementation is implicitly fetched using default search criteria and an algorithm name that's consistent with the context in which it's 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's being used and the function that's been called. 🔑

🎀 Performance 🎀

If you're doing the same operation many times with the same algorithm, the recommendation is to use a single explicit fetch of the algorithm and then reuse that explicitly fetched algorithm every subsequent time. That's typically 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() that 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 those convenience functions to an operation, you're using implicit fetching. If you're converting an application that worked with an OpenSSL version prior to OpenSSL 3.0, consider changing instances of implicit fetching to explicit fetching instead. 💡

If an explicitly fetched object isn't 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. Fr fr, pass the object. 💅

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 💀

Here's the part where we actually fetch algorithm implementations. Vibe check, examples time. 🌈

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 ✨

Okay bestie so like, how do you actually use these algorithms in your app? 🫶

Cryptographic algorithms are made available to applications through use of the "EVP" APIs. Each of the various operations — encryption, digesting, message authentication codes, etc. — has a set of EVP function calls that can be invoked to use them. See the evp(7) page for further details, iykyk.

Most of these follow a common pattern, and honestly it's giving routine in the best way:

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;
}

Like, look at that error-handling. goto err in a C file is so real, no notes 💅

🔑 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. (Overachievers, honestly.)

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". Go read it, bestie, it's the next main-character arc ✨

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>.

Okay and that's the whole page, bestie! 🦋 This one's from OpenSSL version 3.5.6, dated 2026-04-0, and yeah — I noticed the date looks like it got cut off too, but that's literally what's on the page, so I'm keeping it exactly as-is. No cap, no edits. 💖

Byeee, go digest something 🫶✨

📦 where this page comes from

freebsd 15.1: base 15.1
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-04: a language model's rewrite of the original, under its terms