PROVIDER-KDF(7SSL)

📦

version 4.0.3 · 13 versions

✨ PROVIDER-KDF ✨

okay bestie, so this is a page from OpenSSL — manual section 7SSL, the one for miscellaneous/overview stuff. Sit down, grab a strawberry 🍓, because we're talking about provider-kdf, aka The KDF library <-> provider functions. yep, that little <-> is doing a lot of heavy lifting and we're gonna respect it.

provider-kdf — The KDF library <-> provider functions 💖

That's the whole NAME situation. Short, sweet, iconic. 🎀

💅 SYNOPSIS 💅

okay so first you gotta pull in the headers, bestie:

#include <openssl/core_dispatch.h>
#include <openssl/core_names.h>

and then — this is the important part, don't get it twisted 🙈 —

/*
 * None of these are actual functions, but are displayed like this for
 * the function signatures for functions that are offered as function
 * pointers in OSSL_DISPATCH arrays.
 */

like, read that comment fr fr. none of these are real functions you can just call. they're signatures — the shape of things that show up as function pointers inside OSSL_DISPATCH arrays. it's giving blueprint energy, not callable-API energy. 🫶

🌸 Context management 🌸

the whole "who am i, where did i come from, why am i here" of a KDF context:

void *OSSL_FUNC_kdf_newctx(void *provctx);
void OSSL_FUNC_kdf_freectx(void *kctx);
void *OSSL_FUNC_kdf_dupctx(void *src);

🔥 Encryption/decryption 🔥

(yeah that's the comment up there, we're just keeping the lore intact)

int OSSL_FUNC_kdf_reset(void *kctx);
int OSSL_FUNC_kdf_derive(void *kctx, unsigned char *key, size_t keylen,
                         const OSSL_PARAM params[]);
void *OSSL_FUNC_kdf_derive_skey(void *ctx, const char *key_type, void *provctx,
                                OSSL_FUNC_skeymgmt_import_fn *import,
                                size_t keylen, const OSSL_PARAM params[]);

🦋 KDF parameter descriptors 🦋

the "tell me what you're into" part:

const OSSL_PARAM *OSSL_FUNC_kdf_gettable_params(void *provctx);
const OSSL_PARAM *OSSL_FUNC_kdf_gettable_ctx_params(void *kcxt, void *provctx);
const OSSL_PARAM *OSSL_FUNC_kdf_settable_ctx_params(void *kcxt, void *provctx);

💜 KDF parameters 💜

and the actual get-and-set moment:

int OSSL_FUNC_kdf_get_params(OSSL_PARAM params[]);
int OSSL_FUNC_kdf_get_ctx_params(void *kctx, OSSL_PARAM params[]);
int OSSL_FUNC_kdf_set_ctx_params(void *kctx, const OSSL_PARAM params[]);
int OSSL_FUNC_kdf_set_skey(void *kctx, void *skeydata, const char *paramname);

⭐ pro tip for reading this page: every single one of those OSSL_FUNC_* names is a slot a provider can fill in. they all get wired up through OSSL_DISPATCH arrays. no cap. 🧚

kk that's the name and the synopsis, the actual descriptions are up next so don't go anywhere — we're just getting started, bestie 💖

💖 DESCRIPTION 💖

Okay bestie, real talk — this doc is for provider authors, like the people writing the actual crypto guts. If that's not you, go peek at provider(7) for the bigger picture first, mmkay? 🫶

The KDF operation is the whole vibe that lets providers implement KDF algorithms and hand them to applications through the API functions EVP_KDF_CTX_reset(3) and EVP_KDF_derive(3). Slay. ✨

So all the "functions" we're about to yap about? They're passed as function pointers between libcrypto and the provider, inside OSSL_DISPATCH(3) arrays, which themselves come through OSSL_ALGORITHM(3) arrays that the provider's provider_query_operation() function returns — see "Provider Functions" in provider-base(7) if you need the receipts.

