Path: blob/main/secure/lib/libcrypto/man/man7/EVP_KDF-SS.7
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.IX Title "EVP_KDF-SS 7ossl"
.TH EVP_KDF-SS 7ossl 2026-01-27 3.5.5 OpenSSL
.\" For nroff, turn off justification. Always turn off hyphenation; it makes
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.SH NAME
EVP_KDF\-SS \- The Single Step / One Step EVP_KDF implementation
.SH DESCRIPTION
.IX Header "DESCRIPTION"
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 \*(AqConcat KDF\*(Aq.
.PP
The output is considered to be keying material.
.SS "Auxiliary function"
.IX Subsection "Auxiliary function"
The implementation uses a selectable auxiliary function H, which can be one of:
.IP "\fBH(x) = hash(x, digest=md)\fR" 4
.IX Item "H(x) = hash(x, digest=md)"
.PD 0
.IP "\fBH(x) = HMAC_hash(x, key=salt, digest=md)\fR" 4
.IX Item "H(x) = HMAC_hash(x, key=salt, digest=md)"
.IP "\fBH(x) = KMACxxx(x, key=salt, custom=""KDF"", outlen=mac_size)\fR" 4
.IX Item "H(x) = KMACxxx(x, key=salt, custom=""KDF"", outlen=mac_size)"
.PD
.PP
Both the HMAC and KMAC implementations set the key using the \*(Aqsalt\*(Aq value.
The hash and HMAC also require the digest to be set.
.SS Identity
.IX Subsection "Identity"
"SSKDF" is the name for this implementation; it
can be used with the \fBEVP_KDF_fetch()\fR function.
.SS "Supported parameters"
.IX Subsection "Supported parameters"
The supported parameters are:
.IP """properties"" (\fBOSSL_KDF_PARAM_PROPERTIES\fR) <UTF8 string>" 4
.IX Item """properties"" (OSSL_KDF_PARAM_PROPERTIES) <UTF8 string>"
.PD 0
.IP """digest"" (\fBOSSL_KDF_PARAM_DIGEST\fR) <UTF8 string>" 4
.IX Item """digest"" (OSSL_KDF_PARAM_DIGEST) <UTF8 string>"
.PD
This parameter is ignored for KMAC.
.IP """mac"" (\fBOSSL_KDF_PARAM_MAC\fR) <UTF8 string>" 4
.IX Item """mac"" (OSSL_KDF_PARAM_MAC) <UTF8 string>"
.PD 0
.IP """maclen"" (\fBOSSL_KDF_PARAM_MAC_SIZE\fR) <unsigned integer>" 4
.IX Item """maclen"" (OSSL_KDF_PARAM_MAC_SIZE) <unsigned integer>"
.IP """salt"" (\fBOSSL_KDF_PARAM_SALT\fR) <octet string>" 4
.IX Item """salt"" (OSSL_KDF_PARAM_SALT) <octet string>"
.PD
These parameters work as described in "PARAMETERS" in \fBEVP_KDF\fR\|(3).
.IP """key"" (\fBOSSL_KDF_PARAM_SECRET\fR) <octet string>" 4
.IX Item """key"" (OSSL_KDF_PARAM_SECRET) <octet string>"
This parameter set the shared secret that is used for key derivation.
.IP """info"" (\fBOSSL_KDF_PARAM_INFO\fR) <octet string>" 4
.IX Item """info"" (OSSL_KDF_PARAM_INFO) <octet string>"
This parameter sets an optional value for fixedinfo, also known as otherinfo.
.PP
The OpenSSL FIPS provider also supports the following parameters:
.IP """fips\-indicator"" (\fBOSSL_KDF_PARAM_FIPS_APPROVED_INDICATOR\fR) <integer>" 4
.IX Item """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.
.IP """key\-check"" (\fBOSSL_KDF_PARAM_FIPS_KEY_CHECK\fR) <integer>" 4
.IX Item """key-check"" (OSSL_KDF_PARAM_FIPS_KEY_CHECK) <integer>"
The default value of 1 causes an error during \fBEVP_KDF_CTX_set_params()\fR if the
length of used key\-derivation key (\fBOSSL_KDF_PARAM_KEY\fR) 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.
.SH NOTES
.IX Header "NOTES"
A context for SSKDF can be obtained by calling:
.PP
.Vb 2
\& EVP_KDF *kdf = EVP_KDF_fetch(NULL, "SSKDF", NULL);
\& EVP_KDF_CTX *kctx = EVP_KDF_CTX_new(kdf);
.Ve
.PP
The output length of an SSKDF is specified via the \fIkeylen\fR
parameter to the \fBEVP_KDF_derive\fR\|(3) function.
.SH EXAMPLES
.IX Header "EXAMPLES"
This example derives 10 bytes using H(x) = SHA\-256, with the secret key "secret"
and fixedinfo value "label":
.PP
.Vb 4
\& 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);
.Ve
.PP
This example derives 10 bytes using H(x) = HMAC(SHA\-256), with the secret key "secret",
fixedinfo value "label" and salt "salt":
.PP
.Vb 4
\& 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);
.Ve
.PP
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:
.PP
.Vb 4
\& 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);
.Ve
.SH "CONFORMING TO"
.IX Header "CONFORMING TO"
NIST SP800\-56Cr1.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBEVP_KDF\fR\|(3),
\&\fBEVP_KDF_CTX_new\fR\|(3),
\&\fBEVP_KDF_CTX_free\fR\|(3),
\&\fBEVP_KDF_CTX_set_params\fR\|(3),
\&\fBEVP_KDF_CTX_get_kdf_size\fR\|(3),
\&\fBEVP_KDF_derive\fR\|(3),
"PARAMETERS" in \fBEVP_KDF\fR\|(3)
.SH HISTORY
.IX Header "HISTORY"
This functionality was added in OpenSSL 3.0.
.SH COPYRIGHT
.IX Header "COPYRIGHT"
Copyright 2019\-2024 The OpenSSL Project Authors. All Rights Reserved. Copyright
(c) 2019, Oracle and/or its affiliates. All rights reserved.
.PP
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>.