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+=pod
+
+=head1 NAME
+
+EVP_KDF-SS - The Single Step / One Step EVP_KDF implementation
+
+=head1 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 'Concat KDF'.
+
+The output is considered to be keying material.
+
+=head2 Auxiliary function
+
+The implementation uses a selectable auxiliary function H, which can be one of:
+
+=over 4
+
+=item B<H(x) = hash(x, digest=md)>
+
+=item B<H(x) = HMAC_hash(x, key=salt, digest=md)>
+
+=item B<H(x) = KMACxxx(x, key=salt, custom="KDF", outlen=mac_size)>
+
+=back
+
+Both the HMAC and KMAC implementations set the key using the 'salt' value.
+The hash and HMAC also require the digest to be set.
+
+=head2 Identity
+
+"SSKDF" is the name for this implementation; it
+can be used with the EVP_KDF_fetch() function.
+
+=head2 Supported parameters
+
+The supported parameters are:
+
+=over 4
+
+=item "properties" (B<OSSL_KDF_PARAM_PROPERTIES>) <UTF8 string>
+
+=item "digest" (B<OSSL_KDF_PARAM_DIGEST>) <UTF8 string>
+
+This parameter is ignored for KMAC.
+
+=item "mac" (B<OSSL_KDF_PARAM_MAC>) <UTF8 string>
+
+=item "maclen" (B<OSSL_KDF_PARAM_MAC_SIZE>) <unsigned integer>
+
+=item "salt" (B<OSSL_KDF_PARAM_SALT>) <octet string>
+
+These parameters work as described in L<EVP_KDF(3)/PARAMETERS>.
+
+=item "key" (B<OSSL_KDF_PARAM_SECRET>) <octet string>
+
+This parameter set the shared secret that is used for key derivation.
+
+=item "info" (B<OSSL_KDF_PARAM_INFO>) <octet string>
+
+This parameter sets an optional value for fixedinfo, also known as otherinfo.
+
+=back
+
+=head1 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 I<keylen>
+parameter to the L<EVP_KDF_derive(3)> function.
+
+=head1 EXAMPLES
+
+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);
+
+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);
+
+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);
+
+=head1 CONFORMING TO
+
+NIST SP800-56Cr1.
+
+=head1 SEE ALSO
+
+L<EVP_KDF(3)>,
+L<EVP_KDF_CTX_new(3)>,
+L<EVP_KDF_CTX_free(3)>,
+L<EVP_KDF_CTX_set_params(3)>,
+L<EVP_KDF_CTX_get_kdf_size(3)>,
+L<EVP_KDF_derive(3)>,
+L<EVP_KDF(3)/PARAMETERS>
+
+=head1 HISTORY
+
+This functionality was added in OpenSSL 3.0.
+
+=head1 COPYRIGHT
+
+Copyright 2019-2023 The OpenSSL Project Authors. All Rights Reserved.  Copyright
+(c) 2019, Oracle and/or its affiliates.  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
+L<https://www.openssl.org/source/license.html>.
+
+=cut