Every single one of these "functions" has a matching function type definition named OSSL_FUNC_{name}_fn, plus a lil helper to grab the function pointer out of an OSSL_DISPATCH(3) element, named OSSL_FUNC_{name}. Example time, because examples ate 💅 — the "function" OSSL_FUNC_kdf_newctx() has these:

 typedef void *(OSSL_FUNC_kdf_newctx_fn)(void *provctx);
 static ossl_inline OSSL_FUNC_kdf_newctx_fn
     OSSL_FUNC_kdf_newctx(const OSSL_DISPATCH *opf);

And the OSSL_DISPATCH(3) array entries are ID'd by numbers that show up as macros in openssl-core_dispatch.h(7), like so:

 OSSL_FUNC_kdf_newctx               OSSL_FUNC_KDF_NEWCTX
 OSSL_FUNC_kdf_freectx              OSSL_FUNC_KDF_FREECTX
 OSSL_FUNC_kdf_dupctx               OSSL_FUNC_KDF_DUPCTX

 OSSL_FUNC_kdf_reset                OSSL_FUNC_KDF_RESET
 OSSL_FUNC_kdf_derive               OSSL_FUNC_KDF_DERIVE
 OSSL_FUNC_kdf_derive_skey          OSSL_FUNC_KDF_DERIVE_SKEY

 OSSL_FUNC_kdf_get_params           OSSL_FUNC_KDF_GET_PARAMS
 OSSL_FUNC_kdf_get_ctx_params       OSSL_FUNC_KDF_GET_CTX_PARAMS
 OSSL_FUNC_kdf_set_ctx_params       OSSL_FUNC_KDF_SET_CTX_PARAMS

 OSSL_FUNC_kdf_gettable_params      OSSL_FUNC_KDF_GETTABLE_PARAMS
 OSSL_FUNC_kdf_gettable_ctx_params  OSSL_FUNC_KDF_GETTABLE_CTX_PARAMS
 OSSL_FUNC_kdf_settable_ctx_params  OSSL_FUNC_KDF_SETTABLE_CTX_PARAMS
 OSSL_FUNC_kdf_set_skey             OSSL_FUNC_KDF_SET_SKEY

Now, a KDF algorithm implementation doesn't have to implement all of these, no cap. But to be a consistent set of functions, you MUST have at least: OSSL_FUNC_kdf_newctx(), OSSL_FUNC_kdf_freectx(), OSSL_FUNC_kdf_set_ctx_params(), and at least one of OSSL_FUNC_kdf_derive() or OSSL_FUNC_kdf_derive_skey(). Everything else is optional, bestie — bonus points energy. 🎀

🧺 Context Management Functions

OSSL_FUNC_kdf_newctx() should create and return a pointer to a provider side structure that holds context info during a KDF operation. That pointer gets passed back into a bunch of the other KDF operation function calls. The provctx parameter is the provider context made during provider initialisation (check provider(7)).

OSSL_FUNC_kdf_freectx() gets handed a pointer to the provider side KDF context in the kctx parameter. If it receives NULL as the kctx value, it should do literally nothing except return — no drama. Otherwise it should free all resources tied to that context.

OSSL_FUNC_kdf_dupctx() should duplicate the provider side KDF context in the kctx parameter and return the duplicate copy. Clone era. 👑

🔐 Encryption/Decryption Functions

OSSL_FUNC_kdf_reset() initialises a KDF operation given a provider side KDF context in the kctx parameter.

OSSL_FUNC_kdf_derive() performs the KDF operation after processing the params as per OSSL_FUNC_kdf_set_ctx_params(). The kctx parameter holds a pointer to the provider side context. The resulting key of the desired keylen should be written to key. If the algorithm does not support the requested keylen, the function must return error. Not optional, babe.

OSSL_FUNC_kdf_derive_skey() is giving OSSL_FUNC_kdf_derive() but with an opaque object for storing the derived key. It accepts a key_type parameter to give the provider a hint about what type of key (like generic or AES) would be generated, plus the import function from the EVP_SKEYMGMT object to be associated with the key. The EVP_SKEYMGMT object comes from the same provider as the KDF itself. Same-house energy. 🏠

🎛 KDF Parameters

Hit up OSSL_PARAM(3) for the full tea on the parameters structure these functions use.

OSSL_FUNC_kdf_get_params() gets details of parameter values associated with the provider algorithm and stores them in params.

OSSL_FUNC_kdf_set_ctx_params() sets KDF parameters associated with the given provider side KDF context kctx to params. Any parameter settings are additional to whatever was set before — it stacks, it doesn't replace. And passing NULL for params should return true. 💯

OSSL_FUNC_kdf_get_ctx_params() retrieves gettable parameter values associated with the given provider side KDF context kctx and stores them in params. Passing NULL for params should return true.

