EVP_KDF-SS(7ossl)

💅 EVP_KDF-SS 💅

okay so this is the OpenSSL manual page for EVP_KDF-SS, bestie — the Single Step / One Step EVP_KDF implementation. yes it's a 7ossl page, yes we're gonna read it together, and yes it slaps.

✨ NAME ✨

EVP_KDF-SS — the Single Step / One Step EVP_KDF implementation. that's it, that's the name, it ate 💅

🌸 DESCRIPTION 🌸

so the EVP_KDF-SS algorithm implements the Single Step key derivation function (SSKDF). SSKDF derives a key using input such as a shared secret key (that was generated during the execution of a key establishment scheme) and fixedinfo. SSKDF is also informally referred to as 'Concat KDF'. like, that's its nickname, iykyk ✨

the output is considered to be keying material. periodt.

🎀 Auxiliary function 🎀

the implementation uses a selectable auxiliary function H, which can be one of:

H(x) = hash(x, digest=md)
H(x) = HMAC_hash(x, key=salt, digest=md)
H(x) = KMACxxx(x, key=salt, custom="KDF", outlen=mac_size)

both the HMAC and KMAC implementations set the key using the 'salt' value. the hash and HMAC also require the digest to be set. no cap, that's literally the whole vibe 🫶

🦋 Identity 🦋

"SSKDF" is the name for this implementation; it can be used with the EVP_KDF_fetch() function. that's your main character name tag, don't lose it 💖

👑 Supported parameters 👑

the supported parameters are:

"properties" (OSSL_KDF_PARAM_PROPERTIES) <UTF8 string>
"digest" (OSSL_KDF_PARAM_DIGEST) <UTF8 string> — this parameter is ignored for KMAC. like, KMAC said not today 🙈
"mac" (OSSL_KDF_PARAM_MAC) <UTF8 string>
"maclen" (OSSL_KDF_PARAM_MAC_SIZE) <unsigned integer>
"salt" (OSSL_KDF_PARAM_SALT) <octet string> — these parameters work as described in "PARAMETERS" in EVP_KDF(3).
"key" (OSSL_KDF_PARAM_SECRET) <octet string> — this parameter set the shared secret that is used for key derivation. (yes, "set", we're keeping it exactly how the original says it, don't come for me 😭)
"info" (OSSL_KDF_PARAM_INFO) <octet string> — this parameter sets an optional value for fixedinfo, also known as otherinfo.

the OpenSSL FIPS provider also supports the following parameters:

"fips-indicator" (OSSL_KDF_PARAM_FIPS_APPROVED_INDICATOR) <integer> — a getter that returns 1 if the operation is FIPS approved, or 0 otherwise. this may be used after calling EVP_KDF_derive. it returns 0 if "key-check" is set to 0 and the check fails.
"key-check" (OSSL_KDF_PARAM_FIPS_KEY_CHECK) <integer> — the default value of 1 causes an error during EVP_KDF_CTX_set_params() if the length of used key-derivation key (OSSL_KDF_PARAM_KEY) is shorter than 112 bits. setting this to zero will ignore the error and set the approved "fips-indicator" to 0. this option breaks FIPS compliance if it causes the approved "fips-indicator" to return 0. ⚠️ like, that's a real consequence, not a drill.

🌈 NOTES 🌈

a context for SSKDF can be obtained by calling:

EVP_KDF *kdf = EVP_KDF_fetch(NULL, "SSKDF", NULL);
EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);

the output length of an SSKDF is specified via the keylen parameter to the EVP_KDF_derive(3) function. manifest that length, bestie 🧚

💜 EXAMPLES 💜

🍓 example one: SHA-256

this example derives 10 bytes using H(x) = SHA-256, with the secret key "secret" and fixedinfo value "label":

EVP_KDF *kdf;
EVP_KDF_CTX *kctx;
unsigned char out[10];
OSSL_PARAM params[4], *p = params;

kdf = EVP_KDF_fetch(NULL, "SSKDF", NULL);
kctx = EVP_KDF_CTX_new(kdf);
EVP_KDF_free(kdf);

