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EVP_KDF-HKDF(7)			    OpenSSL		       EVP_KDF-HKDF(7)

NAME
       EVP_KDF-HKDF - The HKDF EVP_KDF implementation

DESCRIPTION
       Support for computing the HKDF KDF through the EVP_KDF API.

       The EVP_KDF-HKDF	algorithm implements the HKDF key derivation function.
       HKDF follows the	"extract-then-expand" paradigm,	where the KDF
       logically consists of two modules. The first stage takes	the input
       keying material and "extracts" from it a	fixed-length pseudorandom key
       K. The second stage "expands" the key K into several additional
       pseudorandom keys (the output of	the KDF).

   Identity
       "HKDF" is the name for this implementation; it can be used with the
       EVP_KDF_fetch() function.

   Supported parameters
       The supported parameters	are:

       "properties" (OSSL_KDF_PARAM_PROPERTIES)	<UTF8 string>
       "digest"	(OSSL_KDF_PARAM_DIGEST)	<UTF8 string>
       "key" (OSSL_KDF_PARAM_KEY) <octet string>
       "salt" (OSSL_KDF_PARAM_SALT) <octet string>
	   These parameters work as described in "PARAMETERS" in EVP_KDF(3).

       "info" (OSSL_KDF_PARAM_INFO) <octet string>
	   This	parameter sets the info	value.	The length of the context info
	   buffer cannot exceed	1024 bytes; this should	be more	than enough
	   for any normal use of HKDF.

       "mode" (OSSL_KDF_PARAM_MODE) <UTF8 string> or <integer>
	   This	parameter sets the mode	for the	HKDF operation.	 There are
	   three modes that are	currently defined:

	   EVP_KDF_HKDF_MODE_EXTRACT_AND_EXPAND	"EXTRACT_AND_EXPAND"
	       This is the default mode.  Calling EVP_KDF_derive(3) on an
	       EVP_KDF_CTX set up for HKDF will	perform	an extract followed by
	       an expand operation in one go.  The derived key returned	will
	       be the result after the expand operation. The intermediate
	       fixed-length pseudorandom key K is not returned.

	       In this mode the	digest,	key, salt and info values must be set
	       before a	key is derived otherwise an error will occur.

	   EVP_KDF_HKDF_MODE_EXTRACT_ONLY "EXTRACT_ONLY"
	       In this mode calling EVP_KDF_derive(3) will just	perform	the
	       extract operation. The value returned will be the intermediate
	       fixed-length pseudorandom key K.	 The keylen parameter must
	       match the size of K, which can be looked	up by calling
	       EVP_KDF_size() after setting the	mode and digest.

	       The digest, key and salt	values must be set before a key	is
	       derived otherwise an error will occur.

	   EVP_KDF_HKDF_MODE_EXPAND_ONLY "EXPAND_ONLY"
	       In this mode calling EVP_KDF_derive(3) will just	perform	the
	       expand operation. The input key should be set to	the
	       intermediate fixed-length pseudorandom key K returned from a
	       previous	extract	operation.

	       The digest, key and info	values must be set before a key	is
	       derived otherwise an error will occur.

NOTES
       A context for HKDF can be obtained by calling:

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

       The output length of an HKDF expand operation is	specified via the
       keylen parameter	to the EVP_KDF_derive(3) function.  When using
       EVP_KDF_HKDF_MODE_EXTRACT_ONLY the keylen parameter must	equal the size
       of the intermediate fixed-length	pseudorandom key otherwise an error
       will occur.  For	that mode, the fixed output size can be	looked up by
       calling EVP_KDF_size() after setting the	mode and digest	on the
       EVP_KDF_CTX.

EXAMPLES
       This example derives 10 bytes using SHA-256 with	the secret key
       "secret", salt value "salt" and info value "label":

	EVP_KDF	*kdf;
	EVP_KDF_CTX *kctx;
	unsigned char out[10];
	OSSL_PARAM params[5], *p = params;

	kdf = EVP_KDF_fetch(NULL, "HKDF", 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_octet_string(OSSL_KDF_PARAM_SALT,
						 "salt", (size_t)4);
	*p = OSSL_PARAM_construct_end();
	if (EVP_KDF_CTX_set_params(kctx, params) <= 0) {
	    error("EVP_KDF_CTX_set_params");
	}
	if (EVP_KDF_derive(kctx, out, sizeof(out)) <= 0) {
	    error("EVP_KDF_derive");
	}

	EVP_KDF_CTX_free(kctx);

CONFORMING TO
       RFC 5869

SEE ALSO
       EVP_KDF(3), EVP_KDF_CTX_new(3), EVP_KDF_CTX_free(3), EVP_KDF_size(3),
       EVP_KDF_CTX_set_params(3), EVP_KDF_derive(3), "PARAMETERS" in
       EVP_KDF(3)

COPYRIGHT
       Copyright 2016-2020 The OpenSSL Project Authors.	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
       <https://www.openssl.org/source/license.html>.

3.0.0-alpha6			  2020-08-06		       EVP_KDF-HKDF(7)

NAME | DESCRIPTION | NOTES | EXAMPLES | CONFORMING TO | SEE ALSO | COPYRIGHT

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