OSSL_FUNC_kdf_gettable_params(), OSSL_FUNC_kdf_gettable_ctx_params(), and OSSL_FUNC_kdf_settable_ctx_params() all return constant OSSL_PARAM(3) arrays that describe the parameters that OSSL_FUNC_kdf_get_params(), OSSL_FUNC_kdf_get_ctx_params(), and OSSL_FUNC_kdf_set_ctx_params() can handle, respectively. And get this — OSSL_FUNC_kdf_gettable_ctx_params() and OSSL_FUNC_kdf_settable_ctx_params() will return the parameters associated with the provider side context kctx in its current state if it's not NULL. Otherwise, they return the parameters associated with the provider side algorithm provctx. Iconic behaviour honestly.

OSSL_FUNF_kdf_set_skey() sets the input key for KDF functions that take a key as input.

So here's the list of parameters that built-in KDFs currently recognise. Heads up though: not all parameters are relevant to, or are understood by, all KDFs. Read the room, each KDF has its own taste.

"size" (OSSL_KDF_PARAM_SIZE) <unsigned integer> — Gets the output size from the associated KDF ctx. If the algorithm produces a variable amount of output, SIZE_MAX should be returned. And if the input parameters needed to calculate the fixed output size haven't been supplied yet, 0 should be returned, indicating an error. No guessing.
"key" (OSSL_KDF_PARAM_KEY) <octet string> — Sets the key in the associated KDF ctx.
"secret" (OSSL_KDF_PARAM_SECRET) <octet string> — Sets the secret in the associated KDF ctx.
"pass" (OSSL_KDF_PARAM_PASSWORD) <octet string> — Sets the password in the associated KDF ctx.
"cipher" (OSSL_KDF_PARAM_CIPHER) <UTF8 string> — "digest" (OSSL_KDF_PARAM_DIGEST) <UTF8 string> — "mac" (OSSL_KDF_PARAM_MAC) <UTF8 string> — Sets the name of the underlying cipher, digest or MAC to be used. It must name a suitable algorithm for the KDF that's being used. Pick right or perish. 🫠
"maclen" (OSSL_KDF_PARAM_MAC_SIZE) <octet string> — Sets the length of the MAC in the associated KDF ctx.
"properties" (OSSL_KDF_PARAM_PROPERTIES) <UTF8 string> — Sets the properties to be queried when trying to fetch the underlying algorithm. This must be given together with the algorithm naming parameter to be considered valid. They're a package deal. 💞
"iter" (OSSL_KDF_PARAM_ITER) <unsigned integer> — Sets the number of iterations in the associated KDF ctx.
"mode" (OSSL_KDF_PARAM_MODE) <UTF8 string> — Sets the mode in the associated KDF ctx.
"pkcs5" (OSSL_KDF_PARAM_PKCS5) <integer> — Enables or disables the SP800-132 compliance checks. A mode of 0 enables the compliance checks.

The checks performed are:

the iteration count is at least 1000.
the salt length is at least 128 bits.
the derived key length is at least 112 bits.