*p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_DIGEST,
                                        SN_sha256, strlen(SN_sha256));
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_KEY,
                                         "secret", (size_t)6);
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_INFO,
                                         "label", (size_t)5);
*p = OSSL_PARAM_construct_end();
if (EVP_KDF_derive(kctx, out, sizeof(out), params) <= 0) {
    error("EVP_KDF_derive");
}

EVP_KDF_CTX_free(kctx);

🍓 example two: HMAC(SHA-256)

this example derives 10 bytes using H(x) = HMAC(SHA-256), with the secret key "secret", fixedinfo value "label" and salt "salt":

EVP_KDF *kdf;
EVP_KDF_CTX *kctx;
unsigned char out[10];
OSSL_PARAM params[6], *p = params;

kdf = EVP_KDF_fetch(NULL, "SSKDF", NULL);
kctx = EVP_KDF_CTX_new(kdf);
EVP_KDF_free(kdf);

*p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_MAC,
                                        SN_hmac, strlen(SN_hmac));
*p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_DIGEST,
                                        SN_sha256, strlen(SN_sha256));
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SECRET,
                                         "secret", (size_t)6);
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_INFO,
                                         "label", (size_t)5);
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SALT,
                                         "salt", (size_t)4);
*p = OSSL_PARAM_construct_end();
if (EVP_KDF_derive(kctx, out, sizeof(out), params) <= 0) {
    error("EVP_KDF_derive");
}

EVP_KDF_CTX_free(kctx);

🍓 example three: KMAC128

this example derives 10 bytes using H(x) = KMAC128(x,salt,outlen), with the secret key "secret" fixedinfo value "label", salt of "salt" and KMAC outlen of 20:

EVP_KDF *kdf;
EVP_KDF_CTX *kctx;
unsigned char out[10];
OSSL_PARAM params[6], *p = params;

kdf = EVP_KDF_fetch(NULL, "SSKDF", NULL);
kctx = EVP_KDF_CTX_new(kdf);
EVP_KDF_free(kdf);

*p++ = OSSL_PARAM_construct_utf8_string(OSSL_KDF_PARAM_MAC,
                                        SN_kmac128, strlen(SN_kmac128));
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SECRET,
                                         "secret", (size_t)6);
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_INFO,
                                         "label", (size_t)5);
*p++ = OSSL_PARAM_construct_octet_string(OSSL_KDF_PARAM_SALT,
                                         "salt", (size_t)4);
*p++ = OSSL_PARAM_construct_size_t(OSSL_KDF_PARAM_MAC_SIZE, (size_t)20);
*p = OSSL_PARAM_construct_end();
if (EVP_KDF_derive(kctx, out, sizeof(out), params) <= 0) {
    error("EVP_KDF_derive");
}

EVP_KDF_CTX_free(kctx);

⭐ CONFORMING TO ⭐

NIST SP800-56Cr1. that's the standard, we stan a certified queen 👑

go read those too, they're all linked in the group chat 💖

🦋 HISTORY 🦋

this functionality was added in OpenSSL 3.0. a classic, honestly.

💅 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>.

okay that's the whole page, bestie. every flag, every param, every example — nothing skipped, nothing invented. you're welcome 💖

and that's a wrap from OpenSSL 3.5.0, dated 2026-09-25 — EVP_KDF-SS(7ossl), signing off 🎀✨🫶

📦 where this page comes from

opensuse-leap 16.0: openssl-3 3.5.0-160000.11.1 · Apache-2.0
opensuse-leap 16.0: openssl-3 3.5.0-160000.11.1 · Apache-2.0
opensuse-leap 16.0: openssl-3 3.5.0-160000.11.1 · Apache-2.0
opensuse-leap 16.0: openssl-3 3.5.0-160000.11.1 · Apache-2.0
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-04: a language model's rewrite of the original, under its terms