"ukm" (OSSL_KDF_PARAM_UKM) <octet string> — Sets an optional random string that is provided by the sender called "partyAInfo". In CMS this is the user keying material.
"cekalg" (OSSL_KDF_PARAM_CEK_ALG) <UTF8 string> — Sets the CEK wrapping algorithm name in the associated KDF ctx.
"n" (OSSL_KDF_PARAM_SCRYPT_N) <unsigned integer> — Sets the scrypt work factor parameter N in the associated KDF ctx.
"r" (OSSL_KDF_PARAM_SCRYPT_R) <unsigned integer> — Sets the scrypt work factor parameter r in the associated KDF ctx.
"p" (OSSL_KDF_PARAM_SCRYPT_P) <unsigned integer> — Sets the scrypt work factor parameter p in the associated KDF ctx.
"maxmem_bytes" (OSSL_KDF_PARAM_SCRYPT_MAXMEM) <unsigned integer> — Sets the scrypt work factor parameter maxmem in the associated KDF ctx.
"prefix" (OSSL_KDF_PARAM_PREFIX) <octet string> — Sets the prefix string using by the TLS 1.3 version of HKDF in the associated KDF ctx.
"label" (OSSL_KDF_PARAM_LABEL) <octet string> — Sets the label string using by the TLS 1.3 version of HKDF in the associated KDF ctx.
"data" (OSSL_KDF_PARAM_DATA) <octet string> — Sets the context string using by the TLS 1.3 version of HKDF in the associated KDF ctx.
"info" (OSSL_KDF_PARAM_INFO) <octet string> — Sets the optional shared info in the associated KDF ctx.
"seed" (OSSL_KDF_PARAM_SEED) <octet string> — Sets the IV in the associated KDF ctx.
"xcghash" (OSSL_KDF_PARAM_SSHKDF_XCGHASH) <octet string> — Sets the xcghash in the associated KDF ctx.
"session_id" (OSSL_KDF_PARAM_SSHKDF_SESSION_ID) <octet string> — Sets the session ID in the associated KDF ctx.
"type" (OSSL_KDF_PARAM_SSHKDF_TYPE) <UTF8 string> — Sets the SSH KDF type parameter in the associated KDF ctx. There are six supported types, and yes we're listing all six because they all matter:
- EVP_KDF_SSHKDF_TYPE_INITIAL_IV_CLI_TO_SRV — The Initial IV from client to server. A single char of value 65 (ASCII char 'A').
- EVP_KDF_SSHKDF_TYPE_INITIAL_IV_SRV_TO_CLI — The Initial IV from server to client A single char of value 66 (ASCII char 'B').
- EVP_KDF_SSHKDF_TYPE_ENCRYPTION_KEY_CLI_TO_SRV — The Encryption Key from client to server A single char of value 67 (ASCII char 'C').
- EVP_KDF_SSHKDF_TYPE_ENCRYPTION_KEY_SRV_TO_CLI — The Encryption Key from server to client A single char of value 68 (ASCII char 'D').
- EVP_KDF_SSHKDF_TYPE_INTEGRITY_KEY_CLI_TO_SRV — The Integrity Key from client to server A single char of value 69 (ASCII char 'E').
- EVP_KDF_SSHKDF_TYPE_INTEGRITY_KEY_SRV_TO_CLI — The Integrity Key from client to server A single char of value 70 (ASCII char 'F').
"constant" (OSSL_KDF_PARAM_CONSTANT) <octet string> — Sets the constant value in the associated KDF ctx.
"id" (OSSL_KDF_PARAM_PKCS12_ID) <integer> — Sets the intended usage of the output bits in the associated KDF ctx. It is defined as per RFC 7292 section B.3. 📜

💖 RETURN VALUES 💖

okay so here's the vibe check on what each of these functions hands back to you:

OSSL_FUNC_kdf_newctx() and OSSL_FUNC_kdf_dupctx() — these two cuties should return the newly created provider side KDF context, or <code>NULL</code> if it went wrong. no in-between, bestie, it's giving binary 💅
OSSL_FUNC_kdf_derive_skey() — this one returns the newly created opaque provider-side key object, or <code>NULL</code> on failure. opaque?? yes. mysterious?? extremely. but that's her whole thing ✨
OSSL_FUNC_kdf_derive(), OSSL_FUNC_kdf_get_params(), OSSL_FUNC_kdf_get_ctx_params(), OSSL_FUNC_kdf_set_ctx_params() and OSSL_FUNC_kdf_set_skey() — all of these return <code>1</code> for success or <code>0</code> on error. classic ones-and-zeros energy, iconic 🎀
OSSL_FUNC_kdf_gettable_params(), OSSL_FUNC_kdf_gettable_ctx_params() and OSSL_FUNC_kdf_settable_ctx_params() — these should return a constant OSSL_PARAM(3) array, or <code>NULL</code> if none is offered. like, sometimes the answer is just "girl, there isn't one" and honestly? valid 🙈

🌸 NOTES 🌸

the KDF life-cycle is fully described in life_cycle-kdf(7). go read it, i'm literally begging you 🫶

providers should make sure that the various transitions listed in there are actually supported, okay?? like, all of them. not just the cute ones.

⚠️ heads up bestie: at some point the EVP layer will begin enforcing the listed transitions. so if you've been vibing without them, that era is ending. no cap. 💀

🦋 SEE ALSO 🦋

(the holy trinity, fr fr 👑)

⭐ HISTORY ⭐

the provider KDF interface was introduced in OpenSSL 3.0.
OSSL_FUNC_kdf_derive_skey() and OSSL_FUNC_kdf_set_skey() were added in OpenSSL 3.6. new girls on the block ✨

💜 COPYRIGHT 💜

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>.

and that's a wrap on the whole page, bestie!! 🎀 this was PROVIDER-KDF(7SSL) from OpenSSL 4.0.3, dated 2026-09-29 — go build something gorgeous with it, mwah 💅💖🌸

📦 where this page comes from

ubuntu stonking: openssl 4.0.3-1ubuntu1
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-11-03: a language model's rewrite of the original, under its terms